# RabbitFarm

### 2022-06-19

#### The Weekly Challenge 169 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

Write a script to generate the first 20 Brilliant Numbers.

### Solution

``````
prime_factors(N, L):-
N > 0,
prime_factors(N, L, 2).
prime_factors(1, [], _):-
!.
prime_factors(N, [F|L], F):-
R is N // F,
N =:= R * F,
!,
prime_factors(R, L, F).
prime_factors(N, L, F):-
next_factor(N, F, NF),
prime_factors(N, L, NF).
next_factor(_, 2, 3):-
!.
next_factor(N, F, NF):-
F * F < N,
!,
NF is F + 2.
next_factor(N, _, N).

brilliants(_) --> [].
brilliants(Seen) --> [X], {brilliant(X), \+ member(X, Seen)}, brilliants([X|Seen]).

brilliant(X):-
current_prolog_flag(max_integer, MAX_INTEGER),
between(1, MAX_INTEGER, X),
prime_factors(X, Factors),
length(Factors, 2),
nth(1, Factors, First),
nth(2, Factors, Second),
number_chars(First, FirstChars),
number_chars(Second, SecondChars),
length(FirstChars, FirstCharsLength),
length(SecondChars, SecondCharsLength),
FirstCharsLength == SecondCharsLength.

n_brilliants(N, Brilliants):-
length(Brilliants, N),
phrase(brilliants([]), Brilliants).
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-1.p
| ?- n_brilliants(20, Brilliants).

Brilliants = [4,6,9,10,14,15,21,25,35,49,121,143,169,187,209,221,247,253,289,299] ?
``````

### Notes

The use of a DCG here seems appropriate as we are generating a sequence of numbers of a DCG will allow us to reason on such lists. The logic for inclusion in the sequence is a bit complex and so it further seems natural to break that into its own predicate. That is not required, of course, but in terms of pure style it seems the DCG starts to look clunky or overstuffed when containing a lot of Prolog code in curly braces. Perhaps that is especially true here where we further need additional predicates for computing the prime factors.

## Part 2

Write a script to generate the first 20 Achilles Numbers.

### Solution

``````
prime_factors(N, L):-
N > 0,
prime_factors(N, L, 2).
prime_factors(1, [], _):-
!.
prime_factors(N, [F|L], F):-
R is N // F,
N =:= R * F,
!,
prime_factors(R, L, F).
prime_factors(N, L, F):-
next_factor(N, F, NF),
prime_factors(N, L, NF).
next_factor(_, 2, 3):-
!.
next_factor(N, F, NF):-
F * F < N,
!,
NF is F + 2.
next_factor(N, _, N).

powerful(N, X):-
M is mod(N, X * X),
M == 0.

imperfect(N):-
Sqrt is round(sqrt(N)),
S is Sqrt - 1,
length(I, S),
fd_domain(I, 2, Sqrt),
fd_all_different(I),
fd_labeling(I),!,
maplist(imperfect(N), I).
imperfect(N, X):-
D is log(N) / log(X),
Check is abs(D - round(D)),
\+ Check < 0.000001.

achilles(_) --> [].
achilles(Seen) --> [X], {current_prolog_flag(max_integer, MAX_INTEGER),
between(2, MAX_INTEGER, X), \+ member(X, Seen), achilles(X)},
achilles([X|Seen]).

achilles(X):-
prime_factors(X, Factors),
maplist(powerful(X), Factors),
imperfect(X).

n_achilles(N, Achilles):-
length(Achilles, N),
phrase(achilles([]), Achilles).
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-2.p
| ?- n_achilles(20, Achilles).

Achilles = [72,108,200,288,392,432,500,648,675,800,864,968,972,1125,1152,1323,1352,1372,1568,1800] ?
``````

### Notes

The approach here for the second task is similar to that of the first. Somewhat surprisingly while the conditions of this sequence are more complex the code itself is represented in a cleaner way. I attribute that to the use of `maplist/2` which streamlines the checking of lists for the two criteria of Achilles numbers: that they are powerful but imperfect.

## References

Challenge 169

posted at: 12:39 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-06-12

#### The Weekly Challenge 168 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

Calculate the first 13 Perrin Primes.

### Solution

``````
perrin_primes(A, B, C) --> {D is B + A, fd_not_prime(D)},
perrin_primes(B, C, D).
perrin_primes(A, B, C) --> {D is B + A, fd_prime(D), D \== C},
[D], perrin_primes(B, C, D).
perrin_primes(A, B, C) --> {D is B + A, fd_prime(D), D == C},
[], perrin_primes(B, C, D).
perrin_primes(_, _, _) --> [], !.

n_perrin_primes(N, PerrinPrimes):-
length(PerrinPrimes, N),
phrase(perrin_primes(3, 0, 2), PerrinPrimes).
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-1.p
| ?- n_perrin_primes(13, PerrinPrimes).

PerrinPrimes = [3,2,5,7,17,29,277,367,853,14197,43721,1442968193,792606555396977] ?
``````

### Notes

This is a pretty cut and dry use of a DCG to generate this interesting mathematical sequence. A couple of things that stand out are (1) the condition that `D \== C` which is to remove the duplicate `5` which occurs naturally at the beginning of the sequence. Afterwards all of the terms strictly increase. Also, (2) although the first two terms are indeed `3, 2` it is a convention to sort these and present them as `2, 3` although I did not happen to do so here.

## Part 2

You are given an integer greater than 1. Write a script to find the home prime of the given number.

### Solution

``````
prime_factors(N, L):-
N > 0,
prime_factors(N, L, 2).
prime_factors(1, [], _):-
!.
prime_factors(N, [F|L], F):-
R is N // F,
N =:= R * F,
!,
prime_factors(R, L, F).
prime_factors(N, L, F):-
next_factor(N, F, NF),
prime_factors(N, L, NF).
next_factor(_, 2, 3):-
!.
next_factor(N, F, NF):-
F * F < N,
!,
NF is F + 2.
next_factor(N, _, N).

factor_concat(Factors, Atom):-
factor_concat(Factors, '', Atom).
factor_concat([], Atom, Atom).
factor_concat([H|T], AtomAccum, Atom):-
number_atom(H, A),
atom_concat(AtomAccum, A, UpdatedAtomAccum),
factor_concat(T, UpdatedAtomAccum, Atom).

home_prime(N, HomePrime):-
prime_factors(N, Factors),
factor_concat(Factors, A),
number_atom(Number, A),
fd_not_prime(Number),
home_prime(Number, HomePrime).
home_prime(N, HomePrime):-
prime_factors(N, Factors),
factor_concat(Factors, A),
number_atom(Number, A),
fd_prime(Number),
HomePrime = Number.
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-2.p
| ?- home_prime(16, HomePrime).

HomePrime = 31636373 ?

(4 ms) yes
| ?- home_prime(54, HomePrime).

HomePrime = 2333 ?

(1 ms) yes
| ?- home_prime(108, HomePrime).

HomePrime = 23971 ?

yes
``````

### Notes

Here we are asked to compute the Home Prime of any given number. The process for doing so is, given `N` to take the prime factors for `N` and concatenate them together. If the result is prime then we are done, that is the Home Prime of `N`, typically written `HP(N)`. This is an easy process to repeat, and in many cases the computation is a very quick one. However, in some cases, the size of the interim numbers on the path to HP(N) grow extremely large and the computation bogs down. I have used the prime factorization code here in several other weekly challenges and it is quite performant but even this runs rather slowly as we are faced with extremely large numbers.

## References

Challenge 168

Home Prime

posted at: 19:21 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-05-22

#### The Weekly Challenge 165 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

Plot lines and points in SVG format.

### Solution

``````
svg-->svg_begin, svg_body, svg_end.
svg_body-->[].
svg_body-->svg_line(_, _, _, _), svg_body.
svg_body-->svg_point(_, _), svg_body.
svg_begin-->['<?xml version="1.0" encoding="UTF-8" standalone="yes"?><!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.0//EN" "http://www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd"><svg height="100%" width="100%" xmlns="http://www.w3.org/2000/svg" xmlns:svg="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">'].
svg_point(X, Y)-->['<circle cx="', X,'" cy="', Y, '" r="1" />'].
svg_line(X1, Y1, X2, Y2)-->['<line x1="', X1, '" x2="', X2, '" y1="', Y1, '" y2="', Y2, '" style="stroke:#006600;" />'].
svg_end-->[''].

plot([], SVGAccum, SVG):-
phrase(svg_begin, Begin),
flatten([Begin|SVGAccum], SVG).
plot([H|T], SVGAccum, SVG):-
length(H, 2),
[X, Y] = H,
phrase(svg_point(X, Y), Point),
plot(T, [Point|SVGAccum], SVG).
plot([H|T], SVGAccum, SVG):-
length(H, 4),
[X1, Y1, X2, Y2] = H,
phrase(svg_line(X1, Y1, X2, Y2), Line),
plot(T, [Line|SVGAccum], SVG).
plot(Lines, SVG):-
phrase(svg_end, End),
plot(Lines, [End], SVG).

main:-
plot([[53,10], [53, 10, 23, 30], [23, 30]], SVG),
maplist(write, SVG), nl,
halt.
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-1.p
<?xml version="1.0" encoding="UTF-8" standalone="yes"?><!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.0//EN" "http://www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd"><svg height="100%" width="100%" xmlns="http://www.w3.org/2000/svg" xmlns:svg="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"><circle cx="23" cy="30" r="1" /><line x1="53" x2="23" y1="10" y2="30" /><circle cx="53" cy="10" r="1" /></svg>
``````

### Notes

SVG is an XML based format, and so the use of a DCG like this may be unexpected. After all, the "grammar" is really dictated by a known XML schema. Still, the DCG is helpful in that we can describe the sequence and formatting of the statements we expect. From this we can generate the required SVG and even do some basic validation.

## Part 2

Given a list of numbers, generate the skip summations.

### Solution

``````
avg_difference(Avg, V, Difference):-
Difference is V - Avg.

square(X, XSquared):-
XSquared is X * X.

xy(X, Y, XY):-
XY is X * Y.

linear_regression(Points, RegressionLineEndpoints):-
length(Points, NumberPoints),
% 1. Calculate average of your X variable.
maplist(nth(1), Points, Xs),
msort(Xs, XSorted),
nth(NumberPoints, XSorted, XMax),
sum_list(Xs, XSum),
XAvg is XSum / NumberPoints,
% 2. Calculate the difference between each X and the average X.
maplist(avg_difference(XAvg), Xs, XDifferences),
% 3. Square the differences and add it all up. This is Sx.
maplist(square, XDifferences, XDifferencesSquared),
sum_list(XDifferencesSquared, Sx),
% 4. Calculate average of your Y variable.
maplist(nth(2), Points, Ys),
sum_list(Ys, YSum),
YAvg is YSum / NumberPoints,
% 5. Multiply the differences (of X and Y from their respective averages) and add them all together.  This is Sxy.
maplist(avg_difference(YAvg), Ys, YDifferences),
maplist(xy, XDifferences, YDifferences, XY),
sum_list(XY, Sxy),
% 6. Using Sx and Sxy, you calculate the intercept by subtracting Sx / Sxy * AVG(X) from AVG(Y).
M is Sxy / Sx,
B is YAvg - (Sxy / Sx * XAvg),
EndX is XMax + 10,
EndY is M * EndX + B,
RegressionLineEndpoints = [0, B, EndX, EndY].

main:-
Points = [[333,129], [39, 189], [140, 156], [292, 134], [393, 52], [160, 166], [362, 122], [13, 193], [341, 104], [320, 113], [109, 177], [203, 152], [343, 100], [225, 110], [23, 186], [282, 102], [284, 98], [205, 133], [297, 114], [292, 126], [339, 112], [327, 79], [253, 136], [61, 169], [128, 176], [346, 72], [316, 103], [124, 162], [65, 181], [159, 137], [212, 116], [337, 86], [215, 136], [153, 137], [390, 104], [100, 180], [76, 188], [77, 181], [69, 195], [92, 186], [275, 96], [250, 147], [34, 174], [213, 134], [186, 129], [189, 154], [361, 82], [363, 89]],
linear_regression(Points, RegressionLine),
write(RegressionLine), nl.
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-1.p --consult-file prolog/ch-2.p
| ?- Points = [[333,129], [39, 189], [140, 156], [292, 134], [393, 52], [160, 166], [362, 122], [13, 193], [341, 104], [320, 113], [109, 177], [203, 152], [343, 100], [225, 110], [23, 186], [282, 102], [284, 98], [205, 133], [297, 114], [292, 126], [339, 112], [327, 79], [253, 136], [61, 169], [128, 176], [346, 72], [316, 103], [124, 162], [65, 181], [159, 137], [212, 116], [337, 86], [215, 136], [153, 137], [390, 104], [100, 180], [76, 188], [77, 181], [69, 195], [92, 186], [275, 96], [250, 147], [34, 174], [213, 134], [186, 129], [189, 154], [361, 82], [363, 89]],
linear_regression(Points, RegressionLine), append(Points, [RegressionLine], PointsLine), plot(PointsLine, SVG), maplist(write, SVG), nl.
<?xml version="1.0" encoding="UTF-8" standalone="yes"?><!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.0//EN" "http://www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd"><svg height="100%" width="100%" xmlns="http://www.w3.org/2000/svg" xmlns:svg="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"><line x1="0" x2="403" y1="200.13227253558171" y2="79.249802930305563" style="stroke:#006600;" /><circle cx="363" cy="89" r="1" /><circle cx="361" cy="82" r="1" /><circle cx="189" cy="154" r="1" /><circle cx="186" cy="129" r="1" /><circle cx="213" cy="134" r="1" /><circle cx="34" cy="174" r="1" /><circle cx="250" cy="147" r="1" /><circle cx="275" cy="96" r="1" /><circle cx="92" cy="186" r="1" /><circle cx="69" cy="195" r="1" /><circle cx="77" cy="181" r="1" /><circle cx="76" cy="188" r="1" /><circle cx="100" cy="180" r="1" /><circle cx="390" cy="104" r="1" /><circle cx="153" cy="137" r="1" /><circle cx="215" cy="136" r="1" /><circle cx="337" cy="86" r="1" /><circle cx="212" cy="116" r="1" /><circle cx="159" cy="137" r="1" /><circle cx="65" cy="181" r="1" /><circle cx="124" cy="162" r="1" /><circle cx="316" cy="103" r="1" /><circle cx="346" cy="72" r="1" /><circle cx="128" cy="176" r="1" /><circle cx="61" cy="169" r="1" /><circle cx="253" cy="136" r="1" /><circle cx="327" cy="79" r="1" /><circle cx="339" cy="112" r="1" /><circle cx="292" cy="126" r="1" /><circle cx="297" cy="114" r="1" /><circle cx="205" cy="133" r="1" /><circle cx="284" cy="98" r="1" /><circle cx="282" cy="102" r="1" /><circle cx="23" cy="186" r="1" /><circle cx="225" cy="110" r="1" /><circle cx="343" cy="100" r="1" /><circle cx="203" cy="152" r="1" /><circle cx="109" cy="177" r="1" /><circle cx="320" cy="113" r="1" /><circle cx="341" cy="104" r="1" /><circle cx="13" cy="193" r="1" /><circle cx="362" cy="122" r="1" /><circle cx="160" cy="166" r="1" /><circle cx="393" cy="52" r="1" /><circle cx="292" cy="134" r="1" /><circle cx="140" cy="156" r="1" /><circle cx="39" cy="189" r="1" /><circle cx="333" cy="129" r="1" /></svg>
``````

### Notes

This is mainly an implementation of the same linear regression procedure as used in the Perl solution to the same problem. By consulting the solution to the first problem we can then re-use the same plotting code.

## References

Challenge 165

Linear Regression Calculation

posted at: 23:28 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-05-15

#### The Weekly Challenge 164 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

Write a script to find all prime numbers less than 1000, which are also palindromes in base 10.

### Solution

``````
:-initialization(main).

palindrome(X):-
fd_labeling(X),
number_codes(X, C),
reverse(C, CR),
number_codes(X, CR).

palindrome_primes(N, PalindromePrimes, NumberPrimes):-
fd_labeling(NumberPrimes),
length(Primes, NumberPrimes),
fd_domain(Primes, 1, N),
fd_all_different(Primes),
maplist(palindrome, Primes),
maplist(fd_prime, Primes),
fd_labeling(Primes),
PalindromePrimes = Primes.

palindrome_primes(N, Primes):-
NP is N // 2,
fd_domain(NumberPrimes, 1, NP),
fd_maximize(palindrome_primes(N, Primes, NumberPrimes), NumberPrimes).

palindrome_prime(N, Prime):-
between(1, N, Prime),
palindrome(Prime),
fd_prime(Prime).

pp(_, _) --> [].
pp(N, Seen) --> [X], {palindrome_prime(N, X), \+ member(X, Seen)}, pp(N, [X|Seen]).

main:-
findall(Prime, palindrome_prime(1000, Prime), PalindromePrimes),
write(PalindromePrimes), nl.
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-1.p
[2,3,5,7,11,101,131,151,181,191,313,353,373,383,727,757,787,797,919,929]
| ?- phrase(pp(1000, []), PalindromePrimes).

PalindromePrimes = [] ? ;

PalindromePrimes =  ? ;

PalindromePrimes = [2,3] ? ;

PalindromePrimes = [2,3,5] ? ;

PalindromePrimes = [2,3,5,7] ? ;

PalindromePrimes = [2,3,5,7,11] ? ;

PalindromePrimes = [2,3,5,7,11,101] ? ;

PalindromePrimes = [2,3,5,7,11,101,131] ? ;

PalindromePrimes = [2,3,5,7,11,101,131,151] ? ;

PalindromePrimes = [2,3,5,7,11,101,131,151,181] ? ;

PalindromePrimes = [2,3,5,7,11,101,131,151,181,191] ? ;

PalindromePrimes = [2,3,5,7,11,101,131,151,181,191,313] ?
.
.
.
``````

### Notes

I experimented with a few different ways to generate Palindrome Primes. The quickest and most efficient way is what is used in `main/0`. Now, suppose we wanted to reason about lists of such numbers versus just generating them there is also a DCG option as shown which will generate or validate all possible lists of Palindrome Primes. Finally, there is the extremely inefficient method using constraints to maximize the size of the list of Palindrome Primes under 1000. This works! Now, does it work "well"? Absolutely not! This is not a very good method for performing the task and is many orders of magnitude slower than the other two. I will admit to an odd satisfaction to getting this unusual approach work, however.

## Part 2

Given a list of numbers, generate the skip summations.

### Solution

``````
:-initialization(main).

pdi(0, Total, Total).
pdi(N, Total_, Total):-
N_ is N // 10,
Total__ is Total_ + round(mod(N, 10) ** 2),
pdi(N_, Total__, Total).
pdi(N, Total):-
pdi(N, 0, Total).

happy(1, _).
happy(N, Seen):-
\+ member(N, Seen),
pdi(N, Total),!,
N_ is Total,
happy(N_, [N|Seen]).
happy(N):-
happy(N, []).

happy(_) --> [].
happy(Seen) --> [X], {between(1, 100, X), \+ member(X, Seen), happy(X)}, happy([X|Seen]).

main:-
length(Happy, 8),
phrase(happy([]), Happy),
write(Happy), nl.
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-2.p
[1,7,10,13,19,23,28,31]
``````

### Notes

As with the code in the first part I also implemented this as a DCG. Here a DCG is more practical since we are asked specifically to generate a list of the first 8 Happy Numbers. This is more of a "list reasoning" task than how the Palindrome Prime question was asked.

## References

Challenge 164

posted at: 23:58 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-05-08

#### The Weekly Challenge 163 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

You are given a list of numbers. Write a script to calculate the sum of the bitwise & operator for all unique pairs.

### Solution

``````
and(N, X, And):-
And is N /\ X.
sum_and([], 0).
sum_and([H|T], SumAnd):-
sum_and(T, Sum),
maplist(and(H), T, Ands),
sum_list(Ands, AndSum),
SumAnd is Sum + AndSum.
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-1.p
| ?- sum_and([1, 2, 3], SumAnd).

SumAnd = 3

yes
| ?- sum_and([2, 3, 4], SumAnd).

SumAnd = 2

yes
| ?- sum_and([1, 2, 3], 99).

no
| ?- sum_and([1, 2, 3], 3).

yes

``````

### Notes

It is not too often you see the bitwise operators used in Prolog code! I am kind of a fan of the `/\` operator, I mean, it even looks like an upper case 'A', right?

Other than the novelty of the bitwise and I found this task to be well suited for the use of `maplist/3` for computing the pairwise and operations across the list of numbers. This is something of a "nested loop" in the sense that recursively the head of the list of numbers is removed and upon each recursive step the maplist is re-performed on the increasingly shorter list.

## Part 2

Given a list of numbers, generate the skip summations.

### Solution

``````
skip_sum(Numbers, N, Sum):-
append(Left, [N|_], Numbers),
sum_list(Left, SumLeft),
Sum is N + SumLeft.

skip_summations(Numbers, Summations):-
skip_summations(Numbers, [Numbers], Summations).
skip_summations([], Summations, Summations).
skip_summations([_|T], SummationsAccum, Summations):-
maplist(skip_sum(T), T, Sums),
skip_summations(Sums, [Sums|SummationsAccum], Summations).

print_summation(S):-
write(S),
write(' ').
print_summations([]).
print_summations([H|T]):-
print_summations(T),
maplist(print_summation, H), nl.
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-2.p
| ?- skip_summations([1, 2, 3, 4, 5], Summations), print_summations(Summations).
1 2 3 4 5
2 5 9 14
5 14 28
14 42
42

Summations = [[],,[14,42],[5,14,28],[2,5,9,14],[1,2,3,4,5]] ?

yes
| ?- skip_summations([1, 3, 5, 7, 9], Summations), print_summations(Summations).
1 3 5 7 9
3 8 15 24
8 23 47
23 70
70

Summations = [[],,[23,70],[8,23,47],[3,8,15,24],[1,3,5,7,9]] ?

yes
| ?- skip_summations([1, 3, 5, 7, 9], [[],,[23,70],[8,23,47],[3,8,15,24],[1,3,5,27,9]]).

no
| ?- skip_summations([1, 3, 5, 7, 9], [[],,[23,70],[8,23,47],[3,8,15,24],[1,3,5,7,9]]).

true ?

yes

``````

### Notes

Much like the task above we use a `maplist/3` within a recursive step. Here the `maplist/3` is used to do the partial sums for each "line" of the output. There is also a `maplist/2` used to print the output lines with a space between each element.

Satisfyingly, as shown above, the same code not only generates the skip summations but also validates them as well. This sort of intrinsic Prolog behavior brings joy!

## References

Challenge 163

posted at: 13:52 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-05-01

#### The Weekly Challenge 162 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

Write a script to generate the check digit of a given ISBN-13 code.

### Solution

``````
weight(-1, 1).
weight(1, 3).

check_sum([], _, 0).
check_sum([H|T], I, CheckSum):-
N is I * -1,
check_sum(T, N, C),
weight(I, Weight),
CheckSum is H * Weight + C.
isbn_check_digit(ISBN, CheckDigit):-
check_sum(ISBN, -1, CheckSum),
Check is mod(CheckSum, 10),
CheckDigit is 10 - Check.
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-1.p
| ?- isbn_check_digit([9, 7, 8, 0, 3, 0, 6, 4, 0, 6, 1, 5], CheckDigit).

CheckDigit = 7

(1 ms) yes

\$ gprolog --consult-file prolog/ch-1.p
| ?- isbn_check_digit([9, 7, 8, 0, 3, 0, 6, 4, 0, 6, 1, 5], 3).

(1 ms) no

\$ gprolog --consult-file prolog/ch-1.p
| ?- isbn_check_digit([9, 7, 8, 0, 3, 0, 6, 4, 0, 6, 1, 5], 7).

yes
``````

### Notes

Sometimes when writing this sort of code I feel the urge to really unleash the power of Prolog and make the most general solution. What would that mean here though? Generate ISBNs for which a given check digit would be valid? While possible it seems like a kind of weird thing to do. Is it interesting to reason about ISBNs in this way? I seem to think that is unlikely.

This code does what seems to reasonable: generate a check digit given an ISBN, or, given both an ISBN and a check digit confirm that the given check digit is the correct one.

## References

Challenge 162

posted at: 14:34 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-04-24

#### The Weekly Challenge 161 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

Output or return a list of all abecedarian words in the dictionary, sorted in decreasing order of length.

### Solution

``````
!.
!.
!.
!.
!.
get_code(Stream, NextChar),

at_end_of_stream(Stream).
\+ at_end_of_stream(Stream),
get_code(Stream, Char),
atom_codes(X, Chars),

abecedarian(Words, Abecedarian):-
member(Word, Words),
atom_chars(Word, Chars),
sort(Chars, SortedChars),
atom_chars(W, SortedChars),
W = Word,
Abecedarian = Word.

word_length(Word, LengthWord):-
atom_chars(Word, Chars),
length(Chars, Length),
LengthWord = Length-Word.

abecedarians(Words, Abecedarians):-
findall(Abecedarian, abecedarian(Words, Abecedarian), A),
maplist(word_length, A, AL),
keysort(AL, ALSorted),
reverse(ALSorted, Abecedarians).

main:-
close(Stream),
abecedarians(Dictionary, Abecedarians),
write(Abecedarians), nl,
halt.
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-1.p --entry-goal main
``````

### Notes

Most of the code here is just for reading the provided dictionary of words. Once that is complete Prolog really shines. `abecedarian/2` is the majority of the logic: if a word's characters when sorted and re-assembled are the original word then it is an Abecedarian.

`abecedarians/2` is necessary only to fulfill the requirements of the problem specification which is that all Abecedarians be sorted by length and returned in descending order.

## References

Challenge 161

posted at: 14:27 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-04-17

#### The Weekly Challenge 160 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

You are given a positive number, \$n < 10. Write a script to generate english text sequence starting with the English cardinal representation of the given number, the word "is" and then the English cardinal representation of the count of characters that made up the first word, followed by a comma. Continue until you reach four.

### Solution

``````
cardinal(1, one).
cardinal(2, two).
cardinal(3, three).
cardinal(4, four).
cardinal(5, five).
cardinal(6, six).
cardinal(7, seven).
cardinal(8, eight).
cardinal(9, nine).
cardinal(10, ten).

four_is_magic(N) --> {between(1, 9, N), \+N == 4,
cardinal(N, Cardinal),
atom_codes(Cardinal, Codes),
length(Codes, Count)},
[N-Count], four_is_magic(Count).
four_is_magic(N) -->  {between(1, 9, N), N == 4}, [N-magic].

print_magic([]):-
nl.
print_magic([H|T]):-
N-Count = H,
\+ N == 4,
cardinal(N, Cardinal),
cardinal(Count, CountCardinal),
format("~a is ~a, ", [Cardinal, CountCardinal]),
print_magic(T).
print_magic([H|T]):-
N-_ = H,
N == 4,
cardinal(N, Cardinal),
format("~a is ~a", [Cardinal, magic]),
print_magic(T).

main(N) :-
phrase(four_is_magic(N), FourIsMagic),
print_magic(FourIsMagic).
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-1.p
| ?- main(6).
six is three, three is five, five is four, four is magic

true ?

(1 ms) yes
| ?- main(_).
one is three, three is five, five is four, four is magic

true ? ;
two is three, three is five, five is four, four is magic

true ? ;
three is five, five is four, four is magic

true ? ;
five is four, four is magic

true ? ;
six is three, three is five, five is four, four is magic

true ? ;
seven is five, five is four, four is magic

true ? ;
eight is five, five is four, four is magic

true ?

(1 ms) yes

``````

### Notes

Prolog has an interesting history of generating text. This is one of the more simple applications I will admit. Here I use a DCG to generate the sequence and then a recursive set of predicates to print the associated text.

In typical Prolog fashion, as shown in the sample output, we can not only generate a sequence with a specified starting point, but all such sequences.

## Part 2

You are give an array of integers, @n. Write a script to find out the Equilibrium Index of the given array, if found.

### Solution

``````
equilibrium_index(Numbers, I):-
member(N, Numbers),
append(Left, [N|Right], Numbers),
sum_list(Left, SumLeft),
sum_list(Right, SumRight),
SumLeft == SumRight,
nth(I, Numbers, N).

main:-
(equilibrium_index([1, 3, 5, 7, 9], I), format("~d~n", [I]); format("-1~n", _)),
(equilibrium_index([1, 2, 3, 4, 5], J), format("~d~n", [J]); format("-1~n", _)),
(equilibrium_index([2, 4, 2], K), format("~d~n", [K]); format("-1~n", _)), halt.
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-2.p
| ?- main.
4
-1
2
``````

### Notes

This problem was very "Prolog shaped" in my opinion! Just from reading the problem statement I could picture the standard Prolog predicates that could be used here. Prolog is not a language which naturally lends itself to code golf contests but in terms of simple elegance this is quite a compact solution!

## References

Challenge 160

posted at: 09:59 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-03-20

#### The Weekly Challenge 156 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

Write a script to generate the first 10 Pernicious Numbers.

### Solution

``````
pernicious(_) --> [].
pernicious(Seen) --> [X], x(Seen, X), {set_bits(X, Bits), fd_prime(Bits)}, pernicious([X|Seen]).
x(Seen, X) --> {between(1, 100, X), \+ member(X, Seen)}.

set_bits(N, X):-
set_bits(N, 0, X).
set_bits(0, X, X).
set_bits(N, X_Acc, X):-
B is N /\ 1,
X0 is X_Acc + B,
N0 is N >> 1,
set_bits(N0, X0, X), !.
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-1.p
| ?- length(Pernicious, 10), phrase(pernicious([]), Pernicious).

Pernicious = [3,5,6,7,9,10,11,12,13,14] ?

(115 ms) yes
| ?- phrase(pernicious([]), [3, 5, 6]).

true ?

(95 ms) yes

``````

### Notes

DCGs are great aren't they? The ability to have two modes, one to test and the other to create is a joy! The logic here is pretty straightforward and more or less follows straight fromt he definition.

## Part 2

Write a script to compute the first 10 distinct Padovan Primes.

### Solution

``````
weird(_) --> [].
weird(Seen) --> [X], x(Seen, X), {
findall(F, factor(X, F), Factors), flatten([1, Factors], FlatFactors),
sum_list(FlatFactors, FactorSum),
FactorSum > X,
powerset(FlatFactors, FactorSets),
maplist(sum_list, FactorSets, FactorSetSums),
\+ member(X, FactorSetSums)
},
weird([X|Seen]).
x(Seen, X) --> {between(1, 1000, X), \+ member(X, Seen)}.

powerset(X,Y):- bagof(S, subseq(S,X), Y).
subseq([], []).
subseq([], [_|_]).
subseq([X|Xs], [X|Ys] ):- subseq(Xs, Ys).
subseq([X|Xs], [_|Ys] ):- append(_, [X|Zs], Ys), subseq(Xs, Zs).

factor(N, Factors):-
S is round(sqrt(N)),
fd_domain(X, 2, S),
R #= N rem X,
R #= 0,
Q #= N // X,
Q #\= X,
fd_labeling([Q, X]),
Factors = [Q, X].
factor(N, Factors):-
S is round(sqrt(N)),
fd_domain(X, 2, S),
R #= N rem X,
R #= 0,
Q #= N // X,
Q #= X,
fd_labeling([Q]),
Factors = [Q].
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-2.p
| ?- phrase(weird([]), ).

true ?

yes
| ?- length(Weird, 1), phrase(weird([]), Weird).

Weird =  ?

(4 ms) yes
``````

### Notes

This solution follows the same generate and test approach I used in the Perl Solution, as far as the testing of the powerset of divisors is concerned anyway. (I'll admit I was too lazy to write my own powerset code so I grabbed someone else's. See the references for a link to the source.)

In my ongoing attempts to improve my DCG skills I implemented this as a DCG which is a bit of overkill for this problem, but it is always nice to be able to generate the sequence as well as validate.

## References

Challenge 156

Pernicious Number

Weird Number

Power Set

posted at: 18:26 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-03-06

#### The Weekly Challenge 154 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

Write a script to compute the first 10 distinct Padovan Primes.

### Solution

``````
:-initialization(main).

make_lists([], []).
make_lists([Word|Words], [List|Rest]):-
atom_chars(Word, List),
make_lists(Words, Rest).

missing_permutation(Word, Permutations, Missing):-
atom_chars(Word, Chars),
permutation(Chars, Permutation),
\+ member(Permutation, Permutations),
atom_chars(Missing, Permutation).

main:-
make_lists(['PELR', 'PREL', 'PERL', 'PRLE', 'PLER', 'PLRE', 'EPRL', 'EPLR', 'ERPL',
'ERLP', 'ELPR', 'ELRP', 'RPEL', 'RPLE', 'REPL', 'RELP', 'RLPE', 'RLEP',
'LPER', 'LPRE', 'LEPR', 'LRPE', 'LREP'], Permutations),
missing_permutation('PERL', Permutations, Missing),
write(Missing), nl,
halt.
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-1.p
LERP
``````

### Notes

This is a nice place where Prolog really shines compared to the Perl solution to the same problem. That approach requires a good deal of care to properly generalize. The Prolog solution is completely general without any extra work! Here we need only split the starting word into characters and then backtrack through any possible missing permutations with `permutation/2` and `member/2`. Elegant!

## Part 2

Write a script to compute the first 10 distinct Padovan Primes.

### Solution

``````
padovan_primes(Size, Primes, PrimesAccum, A, B, C) --> {D is B + A, fd_not_prime(D)}, [A], padovan_primes(Size, Primes, PrimesAccum, B, C, D).
padovan_primes(Size, Primes, PrimesAccum, A, B, C) --> {D is B + A, fd_prime(D), append(PrimesAccum, [D], NewPrimesAccum), length(NewPrimesAccum, L), L < Size}, [A], padovan_primes(Size, Primes, NewPrimesAccum, B, C, D).
padovan_primes(Size, Primes, PrimesAccum, A, B, _) --> {D is B + A, fd_prime(D), append(PrimesAccum, [D], NewPrimesAccum), length(NewPrimesAccum, L), L >= Size, Primes = NewPrimesAccum}, [D].

succ(N, X),
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-2.p

Primes = [2,3,5,7,37,151,3329] ?

(113 ms) yes
| ?-
``````

### Notes

If you watch any of the videos on The Power of Prolog YouTube channel you'll learn from Markus Triska that a DCG is the preferable way to handle this sort of problem. Not just because DCGs are a convenient way to process a list in Prolog, but because they can be used to both generate and test solutions. Excellent advice! This code above shows this for a somewhat complicated problem. We must generate the sequence and also determine which of the sequence terms are prime. Primality testing is performed by GNU Prolog's `fd_prime/1` and `fd_not_prime/1`. As the primes are found they are added, along with the most recently computed three sequence terms, as extra arguments.

This solution is very similar to a previous bit of code for Fibonacci Strings.

## References

Challenge 154

The Power of Prolog: YouTube Channel

The Power of Prolog

posted at: 19:25 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-02-06

#### The Weekly Challenge 150 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

You are given two strings having the same number of digits, \$a and \$b. Write a script to generate Fibonacci Words by concatenation of the previous two strings. Print the 51st
of the first term having at least 51 digits.

### Solution

``````
fibonacci_words(Size, A, B) --> {atom_concat(A, B, C), atom_chars(C, Chars), length(Chars, N), N < Size}, [A], fibonacci_words(Size, B, C).
fibonacci_words(Size, A, B) --> {atom_concat(A, B, C), atom_chars(C, Chars), length(Chars, N), N >= Size}, [C].

fibonacci_words_nth_character(A, B, N, NthChar) :-
phrase(fibonacci_words(N, A, B), FibonacciWords),
last(FibonacciWords, LongestTerm),
atom_chars(LongestTerm, LongestTermChars),
nth(N, LongestTermChars, NthChar).
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-1.p
| ?- fibonacci_words_nth_character('1234', '5678', 51, N).

N = '7' ?

(2 ms) yes
| ?-
``````

### Notes

This little bit of code might be the thing I am proudest of in a while! You see, modern Prolog style generally promotes the use of DCGs for almost any list processing you might want to do. The reasons for this are that DCG code is usually easily bimodal and also testing of code is much easier. Furthermore, DCG code can be said to rely more on the Prolog engine itself backtracking versus more manual recursive style code. Of course these are general statements which are not always necessarily true. In any event I default to not using DCGs so generally, but have been making a conscious attempt to do so. This Fibonacci Words task is well suited for DCGs.

`Size`, `A`, and `B` are passed as extra arguments to the DCG. Some parts of the DCG might appear a little mysterious so here is what the DCG code looks like when expanded

``````
fibonacci_words(A, B, C, D, E) :-
atom_concat(B, C, F),
atom_chars(F, G),
length(G, H),
H < A,
D = [B|I],
fibonacci_words(A, C, F, I, E).
fibonacci_words(A, B, C, D, E) :-
atom_concat(B, C, F),
atom_chars(F, G),
length(G, H),
H >= A,
D = [F|E].
``````

while the variable names are changed during the term expansion you can see the behavior which may at first seem odd. The first three variables for each predicate are the "extra" ones which carry the size information, and the most recent two terms of the sequence which are used to compute the next term. In the DCG when we have `[A]` you can see what happens, that it is expanded to unify with the first implicit argument. In the expanded version we see `D = [B|I]`, and in this way we create the recursive relationship to build the sequence list. Well, of course the recursive call which follows is important too, but see how we've now passed as the first implicit argument `I` which represents the uninstantiated tail of a list and on the next call will recursively be added to.

## Part 2

Write a script to generate all square-free integers <= 500.

### Solution

``````
prime_factors(N, L):-
N > 0,
prime_factors(N, L, 2).
prime_factors(1, [], _):-
!.
prime_factors(N, [F|L], F):-
R is N // F,
N =:= R * F,
!,
prime_factors(R, L, F).
prime_factors(N, L, F):-
next_factor(N, F, NF),
prime_factors(N, L, NF).
next_factor(_, 2, 3):-
!.
next_factor(N, F, NF):-
F * F < N,
!,
NF is F + 2.
next_factor(N, _, N).

square_free(N, SquareFree):-
findall(X,
(between(1, N, X),
prime_factors(X, PrimeFactors),
msort(PrimeFactors, PrimeFactorsMSorted),
length(PrimeFactorsMSorted, MSortedLength),
SortedLength == MSortedLength), SquareFree).
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-2.p
| ?- square_free(500, SquareFree).

SquareFree = [1,2,3,5,6,7,10,11,13,14,15,17,19,21,22,23,26,29,30,31,33,34,35,37,38,39,41,42,43,46,47,51,53,55,57,58,59,61,62,65,66,67,69,70,71,73,74,77,78,79,82,83,85,86,87,89,91,93,94,95,97,101,102,103,105,106,107,109,110,111,113,114,115,118,119,122,123,127,129,130,131,133,134,137,138,139,141,142,143,145,146,149,151,154,155,157,158,159,161,163,165,166,167,170,173,174,177,178,179,181,182,183,185,186,187,190,191,193,194,195,197,199,201,202,203,205,206,209,210,211,213,214,215,217,218,219,221,222,223,226,227,229,230,231,233,235,237,238,239,241,246,247,249,251,253,254,255,257,258,259,262,263,265,266,267,269,271,273,274,277,278,281,282,283,285,286,287,290,291,293,295,298,299,301,302,303,305,307,309,310,311,313,314,317,318,319,321,322,323,326,327,329,330,331,334,335,337,339,341,345,346,347,349,353,354,355,357,358,359,362,365,366,367,370,371,373,374,377,379,381,382,383,385,386,389,390,391,393,394,395,397,398,399,401,402,403,406,407,409,410,411,413,415,417,418,419,421,422,426,427,429,430,431,433,434,435,437,438,439,442,443,445,446,447,449,451,453,454,455,457,458,461,462,463,465,466,467,469,470,471,473,474,478,479,481,482,483,485,487,489,491,493,494,497,498,499]

(26 ms) yes
| ?-
``````

### Notes

I am re-using the prime factorization code I have use din the past, most recently in Challenge 123. Getting the factors is really the hardest part. Once that is doine we need to only check to see if a number has duplicate prime factors indicating a square. To do that we sort using `msort/2` and `sort/2` and see if the resulting lists are the same length. Recall that `sort/2` will remove duplicates whereas `msort/2` does not. If the results of both are the same length then we can conclude there were no square factors.

## References

Challenge 150

Squarefree Numbers

posted at: 16:44 by: Adam Russell | path: /prolog | permanent link to this entry

### 2022-01-16

#### The Weekly Challenge 147 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

Write a script to generate first 20 left-truncatable prime numbers in base 10.

### Solution

``````
:-initialization(main).

left_truncatable(X):-
fd_labeling(X),
number_codes(X, C),
\+ member(48, C),
length(C, L),
findall(Truncatable, (
between(1, L, N),
length(T, N),
append(_, T, C),
number_codes(Truncatable, T),
fd_prime(Truncatable)), Truncatables),
length(Truncatables, NumberTruncatable),
L == NumberTruncatable.

first_twenty_left_truncatable(FirstTwenty):-
length(FirstTwenty, 20),
fd_domain(FirstTwenty, 1, 200),
fd_all_different(FirstTwenty),
maplist(left_truncatable, FirstTwenty),
fd_labeling(FirstTwenty).

main:-
first_twenty_left_truncatable(FirstTwenty),
write(FirstTwenty), nl,
halt.
``````

### Sample Run

``````
\$ gplc prolog/ch-1.p
\$ prolog/ch-1
[2,3,5,7,13,17,23,37,43,47,53,67,73,83,97,113,137,167,173,197]
``````

### Notes

I thought quite a while on how to best approach this problem in Prolog. The code here works well. Some knowledge of the size of the left truncatable primes lets us set the upper bound of the domain pretty tightly, but at best that is a very small optimization, if we can even really claim it as such. The change which might most effect performance is to start with a pre-generated list of primes. Especially since the density of primes is much more sparse as the numbers increase the number of unnecessary checks would be greatly reduced.

## Part 2

Write a script to find the first pair of Pentagon Numbers whose sum and difference are also a Pentagon Number.

### Solution

``````
n_pentagon_numbers(0, []).
n_pentagon_numbers(N, [H|T]):-
H #= N * (3 * N - 1) / 2,
Next #= N - 1,
n_pentagon_numbers(Next, T).

first_pair_pentagon(FirstPair):-
n_pentagon_numbers(10000, Pentagons),
fd_domain([X, Y, Sum, AbsoluteDifference], Pentagons),
Sum #= X + Y,
Difference #= X - Y,
((
Difference #< 0,
AbsoluteDifference #= -1 * Difference
); AbsoluteDifference #= Difference),
fd_labeling([X, Y]),
FirstPair = [X, Y].
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-2.p
| ?- first_pair_pentagon(FirstPair).
FirstPair = [7042750,1560090] ?
``````

### Notes

Apparently GNU Prolog does not define an absolute value function for FD vars. Perhaps because of some theoretical limitation I am unaware of? No matter, some extra code takes care of that. Frankly, the bigger issue is that in this case the use of an FD solver doesn't really help much beyond a pure Prolog one. Again, I may be unaware of what is happening under the hood, but performance wise it doesn't seem any better to constrain the domains of the variables versus an outright "generate and test".

## References

Challenge 147

Left Truncatable Primes

Pentagonal Numbers

posted at: 13:06 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-12-19

#### The Weekly Challenge 143 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

_You are given a string, \$s, containing mathematical expression. Write a script to print the result of the mathematical expression. To keep it simple, please only accept + - * ().

### Solution

``````
:-initialization(main).

operand(X) --> [X], {number(X)}.

main:-
calculator([10, (+), 20, (-), 5], AnswerA),
calculator(['(', 10, (+), 20, (-), 5, ')', (*), 2], AnswerB),
halt.
``````

### Sample Run

``````
\$ gplc prolog/ch-1.p
\$ prolog/ch-1
25
50
``````

### Notes

This is the sort of problem which is just so clean and straightforward to implement in Prolog. A DCG is used to describe the expected infix notation of the calculator and that pretty much takes care of it.

## Part 2

You are given a positive number, \$n. Write a script to find out if the given number is a Stealthy Number.

### Solution

``````
:-initialization(main).

stealthy(N):-
fd_domain(S, 2, N),
fd_domain(T, 2, N),
fd_domain(U, 2, N),
fd_domain(V, 2, N),
S * T #= N,
U * V #= N,
S + T #= U + V + 1,
fd_labeling([S, T, U, V]).

main:-
(stealthy(36), format("~d~n", );format("~d~n", )),
(stealthy(12), format("~d~n", );format("~d~n", )),
(stealthy(6), format("~d~n", );format("~d~n", )),
halt.
``````

### Sample Run

``````
\$ gplc prolog/ch-2.p
\$ prolog/ch-2
1
1
0
``````

### Notes

Much like Part 1 of this weeks challenge Prolog really shines in terms of providing a short clean solution. Here we describe the desired property in terms of finite domain variables and Prolog let's us know if any values exist which match those constraints.

## References

Challenge 143

posted at: 19:56 by: Adam Russell | path: /prolog | permanent link to this entry

## Part 1

Advent of Code 2021 Day 13, in summary, involves the simulation of folding a transparent piece of paper. When "folded" properly a correct solution yields points which when plotted reveal a code which is the solution to a puzzle!

Not wanting to spend too much time on plotting I first considered dumping the points to a file and then using a plotting program like gnuplot directly. In the spirit of having a complete self-contained solution, however, I explored what could be done be interfacing with gnuplot using GNU Prolog's `popen/3`. This worked very well!

### Solution

``````
:-dynamic(dots/1).

:-initialization(main).

!.
!.
!.
!.
!.
get_code(Stream, NextChar),

at_end_of_stream(Stream).
\+ at_end_of_stream(Stream),
get_code(Stream, Char),
atom_codes(X, Chars),

gnuplot_command(Command, PlotStream):-
repeat,                           % START REPEAT
length(Command, CommandLength),
between(1, CommandLength, N),
nth(N, Command, CommandCode),
put_code(PlotStream, CommandCode),
N  == CommandLength,              % END REPEAT
nl(PlotStream).

plot_configuration(PlotStream):-
popen('/usr/pkg/bin/gnuplot', 'write', PlotStream),
gnuplot_command("set terminal postscript eps size 9, 2 enhanced color font 'Courier, 11'", PlotStream),
gnuplot_command("set output '13.eps'", PlotStream),
gnuplot_command("set yrange [:] reverse", PlotStream),
gnuplot_command("plot '-' u 1:2 t 'Code' with points pointtype 7 pointsize 2", PlotStream).

plot_dots(PlotStream):-
findall(_,(
dots(X-Y),
atom_codes(X, CodesA),
atom_codes(Y, CodesY),
append(CodesA, [32|CodesY], DataCodes),
length(DataCodes, CodesLength),
findall(_,(
between(1, CodesLength, N),
nth(N, DataCodes, DataCode),
put_code(PlotStream, DataCode)
),_),
nl(PlotStream)
), _).

plot:-
plot_configuration(PlotStream),
plot_dots(PlotStream),
[Exit] = "e",
put_code(PlotStream, Exit),
nl(PlotStream),
close(PlotStream).

make_transparency(Records, Folds):-
make_transparency(Records, [], Folds).
make_transparency([], Folds, Folds).
make_transparency([fold, along, Fold|Records], FoldAccum, Folds):-
make_transparency(Records, [Fold|FoldAccum], Folds).
make_transparency([H|Records], FoldAccum, Folds):-
atom_codes(H, C),
C \== [],
append([Y], Rest, Records),
asserta(dots(H-Y)),
make_transparency(Rest, FoldAccum, Folds).
make_transparency([H|Records], FoldAccum, Folds):-
atom_codes(H, C),
C == [],
make_transparency(Records, FoldAccum, Folds).

fold_up(Line):-
findall(Yn, (dots(X-Y), number_atom(Yn, Y)), Yns),
max_list(Yns, MaxY),
Half is div(MaxY, 2),
number_atom(LineN, Line),
findall(_,(
dots(X-Y),
number_atom(Yn, Y),
Yn > LineN,
Y0 is LineN - Yn + Half,
number_atom(Y0, Ya),
retract(dots(X-Y)),
retractall(dots(X-Ya)),
asserta(dots(X-Ya))
), _).

fold_left(Line):-
findall(Xn, (dots(X-Y), number_atom(Xn, X)), Xns),
max_list(Xns, MaxX),
Half is div(MaxX, 2),
number_atom(LineN, Line),
findall(_,(
dots(X-Y),
number_atom(Xn, X),
Xn > LineN,
X0 is LineN - Xn + Half,
number_atom(X0, Xa),
retract(dots(X-Y)),
retractall(dots(Xa-Y)),
asserta(dots(Xa-Y))
), _).

fold_transparency([], _).
fold_transparency(Folds, Count):-
append(F, [Fold], Folds),
atom_codes(Fold, [Axis, 61|Location]),
atom_codes(Direction, [Axis]),
atom_codes(Line, Location),
((Direction == x, fold_left(Line))
;
(Direction == y, fold_up(Line))),
findall(X-Y,dots(X-Y), Dots),
length(Dots, Count),
fold_transparency(F, _).

transparency(Records, Count):-
make_transparency(Records, Folds),
fold_transparency(Folds, Count),
plot.

main:-
close(Stream),
transparency(Records, _),
halt.
``````

### The results ### Notes

When first plotted the data I did not set the image size and so the default resulted in a squared image which obscured the characters which are supposed to be visible. The `size 9, 2` part of the terminal configuration is to set an aspect ratio for easy reading of the resulting characters. `set yrange [:] reverse` is necessary to re-orient the gnuplot axis to match the axis of the puzzle data.

Anyway, I've never used `popen/3` before and was very happy to find that it was so easy to use to come up quickly with fully working solution. Especially when rushing to complete a coding puzzle as quickly as I could. Another testament to GNU Prolog's clean design!

## References

popen/3

GNU Prolog

gnuplot

posted at: 15:42 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-11-28

#### The Weekly Challenge 140 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

You are given two decimal-coded binary numbers, \$a and \$b. Write a script to simulate the addition of the given binary numbers.

### Solution

``````
:-initialization(main).

sum_carry(0, 0, 1, 1, 1).
sum_carry(1, 0, 1, 0, 0).
sum_carry(0, 1, 1, 1, 0).
sum_carry(0, 1, 0, 1, 1).
sum_carry(0, 0, 0, 0, 0).
sum_carry(1, 0, 0, 0, 1).
sum_carry(0, 1, 1, 0, 1).
sum_carry(1, 0, 0, 1, 0).
sum_carry(0, 1, 1, 1).
sum_carry(1, 0, 0, 1).
sum_carry(0, 0, 0, 0).
sum_carry(1, 0, 1, 0).

number_chars(A, AChars),
number_chars(B, BChars),
reverse(AChars, ACharsReverse),
reverse(BChars, BCharsReverse),
number_chars(Sum, SumAccum).
number_chars(Sum, ['1'|SumAccum]).
number_chars(D, [H]),
sum_carry(S, C, D, Carry),
number_chars(S, [N]),
add_binary(T, [], C, [N | SumAccum], Sum).
number_chars(D0, [H0]),
number_chars(D1, [H1]),
sum_carry(S, C, D0, D1, Carry),
number_chars(S, [N]),
add_binary(T0, T1, C, [N | SumAccum], Sum).

main:-
write(X0), nl,
write(X1), nl,
write(X2), nl,
halt.
``````

### Sample Run

``````
\$ gplc prolog/ch-1.p
\$ prolog/ch-1
100
110
111
``````

### Notes

The approach here may seem just a little strange for "simulating a binary addition", but it seemed like a fun idea, given the small number of combinations, to just store all the intermediate results and then retrieve them as needed. This all seems to work ok, except that maybe going back and forth between numbers and chars is a little clunky.

## Part 2

You are given 3 positive integers, \$i, \$j and \$k. Write a script to print the \$kth element in the sorted multiplication table of \$i and \$j.

### Solution

``````
:-initialization(main).

multiply(I, J, N):-
between(1, I, Ith),
between(1, J, Jth),
N is Ith * Jth.

multiplication_table(I, J, Table):-
bagof(N, multiply(I, J, N), Table).

nth_from_table(I, J, K, N):-
multiplication_table(I, J, Table),
msort(Table, SortedTable),
nth(K, SortedTable, N).

main:-
nth_from_table(2, 3, 4, N0),
write(N0), nl,
nth_from_table(3, 3, 6, N1),
write(N1), nl,
halt.
``````

### Sample Run

``````
\$ gplc prolog/ch-2.p
\$ prolog/ch-2
3
4
``````

### Notes

It's maybe a little confusing, to me anyway, that GNU Prolog's `msort/2` does not merge duplicates but `sort/2` does. Other than that I have to say that I really like this bit of Prolog. It seems very clean to me in that no recursion was required, everything is handled via Prolog itself.

## References

Challenge 140

posted at: 16:59 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-11-21

#### The Weekly Challenge 139 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

You are given a list of numbers. Write a script to implement JortSort. It should return true/false depending if the given list of numbers are already sorted.

### Solution

``````
:-initialization(main).

jort([]).
jort([H0, H1|[]]):-
H1 >= H0.
jort([H0, H1|T]):-
H1 >= H0,
jort([H1|T]).

main:-
(jort([1, 2, 3, 4, 5]), format("1~n", _); format("0~n", _)),
(jort([1, 3, 2, 4, 5]), format("1~n", _); format("0~n", _)),
(jort([1, 2, 3, 4, 5, 6]), format("1~n", _); format("0~n", _)),
halt.
``````

### Sample Run

``````
\$ gplc prolog/ch-1.p
\$ prolog/ch-1
1
0
1
``````

### Notes

I had never heard of a Jort Sort before this week. Once I understand what it was, a joke function which just returns true or false based on whether a given list is already sorted, I am still not sure I really get it. Or, at least, I don't really get the "joke". Apparently it started as a JavaScript thing so maybe there is something inherently funny about the JavaScript code for it.

Anyway, this is pretty easily done in Prolog especially in this case where we only are to be given a list of numbers. The code as written only goes in one direction in that it only verifies a list as requested. This could go in the other direction with a little use of `between/3` and generate a sorted list too. But would that ruin the joke? Make it funnier? I found the whole exercise so unamusing I didn't bother!

Maybe the amusing part of this whole "joke" sort was, ironically, just how stupid I found the whole thing.

## References

Jort Sort

Challenge 139

posted at: 16:33 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-10-31

#### The Weekly Challenge 136 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

You are given 2 positive numbers, \$m and \$n. Write a script to find out if the given two numbers are Two Friendly.

### Solution

``````
:-initialization(main).

two_friendly(M, N):-
current_prolog_flag(max_integer, MAX_INTEGER),
between(1, MAX_INTEGER, M),
between(1, MAX_INTEGER, N),
GCD is gcd(M, N),
P is log(2, GCD),
P0 is ceiling(P),
P1 is floor(P),
P0 == P1.

main:-
(two_friendly(8, 24), format("1~n", _); format("0~n", _)),
(two_friendly(26, 39), format("1~n", _); format("0~n", _)),
(two_friendly(4, 10), format("1~n", _); format("0~n", _)),
halt.
``````

### Sample Run

``````
\$ gplc prolog/ch-1.p
\$ prolog/ch-1
1
0
1
``````

### Notes

Evn after years of writing Prolog I still get quite a kick out of its inherent power. Here the `two_friendly/2` predicate not only verifies for any `M` and `N` but also is multi-modal and can generate pairs for any `M` and `N` as well!

## Part 2

You are given a positive number \$n. Write a script to find how many different sequences you can create using Fibonacci numbers where the sum of unique numbers in each sequence are the same as the given number.

### Solution

``````
fibonaccis_below_n(N, Fibonaccis):-
fibonaccis_below_n(N, Fibonaccis, 0, [1, 1]).
fibonaccis_below_n(-1, Fibonaccis, _, Fibonaccis):- !.
fibonaccis_below_n(N, Fibonaccis, Sum, PartialFibonaccis):-
[H0, H1 | _] = PartialFibonaccis,
F is H0 + H1,
F < N,
fibonaccis_below_n(N, Fibonaccis, Sum, [F|PartialFibonaccis]).
fibonaccis_below_n(N, Fibonaccis, Sum, PartialFibonaccis):-
[H0, H1 | _] = PartialFibonaccis,
F is H0 + H1,
F > N,
fibonaccis_below_n(-1, Fibonaccis, Sum, PartialFibonaccis).

sum_x([], 0).
sum_x([X|Xs], X+Xse):-
sum_x(Xs, Xse).

sum_lists(X, N, Xsub):-
sublist(Xsub, X),
sum_x(Xsub, Xe),
N #= Xe.

fibonacci_sum_clp(N, Summands):-
fibonaccis_below_n(N, Fibonaccis),
sum_lists(Fibonaccis, N, Summands).
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-2.p
| ?- setof(Summands, fibonacci_sum_clp(16, Summands), S).

S = [[8,5,2,1],[8,5,3],[13,2,1],[13,3]]

(1 ms) yes
``````

### Notes

Instead of using a pre-computed list of Fibonacci numbers we generate them as needed. No particular reason other than it's a little more fun, and also it allows us to flexibly allow for virtually any value for `N`.

I just realized as I am looking at the code that I may have slightly misnamed the `fibonacci_sum_clp/2` predicate! I was experimenting with different approaches including a clpfd one. This is clearly not really using clpfd though! Instead `sublist/2` is used to generate and test all possible sublists of the Fibonacci subsequence with values less than `N`.

## References

Challenge 136

posted at: 20:09 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-10-24

#### The Weekly Challenge 135 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

You are given an integer. Write a script find out the middle 3-digits of the given integer, if possible, otherwise show a sensible error message.

### Solution

``````
:-initialization(main).

middle_three(N, Middle3):-
number_chars(N, Chars),
N > 0,
length(Chars, Length),
Length > 2,
IsOdd is Length mod 2, IsOdd == 1,
length(M3, 3),
PrefixLength is ceiling(Length / 2) - 2,
length(Prefix, PrefixLength),
append(Prefix, Middle, Chars),
append(M3, _, Middle),
number_chars(Middle3, M3).

middle_three(N, Middle3):-
(N < 0, N0 is abs(N), middle_three(N0, Middle3));
(number_chars(N, Chars), length(Chars, Length), Length < 3, format("too short~n", _));
(number_chars(N, Chars), length(Chars, Length), IsOdd is Length mod 2, IsOdd == 0, format("even number of digits~n", _));
middle_three(N, Middle3).

main:-
middle_three(1234567, Middle3),
((nonvar(Middle3), format("~d~n", [Middle3]), halt);
halt).
``````

### Sample Run

``````
\$ gplc prolog/ch-1.p
\$ prolog/ch-1
345
``````

### Notes

Interestingly this is one of the rare cases where a Prolog solution follows a fairly similar approach a [Perl solution to the same problem0(http://www.rabbitfarm.com/cgi-bin/blosxom/2021/10/24/perl).

## Part 2

You are given 7-characters alphanumeric SEDOL. Write a script to validate the given SEDOL. Print 1 if it is a valid SEDOL otherwise 0.

### Solution

``````
:-initialization(main).

weight(1, 1).
weight(2, 3).
weight(3, 1).
weight(4, 7).
weight(5, 3).
weight(6, 9).

base --> alphanumeric, alphanumeric, alphanumeric, alphanumeric, alphanumeric, alphanumeric.
alphanumeric --> [AlphaNumeric], {letter_or_digit(AlphaNumeric)}.

sedol(Sedol):-
var(Sedol),
sedol(_, Sedol).
sedol(Sedol):-
nonvar(Sedol),
length(Base, 6),
append(Base, [CheckDigit], Sedol),
phrase(base, Base),
compute_check(Base, ComputedCheckDigit),
CheckDigit == ComputedCheckDigit.

sedol(Base, Sedol):-
phrase(base, Base),
check_digit(Base, Sedol).

letter_or_digit(A):-
nonvar(A),
atom_codes(A, C),
((C >= 66, C =< 90);
(C >= 48, C =< 57)).

letter_or_digit(A):-
var(A),
((between(66, 90, C));  %B through Z
(between(48, 57, C))), %0-9
atom_codes(A, [C]).

compute_check(Base, CheckSum):-
compute_check(Base, 1, CheckSum, 0).
compute_check([], _, CheckSum, PartialCheckSum):-
CheckSum is mod(10 - mod(PartialCheckSum, 10), 10).
compute_check([H|T], Index, CheckSum, PartialCheckSum):-
atom_codes(H, [C]),
between(66, 90, C),
weight(Index, Weight),
LetterValue is C - 64 + 9,
Partial is PartialCheckSum + (LetterValue * Weight),
succ(Index, I),
compute_check(T, I, CheckSum, Partial).
compute_check([H|T], Index, CheckSum, PartialCheckSum):-
atom_codes(H, [C]),
between(48, 57, C),
weight(Index, Weight),
NumeralValue is C - 48,
Partial is PartialCheckSum + (NumeralValue * Weight),
succ(Index, I),
compute_check(T, I, CheckSum, Partial).

check_digit(Base, BaseCheckDigit):-
compute_check(Base, CheckDigit),
append(Base, [CheckDigit], BaseCheckDigit).

main:-
(sedol(['2','9','3','6','9','2',1]), format("1~n", _);
format("0~n", _)),
(sedol(['1','2','3','4','5','6',7]), format("1~n", _);
format("0~n", _)),
(sedol(['B','0','Y','B','K','L',9]), format("1~n", _);
format("0~n", _)),
halt.
``````

### Sample Run

``````
\$ gplc prolog/ch-2.p
\$ prolog/ch-2
1
0
1
``````

### Notes

I had originally hoped to have all code for this using almost exclusively DCGs. Things got a bit unwieldy with the checksum computation and so I dialed that back a bit, so to speak. Here the DCG part is for validating or generating a SEDOL base 6 digits sequence and the check digit is computed using regular Prolog predicates.

The rules around SEDOLs are a bit more complex than this problem lets on. I won't recount them all here, but suffice to say we are dealing with a quite idealized set of validations here. For example, prior to 2004 only numerals were allowed, but since then letters are allowed. But only a numeral can follow a letter. Again, though, those are only rules that apply for a certain time range.

Here we are just checking on length, whether or not the SEDOl contains all numerals and/or (uppercase) letter, and the checksum validation.

## References

Challenge 135

Stock Exchange Daily Official List (SEDOL)

posted at: 15:17 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-10-17

#### The Weekly Challenge 134 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

Write a script to generate first 5 Pandigital Numbers in base 10.

### Solution

``````
:-initialization(first_5_pandigitals).

pandigital(Pandigitals):-
Digits = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
Pandigitals = [A, B, C, D, E, F, G, H, I, J],
fd_domain([A, B, C, D, E, F, G, H, I, J], Digits),
A #= 1,
B #= 0,
fd_labeling(Pandigitals).

first_5_pandigitals:-
setof(P, pandigital(P), Pandigitals),
sort(Pandigitals, [A, B, C, D, E | _ ]),
print_pandigitals([A, B, C, D, E]).

print_pandigitals([]).
print_pandigitals([H|T]):-
maplist(number_codes, H, Codes),
flatten(Codes, DigitCodes),
number_codes(Number, DigitCodes),
write(Number), nl,
print_pandigitals(T).
``````

### Sample Run

``````
\$ gplc prolog/ch-1.p
\$ prolog/ch-1
1023456789
1023456798
1023456879
1023456897
1023456978
``````

### Notes

Rather than recursively iterate over numbers like in the Perl solution I instead use a bit of constraint programming to generate and test a large number of candidates. I then take only the first five as required. A bit of intuition, based on the definition, reduces the number of candidates generated: the first digit will clearly be a 1, the second a 0.

This runs pretty quickly, yet another testimonial to the design and implementation of GNU Prolog's FD solver. On my 2018 Mac Mini it ran to completion in about second. This is much faster than the brute force Perl solution which takes about 20s on the same machine.

## Part 2

You are given 2 positive numbers, \$m and \$n. Write a script to generate multiplication table and display count of distinct terms.

### Solution

``````
:-initialization(main).

table(M, N, K):-
between(1, M, I),
between(1, N, J),
K is I * J.

print_table(M, N, Distinct):-
findall(K, table(M, N, K), Ks),
setof(K, table(M, N, K), Distinct),
print_seperator(M, N),
print_rows(M, N, Ks),
maplist(number_chars, Distinct, DistinctRow),
flatten(DistinctSpaced, DistinctSpacedFlattened),
format("~nDistinct Terms: ~S~n", [DistinctSpacedFlattened]),
length(Distinct, Count),
format("Count: ~d ~n", [Count]).

print_rows(0, _, _).
print_rows(M, N, Products):-
length(Row, N),
append(Rest, Row, Products),
M0 is M - 1,
print_rows(M0, N, Rest),
maplist(number_chars, Row, RowChars),
format(" ~n  ~d | ~S ", [M, CharsSpacedFlattened]).

format("  x | ", _),
N0 is N - 1,
format("~d ", [N]).

print_seperator(_, N):-
format("~n  --+-", _),
print_seperator(N).
print_seperator(0).
print_seperator(N):-
N0 is N - 1,
print_seperator(N0),
format("~a", ['--']).

flatten(C, F),
append(F, [' '], FS),
atom_chars(A, FS),
atom_chars(A, CS).

main:-
print_table(3, 3, _), nl, nl,
print_table(3, 5, _),
halt.
``````

### Sample Run

``````
\$ gplc prolog/ch-2.p
\$ prolog/ch-2
x | 1 2 3
--+-------
1 | 1 2 3
2 | 2 4 6
3 | 3 6 9
Distinct Terms: 1 2 3 4 6 9
Count: 6

x | 1 2 3 4 5
--+-----------
1 | 1 2 3 4 5
2 | 2 4 6 8 10
3 | 3 6 9 12 15
Distinct Terms: 1 2 3 4 5 6 8 9 10 12 15
Count: 11
``````

### Notes

This was an interesting exercise in formatting output in Prolog! As can be seen, the vast majority of the code here is to format the table in the required way. I haven't ever done all that much with `format/2`, but it is quite versatile. As far as formatting output goes it provides a solid set of primitives in the spirit of C's `printf` that allow for us to do whatever we want, albeit with a bit of work.

The actual computation of the value takes no more than the first half dozen or so lines of code in `table/3` and `print_table/3`.

## References

Challenge 134

Pandigital Numbers

GNU Prolog format/2

posted at: 12:55 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-08-29

#### The Weekly Challenge 127 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

You are given two sets with unique numbers. Write a script to figure out if they are disjoint.

### Solution

``````
:-initialization(main).

conflicts(List0, List1):-
member(X, List0),
member(X, List1).

disjoint(List0, List1):-
\+ conflicts(List0, List1).

main:-
((disjoint([1, 2, 5, 3, 4], [4, 6, 7, 8, 9]), write(1)); (write(0))),
nl,
((disjoint([1, 3, 5, 7, 9], [0, 2, 4, 6, 8]), write(1)); (write(0))),
nl,
halt.
``````

### Sample Run

``````
\$ gplc prolog/ch-1.p
\$ prolog/ch-1
0
1
``````

### Notes

The `conflcts/2` predicate establishes whether or not if there is any overlapping elements in the two lists. To establish whether the sets are disjoint `disjoint/2` examines the negation of this result.

Remember, Prolog only backtracks on failure and not success. This fits nicely for what we want. We only care if the lists have any overlap whatsoever. So using backtracking `conflicts/2` will backtrack for all members of List0 until it either succeeds in finding an element that is also in List1 or, if none, fails completely indicating a disjoint set. Since we actually consider the discovery of disjoint sets a success we negate the result with `\+` in `disjoint/2`.

## Part 2

You are given a list of intervals. Write a script to determine conflicts between the intervals.

### Solution

``````
:-initialization(main).

conflicts(Intervals, Conflicts):-
conflicts(Intervals, Conflicts, []).
conflicts([_|[]], Conflicts, ConflictsAccum):-
sort(ConflictsAccum, Conflicts).
conflicts([H|T], Conflicts, ConflictsAccum):-
last(T, Last),
append(Intervals, [Last], T),
append([H], Intervals, CompareIntervals),
comparisons(Last, CompareIntervals, HasConflicts),
append(ConflictsAccum, HasConflicts, ConflictsAccumUpdated),
conflicts(CompareIntervals, Conflicts, ConflictsAccumUpdated).

comparisons(Interval, CompareIntervals, Conflicts):-
comparisons(Interval, CompareIntervals, Conflicts, []).
comparisons(_, [], Conflicts, Conflicts).
comparisons(Interval, [[H0|T0]|T], Conflicts, ConflictsAccum):-
[I|J] = Interval,
I >= H0,
I =< T0,
comparisons([I|J], T, Conflicts, [Interval|ConflictsAccum]).
comparisons([I|J], [_|T], Conflicts, ConflictsAccum):-
comparisons([I|J], T, Conflicts, ConflictsAccum).

main:-
conflicts([[1, 4], [3, 5], [6, 8], [12, 13], [3, 20]], Conflicts0),
write(Conflicts0), nl,
conflicts([[3, 4], [5, 7], [6, 9], [10, 12], [13, 15]], Conflicts1),
write(Conflicts1), nl,
halt.
``````

### Sample Run

``````
\$ gplc prolog/ch-2.p
\$ prolog/ch-2
[[3,5],[3,20]]
[[6,9]]
``````

### Notes

The examples given in the problem statement are with the `[minimum, maximum]` intervals sorted by the maximum value. This makes the problem a bit easier since then we need only check to see if, when working down the sorted list, if the minimum is in one of the other intervals.

This is mostly all handled using recursion which keeps with the nature of the examples in the problem statement. This is fine, and obviously works, but after having worked out the details of finding a solution it seems there is room to make this more, well, logical?

Preferring reasoning over recursion as a principle of Prolog development seems to be reasonable (pun intended!). Here we might, for example, arrange the intervals in Key-Value pairs stored in the Prolog database and make better use of built in backtracking. But even that would include a little extra work as we would need to sort the pairs by value and ultimately the same number of lines of code would be written as shown here, it would just arguably be a bit more Prological.

Sometimes the best solution to a problem is only discovered after working on it for a bit using alternative means!

## References

Challenge 127

Disjoint Sets

posted at: 11:21 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-08-22

#### The Weekly Challenge 126 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

You are given a positive integer \$N. Write a script to print count of numbers from 1 to \$N that don’t contain digit 1.

### Solution

``````
:-initialization(main).

has_1(N):-
number_codes(N, Codes),
memberchk(49, Codes).

count_numbers_without_1(N, Count):-
count_numbers_without_1(N, 0, Count).
count_numbers_without_1(1, Count, Count).
count_numbers_without_1(N, CountAccum, Count):-
\+ has_1(N),
succ(CountAccum, C),
N0 is N - 1,
count_numbers_without_1(N0, C, Count).
count_numbers_without_1(N, CountAccum, Count):-
has_1(N),
N0 is N - 1,
count_numbers_without_1(N0, CountAccum, Count).

main:-
count_numbers_without_1(15, Count0),
write(Count0), nl,
count_numbers_without_1(25, Count1),
write(Count1), nl,
halt.
``````

### Sample Run

``````
\$ gplc prolog/ch-1.p
\$ prolog/ch-1
8
13
``````

### Notes

The `count_numbers_without_1` predicates recurse over the range of numbers and tally the qualifying numbers at each step. `has_1/1` converts the numeral to the list of associated ascii codes and then we see if the ascii code for '1' (49) is present.

## Part 2

You are given a rectangle with points marked with either x or *. Please consider the x as a land mine. Write a script to print a rectangle with numbers and x as in the Minesweeper game.

### Solution

``````
:-initialization(main).

rows(5).
columns(10).
grid([x, '*', '*', '*', x, '*', x, x, x, x,
'*', '*', '*', '*', '*', '*', '*', '*', '*', x,
'*', '*', '*', '*', x, '*', x, '*', x, '*',
'*', '*', '*', x, x, '*', '*', '*', '*', '*',
x, '*', '*', '*', x, '*', '*', '*', '*', x]).

write_grid([H|T]):-
write('\t'),
write_grid([H|T], 0).
write_grid([], _).
write_grid([H|T], Counter):-
columns(Columns),
succ(Counter, C),
M is C mod Columns,
M \== 0,
write(H),
write(' '),
write_grid(T, C).
write_grid([H|T], Counter):-
columns(Columns),
C is Counter + 1,
M is C mod Columns,
M == 0,
write(H),
((rows(Rows),
columns(Columns),
Cells is Rows * Columns,
C \== Cells,
nl,
write('\t'),
write_grid(T, C)
);
(
write_grid(T, C)
)).

minecount(List, MineCount):-
minecount(List, 0, MineCount).
minecount([], MineCount, MineCount).
minecount([H|T], MineCountPartial, MineCount):-
H == x,
MCP is MineCountPartial + 1,
minecount(T, MCP, MineCount).
minecount([H|T], MineCountPartial, MineCount):-
H \== x,
minecount(T, MineCountPartial, MineCount).

columns(Columns),
Columns1 is Columns - 1,
C is Cell + Columns1,
C > 0,
M0 is Cell mod Columns,
M1 is C mod Columns,
((M0 \== 0, M0 \==1, M1 < M0);
(M0 == 0, M1 == Columns1)),

columns(Columns),
Columns1 is Columns - 1,
C is Cell - 1,
C > 0,
M0 is Cell mod Columns,
M1 is C mod Columns,
((M0 \== 0, M0 \==1, M1 < M0);
(M0 == 0, M1 == Columns1)),

columns(Columns),
Columns1 is Columns - 1,
C is Cell - (Columns + 1),
C > 0,
M0 is Cell mod Columns,
M1 is C mod Columns,
((M0 \== 0, M0 \==1, M1 < M0);
(M0 == 0, M1 == Columns1)),

columns(Columns),
Columns1 is Columns - 1,
C is Cell + (Columns + 1),
C > 0,
M0 is Cell mod Columns,
M1 is C mod Columns,
((M1 > M0, M0 \== 0);
(M1 == 0, M0 == Columns1)),

columns(Columns),
Columns1 is Columns - 1,
C is Cell + 1,
C > 0,
M0 is Cell mod Columns,
M1 is C mod Columns,
((M1 > M0, M0 \== 0);
(M1 == 0, M0 == Columns1)),

columns(Columns),
Columns1 is Columns - 1,
C is Cell - Columns1,
M0 is Cell mod Columns,
M1 is C mod Columns,
((M1 > M0, M0 \== 0);
(M1 == 0, M0 == Columns1)),

columns(Columns),
C is Cell - Columns,
C > 0,

columns(Columns),
C is Cell + Columns,
C > 0,

make_grid(Grid, NewGrid):-
make_grid(Grid, 1, [], NewGrid).
make_grid(Grid, Counter, NewGridPartial, NewGrid):-
nth(Counter, Grid, CurrentCell),
CurrentCell \== x,
append(NewGridPartial, [MineCount], NGP),
((rows(Rows),
columns(Columns),
Cells is Rows * Columns,
Counter == Cells,
!,
NewGrid = NGP
);
(succ(Counter, C),
make_grid(Grid, C, NGP, NewGrid))).
make_grid(Grid, Counter, NewGridPartial, NewGrid):-
nth(Counter, Grid, CurrentCell),
CurrentCell == x,
append(NewGridPartial, [CurrentCell], NGP),
((rows(Rows),
columns(Columns),
Cells is Rows * Columns,
Counter == Cells,
!,
NewGrid = NGP
);
(succ(Counter, C),
make_grid(Grid, C, NGP, NewGrid))).

main:-
grid(Grid),
write('Input:'), nl,
write_grid(Grid), nl,
make_grid(Grid, NewGrid),
write('Output:'), nl,
write_grid(NewGrid), nl,
halt.
``````

### Sample Run

``````
\$ gplc prolog/ch-2.p
\$ prolog/ch-2
Input:
x * * * x * x x x x
* * * * * * * * * x
* * * * x * x * x *
* * * x x * * * * *
x * * * x * * * * x
Output:
x 1 0 1 x 2 x x x x
1 1 0 2 2 4 3 5 5 x
0 0 1 3 x 3 x 2 x 2
1 1 1 x x 4 1 2 2 2
x 1 1 3 x 2 0 0 1 x
``````

### Notes

For every cell C which does not contain a mine (x) we need to look at all eight adjacent cells and count the number of mines they contain. The number of mines in the adjacent cells is then set as C's label.

Obtaining the contents of the adjacent cells gets a little tedious. In fact, doing so is the majority of the code here. Care must be taken to make sure that we do not accidentally look for a cell which does not exist or is not actually adjacent. The logic for find the adjacent cells in this way is re-used from a coding challenge I did last year. The Advent of Code 2020 Day 11, in part, had a similar requirement.

After the logic of determining the contents of adjacent cells is worked out, the rest proceeds in a much less complicated way. The contents of all the adjacent cells are examined for mines and the cell labels are set appropriately.

## References

Challenge 126

History of Minesweeper

Advent of Code 2020 Day 11

Prolog Solution: AoC 2020 Day 11 Part 1

posted at: 17:38 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-08-01

#### The Weekly Challenge 123 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

You are given an integer n >= 1. Write a script to find the nth Ugly Number.

### Solution

``````
:-initialization(main).

prime_factors(N, L):-
N > 0,
prime_factors(N, L, 2).
prime_factors(1, [], _):-
!.
prime_factors(N, [F|L], F):-
R is N // F,
N =:= R * F,
!,
prime_factors(R, L, F).
prime_factors(N, L, F):-
next_factor(N, F, NF),
prime_factors(N, L, NF).
next_factor(_, 2, 3):-
!.
next_factor(N, F, NF):-
F * F < N,
!,
NF is F + 2.
next_factor(N, _, N).

ugly(N, UglyNumber):-
ugly(N, 1, 1, _, UglyNumber).
ugly(1, _, _, _, 1).
ugly(N, _, N, UglyNumber, UglyNumber).
ugly(N, X, I, _, UglyNumber):-
prime_factors(X, Factors),
member(Factor, Factors),
(Factor == 2; Factor == 3; Factor == 5),
X0 is X + 1,
I0 is I + 1,
ugly(N, X0, I0, X, UglyNumber).
ugly(N, X, I, UglyNext, UglyNumber):-
X0 is X + 1,
ugly(N, X0, I, UglyNext, UglyNumber).

main:-
ugly(10, UglyNumber),
write(UglyNumber), nl,
halt.
``````

### Sample Run

``````
\$ gplc prolog/ch-1.p
\$ prolog/ch-1
12
``````

### Notes

Here the first N ugly numbers are generated in a pretty routine way. Much of the code is related to computing the prime factors. Once that is out of way the rest of the code seems to be straightforward to follow: recursively counting up each Ugly Number until we reach the Nth one.

## Part 2

You are given coordinates of four points. Write a script to find out if the given four points form a square.

### Solution

``````
:-initialization(main).

dot_product(X0-Y0, X1-Y1, N):-
N0 is X0 * X1,
N is N0 + Y0 * Y1.

swap_key_value([], []).
swap_key_value([A-B|R], [B-A|S]):-
swap_key_value(R, S).

square(Points):-
setof(X, member(X-_, Points),  Xs),
setof(Y, member(_-Y, Points),  Ys),
length(Xs, LXs),
length(Ys, LYs),

keysort(Points, PointsByX),
swap_key_value(Points, Swapped),
keysort(Swapped, PointsByY0),
swap_key_value(PointsByY0, PointsByY),
last(PointsByY, Sx-Sy),
last(PointsByX, Tx-Ty),
nth(1, PointsByY, Ux-Uy),
nth(1, PointsByX, Vx-Vy),
SUx is Sx + Ux,
TVx is Tx + Vx,
SUy is Sy + Uy,
TVy is Ty + Vy,
SUym is Sy - Uy,
TVxm is Tx - Vx,

DVSTx is Sx - Tx,
DVSTy is Sy - Ty,
DVTUx is Tx - Ux,
DVTUy is Ty - Uy,
DVUVx is Ux - Vx,
DVUVy is Uy - Vy,
DVVSx is Vx - Sx,
DVVSy is Vy - Sy,

dot_product(DVSTx-DVSTy, DVTUx-DVTUy, DPSTU),
dot_product(DVTUx-DVTUy, DVUVx-DVUVy, DPTUV),
dot_product(DVUVx-DVUVy, DVVSx-DVVSy, DPUVS),

((LXs == 2, LYs == 2); (SUx == TVx, SUy == TVy, SUym == TVxm, DPSTU == 0, DPTUV == 0, DPUVS == 0)).

main:-
((square([10-20, 20-20, 20-10, 10-10]), write(1)); (write(0))),
nl,
((square([12-24, 16-10, 20-12, 18-16]), write(1)); (write(0))),
nl,
((square([-3-1, 4-2, -(9,-3), -(2,-4)]), write(1)); (write(0))),
nl,
((square([0-0, 2-1, -(3,-1), -(1,-2)]), write(1)); (write(0))),
nl,
halt.
``````

### Sample Run

``````
\$ gplc prolog/ch-2.p
\$ prolog/ch-2
1
0
0
1
``````

### Notes

This is most likely the most tedious Prolog code I have written in a long time! The actual logic of determining if the points determine a square is not so bad:

• Are there only two each of X and Y co-ordinates? Then that is enough to establish that we have a square.
• Otherwise, make sure the side lengths are all equivalent and that the angles between the sides are all 90 degrees.

The tedious part is just all the computation of the distance vectors, the sorting and arranging of the points, and so forth.

The points are represented as pairs. To orient the points they are sorted by X (key) or Y (value). This is both done by the builtin `keysort/2` predicate, with keys and values swapped to facilitate the sorting by value.

In the case of negative co-ordinates we use the alternative `(-)/2` syntax. For example, `-(3,-1)` is used since `3--1` is not valid syntactically.

In the example output we see that the first and last sets of points are both squares. The first is an example of a square with two unique X and Y co-ordinates. The third example is a rhombus and so is a good test to make sure the angles are being checked correctly.

## References

Challenge 123

Rhombus

posted at: 16:58 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-07-25

#### The Weekly Challenge 122 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

You are given a stream of numbers, @N. Write a script to print the average of the stream at every point.

### Solution

``````
:-initialization(main).

moving_average(N):-
moving_average(0, N, 1).
moving_average(Sum, N, I):-
I \== N,
Sum0 is Sum + (I * 10),
Average is Sum0 / I,
write(Average), nl,
I0 is I + 1,
moving_average(Sum0, N, I0).
moving_average(_, N, I):-
I == N.

main:-
moving_average(10),
halt.
``````

### Sample Run

``````
\$ gplc prolog/ch-1.p
\$ prolog/ch-1
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
``````

### Notes

Typically when one thinks of a stream the idea is of a virtually endless source of data. Or, at least, data which is handled as if this were the case. Here the "stream" is simulated by a generated sequence of numbers. For each recursive call to `moving_average/3` we increase the simulated "stream" by 10 and compute the moving average.

(The idea of a stream in Prolog is fairly specific. My preferred Prolog is Gnu Prolog, which has a very nice write up of the subject.)

## Part 2

You are given a score \$S. You can win basketball points e.g. 1 point, 2 points and 3 points. Write a script to find out the different ways you can score \$S.

### Solution

``````
:-initialization(main).

points --> [].
points --> point, points.
point  --> ; ; ; .

length(Points, Goal),
phrase(points, Points),
sum_list(Points, Goal).

zero_remove([], []).
zero_remove([H|T], [ZR_H|ZR_T]):-
delete(H, 0, ZR_H),
zero_remove(T, ZR_T).

main:-
zero_remove(Points, PointsZR),
sort(PointsZR, PointsZR_Unique),
write(PointsZR_Unique), nl,
halt.
``````

### Sample Run

``````
\$ gplc prolog/ch-2.p
\$ prolog/ch-2
[[1,1,1,1],[1,1,2],[1,2,1],[1,3],[2,1,1],[2,2],[3,1]]
``````

### Notes

This is almost exactly identical to a solution for Challenge 112. The only difference is that I changed the predicate and variable names to match the current problem statement!

## References

Challenge 122

posted at: 18:09 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-07-18

#### The Weekly Challenge 121 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

You are given integers 0 <= \$m <= 255 and 1 <= \$n <= 8. Write a script to invert \$n bit from the end of the binary representation of \$m and print the decimal representation of the new binary number.

### Solution

``````
:-initialization(main).

length(Bits, L),

bits(N, Bits):-
bits(N, [], Bits).
bits(0, Bits, Bits).
bits(N, Bit_Accum, Bits):-
B is N /\ 1,
N0 is N >> 1,
bits(N0, [B|Bit_Accum], Bits).

flip_nth_bit(N, Bits, NthFlipped):-
N0 is 9 - N,
N1 is 8 - N,
nth(N0, Bits, B),
Flipped is xor(B, 1),
length(Bits0, N1),
append(Bits0, [B|T], Bits),
append(Bits0, [Flipped|T], NthFlipped).

decimal(Bits, Decimal):-
decimal(Bits, 0, Decimal).
decimal([], Decimal, Decimal).
decimal([H|T], DecimalAccum, Decimal):-
length([H|T], B),
D is (H * 2 ** (B - 1)) + DecimalAccum,
decimal(T, D, Decimal).

main:-
bits(12, B),
decimal(Flipped, Decimal),
write(Decimal), nl,
halt.
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-1.p
GNU Prolog 1.4.5 (32 bits)
Compiled Dec  3 2020, 00:37:14 with gcc
By Daniel Diaz
| ?- consult('prolog/ch-1.p').
8.0
``````

### Notes

This re-uses much code from last week. What is different here is `flip_nth_bit/3` which finds the Nth bit specified, XORs it, and then sets the updated list.

## References

Challenge 121

posted at: 23:35 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-07-11

#### The Weekly Challenge 120 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

You are given a positive integer \$N less than or equal to 255. Write a script to swap the odd positioned bits with the even positioned bits and print the decimal equivalent of the new binary representation.

### Solution

``````
:-initialization(main).

length(Bits, L),

bits(N, Bits):-
bits(N, [], Bits).
bits(0, Bits, Bits).
bits(N, Bit_Accum, Bits):-
B is N /\ 1,
N0 is N >> 1,
bits(N0, [B|Bit_Accum], Bits).

swap([], []).
swap([H0, H1|T], [H1, H0|ST]):-
swap(T, ST).

decimal(Bits, Decimal):-
decimal(Bits, 0, Decimal).
decimal([], Decimal, Decimal).
decimal([H|T], DecimalAccum, Decimal):-
length([H|T], B),
D is (H * 2 ** (B - 1)) + DecimalAccum,
decimal(T, D, Decimal).

main:-
bits(18, B),
decimal(Swapped, Decimal),
write(Decimal), nl,
halt.
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-1.p
GNU Prolog 1.4.5 (32 bits)
Compiled Dec  3 2020, 00:37:14 with gcc
By Daniel Diaz
33.0
``````

### Notes

Working with bitwise operators in Prolog is something I have done a bit before. This code most notably borrows from a solution to Challenge 079. Here that `set_bit/2` predicate is repurposed as `bits/2` which returns a list of bits for any given number.

The list of bits must be padded (`pad/2`), the bits swapped (`swap/2`), and then the decimal value of the swapped bits calculated (`decimal/2`).

## Part 2

You are given time \$T in the format hh:mm. Write a script to find the smaller angle formed by the hands of an analog clock at a given time.

### Solution

``````
:-initialization(main).

clock_angle(Time, Angle):-
append(H, [58|M], Time),
number_codes(Hour, H),
number_codes(Minutes, M),
A is abs(0.5 * (60 * Hour - 11 * Minutes)),
((A > 180, Angle is 360 - A); Angle = A).

main:-
clock_angle("03:10", Angle),
write(Angle), nl,
halt.
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-2.p
GNU Prolog 1.4.5 (32 bits)
Compiled Dec  3 2020, 00:37:14 with gcc
By Daniel Diaz
35.0
``````

### Notes

`clock_angle/2` starts out by splitting the hh:mm formatted time into the hour and minute parts, by way of append. 58 is the character code for the ':'. Once this is done, and that is perhaps the most prological part of the code, the formula for the angle is applied.

## References

Challenge 120

posted at: 14:37 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-06-20

#### The Weekly Challenge 117 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

You are given text file with rows numbered 1-15 in random order but there is a catch one row in missing in the file.

### Solution

``````
:-initialization(main).

!.
!.
!.
!.
get_code(Stream, NextChar),

at_end_of_stream(Stream).
\+ at_end_of_stream(Stream),
get_code(Stream, Char),
atom_codes(X, Chars),

line_numbers([], []).
line_numbers([N0,_|T], [N1|N]):-
number_atom(N1, N0),
line_numbers(T, N).

missing(Contents, Missing):-
line_numbers(Contents, Numbers),
max_list(Numbers, Max),
min_list(Numbers, Min),
between(Min, Max, X),
\+ member(X, Numbers),
Missing = X.

main:-
close(Stream),
missing(Contents, Missing),
format('Missing: ~d ~N', [Missing]),
halt.
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-1.p
GNU Prolog 1.4.5 (32 bits)
Compiled Dec  3 2020, 00:37:14 with gcc
By Daniel Diaz
Missing: 12
``````

### Notes

What interested me the most on this is the majority of the code here is to read in the data file. It's all boilerplate stuff, ordinarily it'd be in some other file of utilities. The work is all done in the seven lines of `missing/2`. Once we have all the line numbers we use `member/2` to determine which one is missing.

As usual there was a second task in this week's challenge but I did not get around to coding up a solution in Prolog. I did get do it in Perl however. Given the nature of that problem my Prolog would have been a fairly close re-implementation of the same algorithm.

Happy Father's Day!

## References

Challenge 117

posted at: 22:53 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-05-30

#### The Weekly Challenge 114 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

You are given a positive integer \$N. Write a script to find out the next Palindrome Number higher than the given integer \$N.

### Solution

``````
:-initialization(main).

next_palindrome(N, NextPalindrome):-
current_prolog_flag(max_integer, MAX_INTEGER),
N0 is N + 1,
between(N0, MAX_INTEGER, X),
number_chars(X, C),
reverse(C, R),
number_chars(NR, R),
NR == X,
NextPalindrome = NR.

main:-
next_palindrome(1234, NextPalindrome0),
write(NextPalindrome0), nl,
next_palindrome(999, NextPalindrome1),
write(NextPalindrome1), nl,
halt.
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-1.p
GNU Prolog 1.4.5 (32 bits)
Compiled Dec  3 2020, 00:37:14 with gcc
By Daniel Diaz
1331
1001
``````

### Notes

The solution to this task is probably the most intuitive: check numbers starting with `N` and for each one reverse and check.

## Part 2

You are given a positive integer \$N. Write a script to find the next higher integer having the same number of 1 bits in binary representation as \$N.

### Solution

``````
:-initialization(main).

set_bits(N, X):-
set_bits(N, 0, X).
set_bits(0, X, X).
set_bits(N, X_Acc, X):-
B is N /\ 1,
X0 is X_Acc + B,
N0 is N >> 1,
set_bits(N0, X0, X), !.

next_same_bits(N, NextSameBits):-
current_prolog_flag(max_integer, MAX_INTEGER),
set_bits(N, NumberBits),
N0 is N + 1,
between(N0, MAX_INTEGER, X),
set_bits(X, B),
B == NumberBits,
NextSameBits = X.

main:-
next_same_bits(3, NextSameBits0),
write(NextSameBits0), nl,
next_same_bits(12, NextSameBits1),
write(NextSameBits1), nl,
halt.
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-2.p
GNU Prolog 1.4.5 (32 bits)
Compiled Dec  3 2020, 00:37:14 with gcc
By Daniel Diaz
5
17
``````

### Notes

`set_bits/2` is code re-used from Challenge 079. Otherwise, the code is very similar to the solution to the first task.

## References

Challenge 114

posted at: 15:59 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-05-23

#### The Weekly Challenge 113 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 1

You are given a positive integer \$N and a digit \$D. Write a script to check if \$N can be represented as a sum of positive integers having \$D at least once. If check passes print 1 otherwise 0.

### Solution

``````
:-initialization(main).

contains([], _, []).
contains([H|T], Digit, [N|R]):-
number_chars(H, C),
number_chars(Digit, [D]),
member(D, C),
N = H,
contains(T, Digit, R).
contains([H|T], Digit, Contains):-
number_chars(H, C),
number_chars(Digit, [D]),
\+ member(D, C),
contains(T, Digit, Contains).

represented(N, D):-
findall(X, between(1, N, X), Numbers),
contains(Numbers, D, Contains),
sum_list(Contains, N).

main:-
(((represented(25, 7), write(1)); write(0)), nl),
(((represented(24, 7), write(1)); write(0)), nl),
halt.
``````

### Sample Run

``````
\$ gplc prolog/ch-1.p
\$ prolog/ch-1
0
1
``````

### Notes

This is pretty straightforward Prolog. `contains/3` is a list filter that gets the numbers from the list which contain `Digit`. After that is done we need only check to see if they sum to `N`. If `represented/2` succeeds we print 1 and 0 otherwise.

## Part 2

You are given a Binary Tree. Write a script to replace each node of the tree with the sum of all the remaining nodes.

### Solution

``````
:-dynamic(edge/3).

:-initialization(main).

root(1).
edge(old, 1, 2).
edge(old, 2, 4).
edge(old, 4, 7).
edge(old, 1, 3).
edge(old, 3, 5).
edge(old, 3, 6).

dfs_replace(GraphOld, GraphNew, Vertex):-
dfs_replace(GraphOld, GraphNew, Vertex, _).
dfs_replace(GraphOld, GraphNew, Vertex, VertexPrevious):-
(var(VertexPrevious),
edge(GraphOld, Vertex, VertexNext),
sum_remaining(GraphOld, Vertex, SumRemaining),
dfs_replace(GraphOld, GraphNew, VertexNext, SumRemaining));
sum_remaining(GraphOld, Vertex, SumRemaining),
assertz(edge(GraphNew, VertexPrevious, SumRemaining)),
findall(V, edge(GraphOld, _, V), VerticesOld),
findall(V, edge(GraphNew, _, V), VerticesNew),
length(VerticesOld, VOL),
length(VerticesNew, VNL),
VOL \== VNL,
edge(GraphOld, Vertex, VertexNext),
dfs_replace(GraphOld, GraphNew, VertexNext, SumRemaining).
dfs_replace(GraphOld, GraphNew, _, _):-
findall(V, edge(GraphOld, _, V), VerticesOld),
findall(V, edge(GraphNew, _, V), VerticesNew),
length(VerticesOld, VOL),
length(VerticesNew, VNL),
VOL == VNL.

sum_remaining(GraphOld, Vertex, SumRemaining):-
findall(V, edge(GraphOld, _, V), Vertices),
root(Root),
delete([Root|Vertices], Vertex, RemainingVertices),
sum_list(RemainingVertices, SumRemaining).

main:-
root(Root),
dfs_replace(old, new, Root),
listing(edge/3),
halt.
``````

### Sample Run

``````
\$ gplc prolog/ch-2.p
\$ prolog/ch-2

edge(old, 1, 2).
edge(old, 2, 4).
edge(old, 4, 7).
edge(old, 1, 3).
edge(old, 3, 5).
edge(old, 3, 6).
edge(new, 27, 26).
edge(new, 26, 24).
edge(new, 24, 21).
edge(new, 27, 25).
edge(new, 25, 23).
edge(new, 25, 22).
``````

### Notes

There are several ways to represent trees and graphs in Prolog. Here I chose to store the edges in the Prolog database along with a label containing the graph name. `old` is the original tree and `new` is the one containing the updated values fort eh vertices.

The overal approach is the same as I did for the Perl solution to this problem.

• Perform a Depth First traversal
• As each vertex is visited construct a new edge with the updated values in a new tree
• When the number of updated vertices is the same as the original tree then we are done
• `listing/1` is used to show the original and updated trees

If we did not have the check on the number of updated vertices then `dfs_replace/3` would simply fail when the traversal was complete. As thise code is designed this should instead succeed when complete.

## References

Challenge 112

Depth First Traversal

posted at: 15:32 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-05-16

#### The Weekly Challenge 112 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

## Part 2

You are given \$n steps to climb. Write a script to find out the distinct ways to climb to the top. You are allowed to climb either 1 or 2 steps at a time.

### Solution

``````
:-initialization(main).

steps --> [].
steps --> step, steps.
step  --> ; ; .

sum_steps(Steps, Goal):-
length(Steps, Goal),
phrase(steps, Steps),
sum_list(Steps, Goal).

zero_remove([], []).
zero_remove([H|T], [ZR_H|ZR_T]):-
delete(H, 0, ZR_H),
zero_remove(T, ZR_T).

main:-
findall(Steps, sum_steps(Steps, 4), S),
zero_remove(S, SZR),
sort(SZR, SZR_Unique),
write(SZR_Unique), nl,
halt.
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-2.p
GNU Prolog 1.4.5 (32 bits)
Compiled Dec  3 2020, 00:37:14 with gcc
By Daniel Diaz
[[1,1,1,1],[1,1,2],[1,2,1],[2,1,1],[2,2]]
``````

### Notes

I've been trying to do more with DCGs. This is not the most algorithmically pure use of DCG's but the point here was just to try something new.

Overview of this brute force approach with DCGs:

• Generate all lists of numbers of length `Goal` using digits 0, 1, and 2.
• Keep all those lists that sum to `Goal`
• Remove zeroes from these matching lists
• Remove duplicate lists (using `sort/2`)

## References

Challenge 112

posted at: 18:20 by: Adam Russell | path: /prolog | permanent link to this entry

### 2021-05-02

#### The Weekly Challenge 110 (Prolog Solutions)

The examples used here are from the weekly challenge problem statement and demonstrate the working solution. Also, the challenge statements are given in a Perl context although for Prolog solutions some adjustments are made to account for the differing semantics between the two languages.

## Part 1

You are given a text file. Write a script to display all valid phone numbers in the given text file.

### Solution

``````
:-initialization(main).

test('0044 1148820341').
test('+44 1148820341').
test('44-11-4882-0341').
test('(44) 1148820341').
test('00 1148820341').

phone_number --> prefx,  space, area_exchange_subscriber.
prefx  --> ['('], digit, digit, [')'].
prefx --> ['+'], digit, digit.
prefx --> digit, digit, digit, digit.
space --> [' '].
area_exchange_subscriber --> digit, digit, digit, digit, digit, digit, digit, digit, digit, digit.
digit --> ['0']; ['1']; ['2']; ['3']; ['4']; ['5']; ['6']; ['7']; ['8']; ['9'].

run_tests:-
test(T),
atom_chars(T, C),
phrase(phone_number, C),
write(T), nl.

main:-
run_tests.
``````

### Sample Run

``````
\$\$ gprolog --consult-file prolog/ch-1.p
GNU Prolog 1.4.5 (32 bits)
Compiled Dec  3 2020, 00:37:14 with gcc
By Daniel Diaz
| ?- run_tests.
0044 1148820341

true ? ;
+44 1148820341

true ? ;
(44) 1148820341

true ? ;

(1 ms) no
``````

I skipped anything to do with actually reading the numbers from a file since that doesn’t really add to the interesting part of the problem. Instead the possible phone numbers to test are just stored in the Prolog db.

Testing is done via DCG. The DCG is a bit rotely written, for sure! DCG’s are a bit hard to debug so I thought I’d start out writing things out as obviously as possible and then come back to tighten things up. Coming back to the DCG later on, I thought there was something kind of attractive to the thing, as it is a bit close to the grammar I wrote in the Perl version of the solution. Perhaps someone looking at the Perl grammar would look at this and even not knowing Prolog be able to make perfect sense of it.

## Part 2

You are given a text file. Write a script to transpose the contents of the given file.

### Solution

``````
:-initialization(main).

transpose(Columns, Transposed):-
transpose(Columns, _, Transposed).
transpose([], Transposed, Transposed).
transpose([H|T], TransposedAccum, Transposed):-
transpose_row(H, TransposedAccum, TransposedAccumNew),
reverse(TransposedAccumNew, TransposedAccumNewReversed),
transpose(T, TransposedAccumNewReversed, Transposed).

transpose_row(Row, TransposedAccum, RowTranspose):-
transpose_row(Row, TransposedAccum, [], RowTranspose).
transpose_row([], [], RowTranspose, RowTranspose).
transpose_row([H|T], [HAccum|TAccum], RowTransposeAccum, RowTranspose):-
append(HAccum, H, HAccum0),
flatten(HAccum0, HAccum1),
transpose_row(T, TAccum, [HAccum1 | RowTransposeAccum], RowTranspose).

main:-
transpose([[1,2,3],[4,5,6]], Transposed),
write(Transposed), nl.
``````

### Sample Run

``````
\$ gprolog --consult-file prolog/ch-2.p
GNU Prolog 1.4.5 (32 bits)
Compiled Dec  3 2020, 00:37:14 with gcc
By Daniel Diaz