RabbitFarm

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  --> [0]; [1]; [2]; [3].

basketball_points(Points, Goal):-
    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:-
    findall(Ps, basketball_points(Ps, 4), Points),
    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