CODEWITHSUNDEEP
prolog
Elviii
Elviii

Reputation: 21

SWI-Prolog Creating/Printing lists, Recursion etc

I'm trying to teach myself some Prolog, however right now i'm really struggling just adapting to the declarative style having never done declarative programming before.

I'm attempting to get my program to come up with a two positive integer numbers, A & B, where A + B =< 50 and B > A. Obviously there are lots of solutions (e.g. A = 5 & B = 12 or A = 15 & B = 17) and i want my program to print all the different solutions.

I honestly don't really know where to begin and would appreciate some guidance or some example code of how to do something as explained above.

Cheers!

Upvotes: 0

Views: 393

Answers (2)

Sergii Dymchenko
Sergii Dymchenko

Reputation: 7209

Looks like a good problem to use constraint logic programming:

:- use_module(library(clpfd)).
model(A, B) :-
    A #> 0, B #> 0,
    A + B #=< 50,
    B #> A.

(I assume you want only positive integer solutions, otherwise there will be infinite number of them). Look how the model code directly reflects the problem statement.

After you have the model you can use it to find all solutions:

?- findall(_, (model(A, B), label([A, B]), writeln([A, B])), _).
[1,2]
[1,3]
[1,4]
[1,5]
[1,6]
... skipped many lines ...
[24,25]
[24,26]
true.

A more traditional Prolog solution without constraint programming (with the same results):

model2(A, B) :-
    between(1, 50, A), 
    between(1, 50, B), 
    A + B =< 50,
    B > A.

?- findall(_, (model2(A, B), writeln([A, B])), _).

Upvotes: 1

Nicholas Carey
Nicholas Carey

Reputation: 74267

You could do something like this:

combos(A,B) :-
  between(1,50,A) ,
  between(1,50,B) ,
  S is A+B ,
  S =< 50
  .

This, on backtracking, will successively find all the solutions.

Use findall/3 to collect the results into a list:

findall(A+B,combos(A,B),X).

Upvotes: 0

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