Prolog不结束计算？

Question

This is rather a technical question I think, I am trying to write a program that will find me all sub-sets of size K of the integers 1,2,...,N.

In here I've asked about a sub-set function that I'm using. The fixed version is:

subs(0,[],X).
subs(N,[A|R1],[A|R2]):-
   N>0,
   N1 is N-1,
   subs(N1,R1,R2).
subs(N,[A|R1],[B|R2]):-
   N>0,
   subs(N,[A|R1],R2).

Later I wrote two functions to help me find the last element in a set and the sub-set of all element except the last (because [A|Rest] means A is the first and Rest is from number 2 to last, but I'd like the opposite - having the last elements and all the elements from the first to the one before the last). The functions are:

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

subLast([],[X]).
subLast([A|R1],[A|R2]):-
   subLast(R1,R2).

Now I wrote a function that creates a list of the first N natural numbers:

setOf(0,[]).
setOf(N,Nums):-
   lastOf(N,Nums),
   N>0, N1 is N-1,
   subLast(NeoNums,Nums),
   setOf(N1, NeoNums).

To combine all the above I have:

choose(K,N,X):-
   setOf(N,Y),
   subs(K,X,Y).

Running it, for example on 2 and 4 , I get:

?-choose(2,4,X).
X = [1, 2] ;
X = [1, 3] ;
X = [1, 4] ;
X = [2, 3] ;
X = [2, 4] ;
X = [3, 4] ;

abort
% Execution Aborted
14 ?- ERROR: Stream user_input:6:143 Syntax error: Unexpected end of clause

These are all the correct outputs, but the problem is that after every time I press enter for a (possible) next answer, I get the next one, apart from the last, in which I have to forcefully abort, as it seems like the programs gets stuck in an infinite loop of some sort.

Can anyone assist?

I'm using SWI-Prolog.

Answer 1

If you're using SWI-Prolog , you can also use clpfd ! Here's a clpfd variant of choose/3 :

:- use_module(library(clpfd)).

choose(K,N,Zs) :-
   length(Zs,K), 
   Zs ins 1..N,
   chain(Zs,#<),
   labeling([],Zs).

That's it! And here's the query you gave in the question:

?- choose(2,4,Zs).
Zs = [1,2] ;
Zs = [1,3] ;
Zs = [1,4] ;
Zs = [2,3] ;
Zs = [2,4] ;
Zs = [3,4].              % the goal `choose(2,4,Zs)` terminates

Answer 2

The setOf is the problem here. More specifically - lastOf , which is generating an infinite number of possible lists ending with N . Anyway, setOf can be implemented much easier and in much more readable way (and which is terminating):

setOf(0, []).
setOf(N, [N|T]) :- 
    N > 0,
    N1 is N-1,
    setOf(N1, T).

This is if you don't care about the reverse order of the numbers. Otherwise by introducing a helper predicate:

setOf(N, X) :- range(1, N, X).
% range(LowerBound, UpperBound, ResultList)
range(L, L, [L]).
range(L, U, [L|T]) :-
   L < U,
   L1 is L + 1,
   range(L1, U, T).