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Asked: 2026-06-10T06:55:16+00:00

I’m big newbie to Prolog and I’m trying to write simple generator which finds

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I’m big newbie to Prolog and I’m trying to write simple generator which finds the integers below 5.

gen(0).
gen(X):- X<5

When I run the program with gen(X) it only prints X = 0 and prompts me to enter something and when I press enter the ‘?-‘ is shown again.

How to make him generate the numbers from zero to five ? I’m using SWI-Prolog. Thanks

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1 Answer

  1. Editorial Team

    The builtin predicate devoted to enumerate integers is between(Low,High,Num).

    Using that you would write gen(X) :- between(0,4,X).

    I’ve reimplemented between in this way

    gen(X) :- between_(0, 4, X).

/* between_(I,J,K) is true if K is an integer between I and J inclusive.    */
between_(I,J,I) :- I =< J.
between_(I,J,K) :- I < J, I1 is I+1, between_(I1,J,K).

    The interesting part of the question is to understand why the naive implementation with just an argument loops forever…

    gen(0).
gen(X) :- gen(Y), X is Y + 1, X < 5.

?- gen(X).
X = 0 ;
X = 1 ;
X = 2 ;
X = 3 ;
X = 4 ;
^CAction (h for help) ? goals
[374,153] 3<5
[374,152] gen(3)
[374,151] gen(_G1013)
[374,150] gen(_G1013)
[374,149] gen(_G1013)
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