Prolog: Variable with multiple values

Question

I'm trying to implement a program that takes a variable with multiple values and evaluates all the values. For instance:

foo(X,R) :-
  X > 2,
  Z is R + 1,
  R = Z.

This program might not be valid, but it will help me ask the question regardless.

My question: If X has multiple values, how would I increment the counter for each value X > 2?

Wouter Beek · Accepted Answer · 2014-12-08 09:01:09Z

1

In order to instantiate X to increasingly larger integers you can use the following:

?- between(0, inf, X).
X = 0 ;
X = 1 ;
X = 2 ;
X = 3 ;
X = 4 ;
<ETC.>

PS1: Notice that you have to instantiate R as well since it is used in the arithmetic expression Z is R + 1.

PS2: Notice that your program fails for all instantiations of X and R since R =\= R + 1 for finite R. The for instance means that the following query will not terminate:

?- between(0, inf, X), foo(X, 1).

Alternatively, the program can be rewritten in library CLP(FD) (created by Markus Triska):

:- use_module(library(clpfd)).

foo(X,R):-
  X #> 2,
  Z #= R + 1,
  R #= Z.

edited Dec 8, 2014 at 9:01

answered Dec 8, 2014 at 8:40

Wouter Beek

3,40719 silver badges30 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

user2962883 Over a year ago

Thanks for the advice. My question was more centered on if I have a variable with multiple values, would prolog evaluate every value and increment the counter for every value where X > 2?

Wouter Beek Over a year ago

@user2962883 Ah, I see. I have added an alternative solution to my answer which shows the same program in CLP(FD) where you can can express that a variable belongs to a certain range (e.g., X #> 2 means that X is one of [2,+inf)).

Collectives™ on Stack Overflow

Prolog: Variable with multiple values

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related