Prolog: Multiplication(X,Y)

Question

double(X, Y) :- X is Y/2, Y is X*2.

I'm trying to execute this but its given error always

Arguments are not sufficiently instantiated In: [2] 4 is _1604/2 [1] double(4,_1662) at line 2

how can I get double of two variables.

gusbro gusbro · Accepted Answer · 2021-03-08T01:36:32

You are trying to make a bidirectional procedure, where at least one of the parameters is instantiated. You may use CLP(fd) like this:

double(X, Y):- Y #= X*2.

Note this will only work with integer values, so for example

?- double(2, Y).
Y = 4.

?- double(X, 4).
X = 2.

but

?- double(2.5, Y).
ERROR: Domain error: `clpfd_expression' expected, found `2.5'
ERROR: In:
ERROR:   [14] throw(error(domain_error(clpfd_expression,2.5),_2162))
ERROR:   [11] clpfd:parse_clpfd(2.5*2,_2200) at c:/swi/swi8/library/clp/clpfd.pl:7359
ERROR:    [9] clpfd:clpfd_equal(_2236,2.5*2) at c:/swi/swi8/library/clp/clpfd.pl:2795
ERROR:    [7] <user>
ERROR: 
ERROR: Note: some frames are missing due to last-call optimization.
ERROR: Re-run your program in debug mode (:- debug.) to get more detail.
?- double(X, 5).
false.

or if you want to use is/2 then you should make sure that the right hand side of is/2 is bound to an number, for example like this:

double(X, Y) :-
    (   number(X)
    ->  Y is X*2
    ;   number(Y)
    ->  X is Y/2
    ).

The procedure using CLP(fd) is clearly more powerful as it does not require the right hand arithmetic expression to be instantiated prior to issue the constraint, and thus allows queries like double(X,Y) to succeed (giving the remaining constraints upon querying).

You may also try CLP(r) which works over reals:

:- use_module(library(clpr)).
double(X, Y):- { Y=X*2 }.

sample runs:

?- double(2, Y).
Y = 4.

?- double(X, 4).
X = 2.

?- double(2.5, Y).
Y = 5.0.

?- double(X, 5).
X = 2.5.

Prolog: Multiplication(X,Y)

1 Answers