CODEWITHSUNDEEP
prologexpression
cloe1234
cloe1234

Reputation: 1

test if the term N is a number or an atom an compute if it number in prolog

I am doing an arithmetic expression in prolog. the predicate is like expression(expr, _Val, _Var, N). If I test

?-expression(y*(1+y*y),3,y,R)

the result would be

R=30

It work find. But if I want to test

?- expression(x*(1+y*y),3,y,R)

the result would be

R=x*10

But it is does not work for me. It seems like I have to test the term N whether it is a number or an atom. if number it will compute otherwise it the case should be left as is in output R=x*10 I am glad if someone could help me.

the code is something like this:

type(N) :-
   functor(N,_,_),

   number(N).

expression(V,Val,V,Val).

expression(expr, Val, _Var, N).
expression(N, _Var, _Val, N) :-
   type(N).
expression(A*B, Var,Val, R) :-
   expression(A, Var, Val, RA),
   expression(B, Var, Val, RB),
   R is RA*RB.
expression(A+B, Var,Val, R) :-
   expression(A, Var, Val, RA),
   expression(B, Var, Val, RB),
   R is RA+RB.

Upvotes: 0

Views: 79

Answers (1)

CapelliC
CapelliC

Reputation: 60004

this

expression(expr, Val, _Var, N).

seems completely useless. It's part of 'specification space', not 'solution space', just to say.

Before evaluating by means of is/2, you should check if the recursively evaluated subexpressions are numeric:

expression(A+B, Var,Val, R) :-
   expression(A, Var, Val, RA),
   expression(B, Var, Val, RB),
   ( number(RA), number(RB) ->
     R is RA+RB
   ; R = RA+RB
   ).

Upvotes: 1

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