Building a base 2 exponent calculator in Prolog

log2(I,E):-
  I is 2.0**E,
  E is log(I)/log(2).

I am trying to use Prolog to compute either the power 2 was raised to 'I' or 2 raised to the 'E' power equals 'I'. I am extremely new to this language and from my understanding it infers the answer based on the information provided.

Queries:
log2(I,3).
-->false.

log2(I,3.0).
-->I = 8.0.

log2(8,E).
-->ERROR: is/2: Arguments are not sufficiently instantiated

log2(8,E).
-->ERROR: is/2: Arguments are not sufficiently instantiated

I'm confused why I have to provide a float in the first circumstance to get the correct answer and why Prolog is unable to infer the answer from the second circumstance at all.

Solution

What you have there is a conjunction. In Prolog, a conjunction a, b means:

Evaluate a, and if it succeeds, evaluate b.

You are trying to do something else, maybe:

Try a, and if it doesn't succeed, try b.

The first thing you should consider is using library(clpr), if it is available in your Prolog implementation.

With SWI-Prolog:

?- use_module(library(clpr)).
true.

?- {I = 2^3}.
I = 8.0 ;
false.

?- {8 = 2^E}.
E = 3.0 ;
false.

You literally have no problem any more.

If this is not an option, you need to do something along these lines:

log2(I, E) :-
    (   number(I)
    ->  E is /* expression here, now that I is a number */
    ;   number(E)
    ->  I is /* expression here, now that E is a number */
    ;   /* what do you do if both are variables? */
    ).

Note that X is Expr will work even if Expr is an expression and not a number. If you want to allow this, then you need to maybe try eval(Expr) first and catch the error or something along these lines.