Prolog without if and else statements

Question

I am currently trying to learn some basic prolog. As I learn I want to stay away from if else statements to really understand the language. I am having trouble doing this th

Answer 1

Your code has both syntactic and semantic issues.

Predicates starts lower case, and the comma represent a conjunction. That is, you could read your clause as

sample(A,B,C,What) if
    greaterthan(A,B,1.0) and lessthan(A,B,-1.0) and equal(A,B,C).

then note that the What argument is useless, since it doesn't get a value - it's called a singleton.

A possible way of writing disjunction (i.e. OR)

sample(A,B,_,1) :- A > B.
sample(A,B,C,C) :- A == B.
sample(A,B,_,-1) :- A < B.

Note the test A < B to guard the assignment of value -1. That's necessary because Prolog will execute all clause if required. The basic construct to force Prolog to avoid some computation we know should not be done it's the cut:

sample(A,B,_,1) :- A > B, !.
sample(A,B,C,C) :- A == B, !.
sample(A,B,_,-1).

Anyway, I think you should use the if/then/else syntax, even while learning.

sample(A,B,C,W) :- A > B -> W = 1 ; A == B -> W = C ; W = -1.