Prolog Program To Check If A Number Is Prime

Question

I wrote the following program based on the logic that a prime number is only divisible by 1 and itself. So I just go through the process of dividing it to all numbers that are greater than one and less than itself, but I seem to have a problem since I get all entered numbers as true. Here's my code... ``` divisible(X,Y) :- Y < X, X mod Y is 0, Y1 is Y+1, divisible(X,Y1). isprime(X) :- integer(X), X > 1, \+ divisible(X,2). ``` Thanks in advance :)

Accepted Answer

I'm a beginner in Prolog but managed to fix your problem.

```
divisible(X,Y) :- 0 is X mod Y, !.

divisible(X,Y) :- X > Y+1, divisible(X, Y+1).

isPrime(2) :- true,!.
isPrime(X) :- X < 2,!,false.
isPrime(X) :- not(divisible(X, 2)).
```

The main issue was the statement `X mod Y is 0`. Predicate `is` has two (left and right) arguments, but the left argument has to be a constant or a variable that is already unified at the moment that the predicate is executing. I just swapped these values. The rest of the code is for checking number 2 (which is prime) and number less than 2 (that are not primes)

I forgot to mention that the comparison `Y < X` is buggy, because you want to test for all numbers between 2 and X-1, that comparison includes X.

Prolog Program To Check If A Number Is Prime

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