A simple recursion function prints weird stuff - c

Question

So I need to code a recursive function that validates if the number is a prime number.
The algorithm is pretty simple and works correctly, it's just that when I print the function rather than showing 1 or 0 it shows random crap (maybe addresses?) and I couldn't find out why.
The code:
int isPrimal(int n, int p) {
if (p == 1) {
return 1;
}
if (n % p == 0) {
return 0;
}
isPrimal(n, p - 1);
printf("n = %d i = %d\n", n, p);
}
int main() {
int numcase, *A, sizeA = 0, i = 0, cnt3dig = 0, n, p;
printf("Enter a number to check for primality\n");
scanf("%d", &n);
p = (n - 1);
printf("The result is 1 if the number is a prime, 0 otherwise\n");
isPrimal(n, p);
printf("The result is %d\n", isPrimal);
}

Answer 1

You are printing the address of the isPrimal() function, you should change it to
printf("The result is %d\n", isPrimal(n, p));
and don't forget to check scanf()'s return value.

Answer 2

The printf in main invokes undefined behavior because you pass the address of the function isPrimal instead of its result: change printf("The result is %d\n", isPrimal); to
printf("The result is %d\n", isPrimal(n, p));
Furthermore, your algorithm is very inefficient. Function isPrimal calls itself recursively n-2 times. If you remove the printf statement, the compiler notices the tail call to itself and turns this recursion into a loop. When you have the printf statement it cannot do this and for large values of n you probably recurse too deep and cause a Stackoverflow.

Answer 3

There are 2 changes that are necessary
1. In main, While printing the output make a function call -not a reference to the location of the function.
2. In isPrimal, Call the isprimal recursively after the print statement.
#include<stdio.h>
#include<stdlib.h>
int isPrimal(int n, int p) {
if (p == 1) {
return 1;
}
if (n % p == 0) {
return 0;
}
printf("n = %d i = %d\n", n, p);
isPrimal(n, p - 1);
}
int main() {
int numcase, *A, sizeA = 0, i = 0, cnt3dig = 0, n, p;
printf("Enter a number to check for primality\n");
scanf("%d", &n);
p = (n - 1);
printf("The result is 1 if the number is a prime, 0 otherwise\n");
printf("The result is %d\n", isPrimal(n, p));
return 1;
}