How do you write a C program that inputs an integer and identify whether it is prime or composite?

Answer 1

#include<stdio.h>

#include<math.h>

bool is_prime (unsigned num) { if (num<2) // 0 and 1 are composite

return false;

if (!(num%2)) // 2 is the only even prime

return num==2;

const unsigned max_div = sqrt (num) + 1;

for (unsigned div=3; div<max_div; div+=2) {

if (!(num%div)) // the number is composite

return false;

}

// if we get this far, the number is prime

return true;

}

int main (void) {

unsigned num;

printf ("Enter a positive integer: ");

scanf ("%u", &num);

printf ("\n%u is %s\n", num, is_prime (num) ? "prime" : "composite");

return 0;

}