isprime.c

为stanford大学donoho教授所编写的redgit变换源代码。是用c编写的

/*
 * Check if P is a prime number.
 *   Return 1 when p is a prime number and return 0 when it is not.
 */

#include <math.h>

int isprime(int p)
{
    int i, q;    

    /* Negative number is not a prime */
    if (p < 0)
	return 0;

    q = (int) sqrt(p);

    for (i = 2; i < q; i++)
	if ((p % i) == 0)
	    return 0;

    /* No factor of p is found */
    return 1;
}