程序代码：

#include <stdio.h>

int main(void) {
    int n, i;
    printf("INPUT N: ");
    scanf("%d", &n);

    for(i = 2; i < n / 2; i++)
        if(0 == n % i) {
            i = 0;
            break; //能被1和自身之外的数整除 非素数 结束循环 i当作判断标识置为0
        }

    //i非0时 打印为素数 否则非素数
    if(i) printf("%d is a prime number.\n", n);

    else printf("%d is not a prime number.\n", n);

    return 0;
}

/*
//// 或者用GOTO 更易理解
[color=#0000FF]#include <stdio.h>

int main(void) {
    int n, i;
    printf("INPUT N: ");
    scanf("%d", &n);

    for(i = 2; i < n / 2; i++)
        if(0 == n % i) {
            printf("%d is not a prime number.\n");
            goto END;
        }

    printf("%d is a prime number.\n", n);
END:
    return 0;
}
[/color]*/