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아래건 소스...

#include <stdio.h>
#include <time.h>

#define TRUE 1

int is_prime1(int n)
{
        int i;

        for(i=2; i<n; i++)
        {
                if(n%i==0)
                        return !TRUE;
                return TRUE;
        }
}

int is_prime2(int n)
{
        int i, sqrn;

        sqrn=(int)sqrt(n);

        for(i=2; i<=sqrn; i++)
        {
                if(n%i==0)
                        return !TRUE;
                return TRUE;
        }
}

void result(int i, int n, int r, double t)
{
        printf("\n Prime%d Ans : %d is%s prime number in %.3f ticks.", i, n, r?"" : " not", t);
}

#define LOOP 1000

int main(void)
{
        int n;
        double t1, t2;        // 시간 저장용
        int r;        // 결과 저장용
        int i;

        puts("PRIME1 : Test that input is prime or not.");
        puts("\n           Input 0 to end program.");

        while(TRUE)
        {
                fputs("\nInput number to test -> ", stdout);
                scanf("%d", &n);

                if(n<0)        // n이 음수이면 재입력
                        continue;
                else if(n==0)        // n이 0이면 끝낸다.
                        break;

                t1=clock();
                for(i=0; i<10000000; i++)
                        r=is_prime1(n);
                t2=clock();
                result(1, n, r, ((t2-t1)/CLK_TCK));

                t1=clock();
                for(i=0; i<10000000; i++)
                        r=is_prime2(n);
                t2=clock();
                result(2, n, r, ((t2-t1)/CLK_TCK));




                //printf("\n Ans : %d is %s prime number", n, is_prime1(n)?"":"not");
        }
}