9주차 퀴즈

1번

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
 
int main(void)
{
    int i;
    int r;
    int n;
    int sum;
    double prob;
 
    for (n = 10; n < 10000000; n *= 10) {
        sum = 0;
        for (i = 0; i < n; i++) {
            r = rand() % 10 + 11;//11~20
            sum = sum + r;
        }
        prob = sum / (double)i;
        printf("n = %7d avg = %lf\n", i, prob);
    }
    return 0;
}

#include <stdio.h>
 
int isPrime(int n);
 
int main(void)
{
    int T;
    int i;
    int n;
 
    scanf("%d", &T);
 
    for (i = 0; i < T; i++) {
        scanf("%d", &n);
 
        if (isPrime(n) == 0) {//소수아닐때
            printf("NO\n");
        } 
        else {//소수일때
            int count = 0;
            for (int j = 2; j <= n; j++) {
                if (isPrime(j) == 1) {
                    count++;
                }
            }
            printf("%d\n", count);
        }
    }
}
 
int isPrime(int n)
{
    int i;
 
    for (i = 2; i < n; i++) {
        if (n % i == 0) {
            return 0;//소수x
        }
    }
    return 1;//소수o
}