(数论2.1.3)UVA 10533 Digit Primes(埃拉托斯特尼筛法)

/*
* UVA_10533.cpp
*
*  Created on: 2013年10月7日
*      Author: Administrator
*/

#include <iostream>
#include <cstdio>
#include <cstring>

using namespace std;

const int maxn = 1000001;
bool u[maxn];
int u2[maxn];

void prepare() {
int i, j;
memset(u, true, sizeof(u));

for (i = 2; i <= maxn; ++i) {
if (u[i]) {
for (j = 2; j * i <= maxn; ++j) {
u[j * i] = false;
}
}
}

}

bool ok(int x) {

int k = 0;
while (x) {
k += x % 10;
x /= 10;
}

return u[k];
}

int main() {
prepare();
int t;
scanf("%d", &t);
int i, j;
for (i = 2; i <= maxn; ++i) {
if (u[i] && ok(i)) {
u2[i] = 1;
}
}

for (i = 2; i <= maxn; ++i) {
u2[i] += u2[i - 1];
}
while (t--) {
scanf("%d%d", &i, &j);
printf("%d\n", u2[j] - u2[i - 1]);
}

return 0;
}