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Sunday, January 11, 2015

FINDPRM-Finding Primes

Finding Primes

given below code is for findprm or finding primes spoj .

Here I have applied basic sieve to find primes . Here you have to tell the numbers of prime between N/2 and N . because all primes before N/2 are not common with second sieve(stated in question).

#include <bits/stdc++.h>
using namespace std;
bool primes[10000009];
int cum[10000009];
void pre(){
    for(int i = 3 ; i<= 3163 ; i+=2){
            for(int j = i*i ; j<= 10000000 ; j+=2*i)
                primes[j] = 1;
    cum[2] = 1;
    for(int i = 3 ; i<=10000000 ; i++){
        if(primes[i] == 0 && i%2 != 0)
            cum[i] += cum[i-1] + 1;
            cum[i] = cum[i-1];
int main(){
    int t;
        int n;
        printf("%d\n" , cum[n] - cum[(n>>1)]);
    return 0;