# Answer to Question #18389 in C++ for Ige

Question #18389

how to write a C++ program to find all the prime numbers between 1 and 20000

Expert's answer

#include <stdlib.h>

#include <math.h>

#include <iostream>

#include <time.h>

using namespace std;

#include <vector>

int Eratosfen (int n)

{

vector <bool> pA (n+1); //масив використовується з індекса 2

for (int i=0 ; i<n ; ++i)

& pA[i]=0;

int count = n-1;

for (int i=2; i*i<=n ; i++)

{

&

& if (pA[i]==0)

& for (int j=i*i; j<=n ; j+=i)

& if (pA[j]==0)

& {

&

& pA[j]=1;

& count--;

& }

}

for (int i = 0 ; i < pA.size() ; i++){

& if (pA[i] == false) cout<<i<<endl;

}

return count;

}

int main ()

{

Eratosfen(20000);

system ("pause");

return 0;

}

#include <math.h>

#include <iostream>

#include <time.h>

using namespace std;

#include <vector>

int Eratosfen (int n)

{

vector <bool> pA (n+1); //масив використовується з індекса 2

for (int i=0 ; i<n ; ++i)

& pA[i]=0;

int count = n-1;

for (int i=2; i*i<=n ; i++)

{

&

& if (pA[i]==0)

& for (int j=i*i; j<=n ; j+=i)

& if (pA[j]==0)

& {

&

& pA[j]=1;

& count--;

& }

}

for (int i = 0 ; i < pA.size() ; i++){

& if (pA[i] == false) cout<<i<<endl;

}

return count;

}

int main ()

{

Eratosfen(20000);

system ("pause");

return 0;

}

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