# Sieve of Eratosthenes [prime count]

Bjarne Stroustrup “Programming Principles and Practice Using C++”
Chapter 4 Exercise 15
Using std_lib_facilities.h by Bjarne Stroustrup.

```//	Philipp Siedler
//	Bjarne Stroustrup's PP
//	Chapter 4 Exercise 15

#include "std_lib_facilities.h"
#include "math.h"

int main()
try
{
int max = 100;
int num = 5;
int counter = 0;
string s;
vector<int> seriesNum;

cout << "Enter a maximum value for the prime number generator: \n";
if (cin >> max) {}
else {
cin.clear();
cin >> s;
}

cout << "How many prime numbers do you want to display? \n";
if (cin >> num) {}
else {
cin.clear();
cin >> s;
}

//fill series of numbers
for (int i = 2; i < max; i++) {
seriesNum.push_back(i);
}

//check for primes
for (int j = 2; j < 6; j++) {
for (int i = 0; i < seriesNum.size() - 1; i++) {
if (seriesNum[i] % j == 0 && seriesNum[i] != j) {
seriesNum[i] = 0;
}
}
}

//print primes
for (int i = 0; i < seriesNum.size(); i++) {
if (seriesNum[i] != 0 && counter < num) {
cout << seriesNum[i] << "\n";
counter++;
}
}

keep_window_open(".");
}

catch (runtime_error e) {
cout << e.what() << "\n";
keep_window_open(".");
}
catch (...) {
cout << "Exiting.\n";
keep_window_open(".");
}
```
```Output:
Enter a maximum value for the prime number generator:
100
How many prime numbers do you want to display?
10
2
3
5
7
11
13
17
19
23
29