# how do you count prime numbers really fast?

So it's a fine morning, the birds and singing, flowers blooming, and you're asked: "Write a program to count how many prime numbers exist between 1 and 2 million"

I guess that sort of thing doesn't happen to many people...

In my case, this was a voluntary action when I embarked upon trying problems from https://projecteuler.net, and problem 10 is precisely this:

The sum of the primes below 10 is 2 + 3 + 5 + 7 = 17. Find the sum of all the primes below two million

Now, you'll rush off into Visual Studio (or vim :) or even emacs! eek) and type away, but when you run your program, if you've written it the brute force way, it's going to take about 22 minutes to run.

To people like me, that's just unacceptable! There has to be a better way right? Yes, if you guessed yes. Behold the solution.. well .... sort of...

http://en.wikipedia.org/wiki/Sieve_of_eratosthenes

Here is the whole problem solved off in C# based on the sieve algorithm. It takes only ~8 milliseconds from 22 minutes :D

```﻿/*
The sum of the primes below 10 is 2 + 3 + 5 + 7 = 17.
Find the sum of all the primes below two million.
http://projecteuler.net/problem=10
* */
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ProjectEuler.Problems
{
//A general IsPrime function will take too long.
//It takes about 22 minutes for the whole program to run
//I used the Seive of Eratosthenes algorithm which
//turns out to take ~79 milliseconds! =D

//ignore IProblem..
public class Problem10 : IProblem
{
public void Run()
{

long sum = 0;
long n = 2000000;
//by default they're all false
bool[] e = new bool[n];
for (int i = 2; i < n; i++)
{
e[i] = true;//set all numbers to true
}
//weed out the non primes by finding mutiples
for (int j = 2; j < n; j++)
{
if (e[j])//is true
{
for (long p = 2; (p*j) < n; p++)
{
e[p * j] = false;
}
}
}
//Uptill here e[] sorta of contains a list of primes
//the index represent the actual number and
//the value at the index represents if the number is prime
//Example:
//e[4], e[100] will all be false since 2,4,100 are not primes
//e[5], e[7], e[11], e[13] will all be true because
//5,7,11,13 are all prime numbers

for (long r = 2; r < n; r++)
{
if (e[r])
{
sum += r;
}
}
Console.WriteLine("Sum:" + sum);
}
}
}```

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