Jan 24 2010
Sieve (of Eratosthenes) algorithm for generating prime numbers
A prime number is divisible by 1 or by itself. To generate prime numbers to 20 we’ll use Sieve (of Eratosthenes) algorithm, which is really simple to explain:
Strike out all multiples of 2’s:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Strike out all multiples of 3’s:
2 3 5 7 9 11 13 15 17 19
And so on. Here is a C# code that does exactly that:
private static void PrintPrimesTo(int num) {
var primes = new bool[num + 1];
// init candidates
for (int i = 2; i <= num; i++) { primes[i] = true; }
// cross-out 2's:
// 2, 4, 6, 8, 10...
// cross-out 3's:
// 9, 12, 15...
for (int i = 2; (i * i) <= num; i++ ) {
Console.WriteLine("Crossing out {0}'s to {1}:", i, num);
for (int j = i * i; j <= num; j = j + i) {
Console.WriteLine(" * crossing-out: {0}", j);
primes[j] = false;
}
}
Console.WriteLine("Results: prime numbers between 2 and {0}:", num);
for (int i = 2; i <= num; i++) {
if (primes[i]) Console.WriteLine(" * {0} ", i);
}
}
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Masakatsu, “True Victory”.
“The most important thing for a human being is not what is between his or her ears; it is what is in his or her heart. If the spirit is strong, one can accomplish anything.
I am (re)reading a book on Aikido called “KI In Daily Life” by Koichi Tohei. Aikido has a very deep personal meaning to me and it has definitely shaped the way I go through life and interact with others.
Today was one of those memorable days I’ll always remember! My bboy crew, “Flovolution” performed at Big Easy for the RedBull “Flight Club” theme party. We shared the stage with breakers from Tri-Cities, and the “Circle Of Fire” (Seattle). Crazy Legs was there too, but he didn’t perform. Everyone was pretty sore from his workshop a day before. The Red Bull and Big Easy kindly provided us with Davenport hotel rooms, hats …