primes

For a library, I need to store the first primes numbers up to a limit L. This collection must have a O(1) lookup time (to check whether a number is prime or not) and it must be easy, given a number, to find the next prime number (assuming it is smaller than L). Given that L is fixed, an Eratostene sieve to generate the list is fine. Right now, I use a packed boolean array to store the list, which contains only entries for odd numbers between 3 and L (inclusive). This takes (L-2)/2 bits of memory. I would like to be able to statically increase L without using more memory. Is there a data

Possible Duplicate: How to determine if a number is a prime with regex? This page claims that this regular expression discovers non-prime numbers (and by counter-example: primes): /^1?$|^(11+?)\1+$/ How does this find primes? Matchu I think the article explains it rather well, but I'll try my hand at it as well. Input is in unary form. 1 is 1 , 2 is 11 , 3 is 111 , etc. Zero is an empty string. The first part of the regex matches 0 and 1 as non-prime. The second is where the magic kicks in. (11+?) starts by finding divisors. It starts by being defined as 11 , or 2. \1 is a variable referring

I wrote this code to find the prime numbers less than the given number i in scala. def findPrime(i : Int) : List[Int] = i match { case 2 => List(2) case _ => { val primeList = findPrime(i-1) if(isPrime(i, primeList)) i :: primeList else primeList } } def isPrime(num : Int, prePrimes : List[Int]) : Boolean = prePrimes.forall(num % _ != 0) But, I got a feeling the findPrime function, especially this part: case _ => { val primeList = findPrime(i-1) if(isPrime(i, primeList)) i :: primeList else primeList } is not quite in the functional style. I am still learning functional programming. Can anyone

I am trying to write a prime number function in C# and I am wondering if the follow code will work. It "appears" to work with the first 50 numbers or so. I just want to make sure it will work no matter how big the number is: static bool IsPrime(int number) { if ((number == 2) || (number == 3) || (number == 5) || (number == 7) || (number == 9)) return true; if ((number % 2 != 0) && (number % 3 != 0) && (number % 5 != 0) && (number % 7 != 0) && (number % 9 != 0) && (number % 4 != 0) && (number % 6 != 0)) return true; return false; } No it won't work! Try 121 = 11 * 11 for example which obviously

From this article , /^1?$|^(11+?)\1+$/ checks whether a number(its value in unary) is prime or not. Using this, perl -l -e '(1 x $_) !~ /^1?$|^(11+?)\1+$/ && print while ++$_;' returns a list of prime numbers. I do not have enough experience with Perl, but what I understand is that the regular expression will be true for a number that is not prime. So, if we print all numbers that do not produce a true with this expression, we have a list of prime numbers. Thats what the perl query is trying to do. About the regex part, ^1?$ part is for counting 1 as not prime ^(11+?)\1+$ is for matching not