Find Prime Numbers in Range (Javascript)

Category: Javascript

Will find and identify any prime numbers between the given start and end values. Primes are identified using a factorial.

Originally written as part of a solution for my May 2016 puzzle. The original solution was written in Python, this snippet is part of a (successful) attempt to rewrite that in javascript so that the solution can simply be dropped into the Developer Tools console to solve the puzzle in browser.

Similar To

Details

Snippet

// Calculate all primes between a lower and upper bound
function calculatePrimesForRange(l,h){
      var primes=[];
      for (i=l; i<h; i++){

          if (checkNumPrime(i)){
            primes.push(i);
          }

      }
      return primes;
}

// Check whether a number is a prime
function checkNumPrime(n){

      // Check whether the number is 1,2 or 3
      if (n<4){
          return true;
      }

      // Check the number isn't directly divisible by 2 or 3
      if (n%2 == 0 || n%3 == 0){
          return false;
      }

      var di=2;
      var i = 5;

      // Don't calculcate higher than the square root (rounded down if needed)
      var lim = Math.floor(Math.sqrt(n));

      while (i < lim){

        if (n%i == 0){
            return false;
        }
        i=i+di;
        di=6-di;
      }

      // If we haven't already returned, n is prime
      return true;

}

Usage Example

calculatePrimesForRange(1,100);