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I began programming recently. I am trying to implement a simple trial-division algorithm for finding all primes up to some number (it is much more "primitive" algorithm than the Sieve of Eratosthenes). Can you please find what's wrong with my code?

#range function:

range = function (a,b,c){
    var range1=[]
    for (i=a; i<b; i=i+c){
        range1.push(i);
    }
    return range1;
}

#The algorithm:

n=prompt("n");
var numbers=range(2,n,1);
var primes=[];
for (number in numbers){
 var sublist=range(2,number,1);
 console.log(sublist);
 for (x in sublist){
  if (number%x ===0){
   break;
  }
 primes.push(number); 
 }
}

Thanks in advance!

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1  
Is this code working? I'm a little confused at the request "Can you please find what's wrong with my code?" – Jeff Vanzella Sep 18 '12 at 23:26

1 Answer 1

up vote 1 down vote

Try using this instead.

// `isPrime` adopted from http://www.javascripter.net/faq/numberisprime.htm
var isPrime = function (n) {
    if (isNaN(n) || !isFinite(n) || n % 1 || n < 2) {
        return false;
    }
    if (n % 2 === 0){
        return (n === 2);
    }
    if (n % 3 === 0){
        return (n === 3);
    }
    for (var i = 5, m = Math.sqrt(n); i <= m; i += 6) {
        if ((n % i === 0) || (n % (i + 2) === 0)){
            return false;
        }
    }
    return true;
}
var getPrimesUntilN = function (n) {
    n = Math.abs(n);
    var primes = (1 < n) ? [2] : [];
    if (isNaN(n) || !isFinite(n)) {
        return primes;
    }
    for (var i = 3; i <= n; i+=2) {
        if (isPrime(i)) {
            primes.push(i);
        }
    }
    return primes;
};

Input:

getPrimesUntilN(50);

Output:

[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47]

share|improve this answer
    
Thank you for your answer. – shauli Sep 1 '12 at 17:21

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