Problem 3
The prime factors of 13195 are 5, 7, 13 and 29.
What is the largest prime factor of the number 600851475143 ?
Solution
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Project Euler 003</title>
</head>
<body >
<script type="text/javascript">
function PrimeFactor(number)
{
//here is our list of prime factors
factors = new Array();
//assume the number is prime
//unless we find a factor
isPrime = true;
//here is the upper limit of what we need to test
factorCandidate = Math.round(Math.sqrt(number), 0);
while (factorCandidate > 1)
{
//is this a factor?
if (number % factorCandidate == 0)
{
//if so, what are it’s factors?
divisor = number / factorCandidate;
factors.concat(PrimeFactor(factorCandidate));
factors.concat(PrimeFactor(divisor));
isPrime = false;
}
//next
factorCandidate–;
}
//we are only adding prime factors
if (isPrime) { factors.push(number); }
return factors;
}
factors = PrimeFactor(600851475143);
// this sorts the numbers in ascending order
factors.sort();
// this reverses the list so the biggest one
// is on top.
factors.reverse();
answer = factors[0];
document.write("<b>" + answer + "</b>");
</script>
</body>
</html>
Discussion
Debugging JavaScript is not my forte. The concat function was handy here, but I don’t think that JavaScript is going to get much better for me unless I spend some time getting better tools.
If you have questions, leave a comment or please find me on Twitter (@azzlsoft) or email (rich@azzlsoft.com).
azzlsoft.com 
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