Project Euler 003 – JavaScript

18 02 2011

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"&gt;

<html xmlns="http://www.w3.org/1999/xhtml"&gt;

 

<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).

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