I'm a professional game developer from Wakefield, England, working as a senior programmer for Rebellion North.

I'm a married father of five and I a also sometimes do Retroburn stuff.

Martin 'Bytrix' Caine

Father. C++ Games Programmer. Cyclist. Guitarist.

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Project Euler - problem 4

Another pretty easy one from Project Euler, puzzle 4 simply required building an array of values based on calculations and sorting them to find the highest values:

A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 99.

Find the largest palindrome made from the product of two 3-digit numbers.

I basically had two variables looping from 999 to 100, multiplying them together for each value. I then converted the value to a string and reversed it, if the reversed string equals the original string the number is palindromic so I added it to an array (making sure to add the number and not the string or ordering the array would work alphabetically rather than numerically).

I then reversed the order of the array and output the first element:



<?
/******************************************************************
*  ProjectEuler.net  *  Problem 4  *  2010-05-31  *  Martin Caine *
******************************************************************/

//create an array to store the results
$numbers = array();

// loop from 999 to 100
for( $x=999; $x>99; $x-- )
{
    // loop the second value from 999 to 100
    for( $y=999; $y>99; $y-- )
    {
        $number = $x * $y;
        // test if the number is palindromic
        if( strval( $number ) == strrev( strval( $number ) ) )
        {
            // if it's not already in our array, add it now
            if( !in_array( $number, $numbers ) )
            {
                $numbers[] = $number;
            }
        }
    }
}

// sort the numbers in reverse order
rsort( $numbers );

// output the highest number
echo $numbers[0];

?>

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