I'm a professional game developer from Wakefield, England, working for TickTock Games.
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Martin 'Bytrix' Caine
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Monday, May 31st 2010 / Web

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$number ) == strrevstrval$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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