I've written a subroutine to validate a 12-digit UPC. This involves a mathematical checksum built into the UPC-A standard...rather than going into full detail, the process is explained pretty well here.

This whole process involves seperately summing the odd digits of the upc and multiplying it by 3, then the even digits, then summing the two results. Right now, I'm accomplishing this by split()ing the upc into an array and checking a modulus of 2 on each index: 

# returns error message on failure, undefined value on success
sub checkUPC
{
  # grab and immediately split upc into array, 1 char per element
  my @chars = split(//, shift);
  
  # return error message if incorrect length
  if( $#chars != 11 )
  { return "should be 12 digits in length"; }
  
  # loop through to seperately sum even and odd chars
  foreach (0..10)
  {
    if($_ % 2 == 0)
    { $odd += $chars[$_]; }
    else
    { $even += $chars[$_]; }
  }
  
  # calculate correct check digit
  $mult = $check = ($odd * 3) + $even;
  while($mult % 10 != 0)
  { $mult++; }
  $check = $mult-$check;
  
  # return error message if wrong check digit was initially given
  if($check != $chars[11])
  { return "invalid checkdigit...should be $check"; }
  
  # otherwise, if validated, return undefined
  return;
}

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Now, this works just fine, but I find the split() and looping seems...messy. Perhaps a fellow monk would know a more elegant and efficient way?

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In reply to Summing the odd/even digits of a larger number (for upc check digits)...a better way? by EvanK

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