Decrementing alphabetical values

Question

I'm trying to figure out how to shift a bunch of letter values in an array down one step. For example, my array contains values ("d", "e", "f", "g", "h") and I want to change this to ("c", "d", "e", "f", "g"). Here's the code I'm working with:

function move_up_left($x) {
    if($x['orientation'] == "down") {
        foreach($x[0] as &$value) {
            $value = --$value; 
        }
    } else {
        foreach($x[1] as &$value) {
            $value = --$value;
        }
    }

    return $x;
}

When I use positive values, the letters change; however the negative numbers do not seem to be working at all.

Answer 1

PHP has overloaded ++ for strings; this isn't the case for --. You can do the same thing with much cleaner code with chr, ord, and array_map:

function decrementLetter($l) {
    return chr(ord($l) - 1);
}

function move_up_left($x) {
    if($x['orientation'] === 'down') $arr = &$x[0];
    else $arr = &$x[1];

    $arr = array_map('decrementLetter', $arr);

    return $x;
}

Here's a demo. Note that you may need to add a special case for decrementing a - I'm not sure how you want to deal with that.

Answer 2

If you need to decrement Excel-like variables ('A', 'AA', ...), here's the function I came to. It doesn't work with special characters but is case insensitive. It returns null if you try to decrement 'a' or 'A'.

function decrementLetter($char) {
     $len = strlen($char);
     // last character is A or a
     if(ord($char[$len - 1]) === 65 || ord($char[$len - 1]) === 97){ 
          if($len === 1){ // one character left
               return null;
           }
           else{ // 'ABA'--;  => 'AAZ'; recursive call
               $char = decrementLetter(substr($char, 0, -1)).'Z';
            }
     }
     else{
         $char[$len - 1] = chr(ord($char[$len - 1]) - 1);
     }
     return $char;
}