Refactoring functions that generate string with random letters, numbers or mixed chars?

Question

The bellow functions are used to generate a string with random characters, and they have a parameter to specify the string length.

The string can be composed of letters, numbers or a conjunction of both.

I've been using this for a very long time now, and currently, I look at it and it seems to damn extensive for the end result.

Functions

function assign_rand_value($num) {

    // accepts 1 - 36
    switch($num) {
        case "1"  : $rand_value = "a"; break;
        case "2"  : $rand_value = "b"; break;
        case "3"  : $rand_value = "c"; break;
        case "4"  : $rand_value = "d"; break;
        case "5"  : $rand_value = "e"; break;
        case "6"  : $rand_value = "f"; break;
        case "7"  : $rand_value = "g"; break;
        case "8"  : $rand_value = "h"; break;
        case "9"  : $rand_value = "i"; break;
        case "10" : $rand_value = "j"; break;
        case "11" : $rand_value = "k"; break;
        case "12" : $rand_value = "l"; break;
        case "13" : $rand_value = "m"; break;
        case "14" : $rand_value = "n"; break;
        case "15" : $rand_value = "o"; break;
        case "16" : $rand_value = "p"; break;
        case "17" : $rand_value = "q"; break;
        case "18" : $rand_value = "r"; break;
        case "19" : $rand_value = "s"; break;
        case "20" : $rand_value = "t"; break;
        case "21" : $rand_value = "u"; break;
        case "22" : $rand_value = "v"; break;
        case "23" : $rand_value = "w"; break;
        case "24" : $rand_value = "x"; break;
        case "25" : $rand_value = "y"; break;
        case "26" : $rand_value = "z"; break;
        case "27" : $rand_value = "0"; break;
        case "28" : $rand_value = "1"; break;
        case "29" : $rand_value = "2"; break;
        case "30" : $rand_value = "3"; break;
        case "31" : $rand_value = "4"; break;
        case "32" : $rand_value = "5"; break;
        case "33" : $rand_value = "6"; break;
        case "34" : $rand_value = "7"; break;
        case "35" : $rand_value = "8"; break;
        case "36" : $rand_value = "9"; break;
    }
    return $rand_value;
}

function get_rand_alphanumeric($length) {
    if ($length>0) {
        $rand_str="";
        for ($i=1; $i<=$length; $i++) {
            mt_srand((double)microtime() * 1000000);
            $num = mt_rand(1,36);
            $rand_str .= assign_rand_value($num);
        }
    }
    return $rand_str;
}

function get_rand_numbers($length) {
    if ($length>0) {
        $rand_str="";
        for($i=1; $i<=$length; $i++) {
            mt_srand((double)microtime() * 1000000);
            $num = mt_rand(27,36);
            $rand_str .= assign_rand_value($num);
        }
    }
    return $rand_str;
}

function get_rand_letters($length) {
    if ($length>0) {
        $rand_str="";
        for($i=1; $i<=$length; $i++) {
            mt_srand((double)microtime() * 1000000);
            $num = mt_rand(1,26);
            $rand_str .= assign_rand_value($num);
        }
    }
    return $rand_str;
}

---

Usage:

Basically I have a main function with the values, then I call secondary functions to build my string based on the length parameter:

Letters:

$str = get_rand_letters(8); // Only Letters

Numbers:

$str = get_rand_numbers(8); // Only Numbers

AlphaNumeric:

$str = get_rand_alphanumeric(8); // Numbers and Letters

---

My question:

How would I go, as to reduce the amount of code, keeping the end result the same?

Answer 1

First: I'm sure there's even simpler way than this (i.e. a library that just does what you need, period), but heck, I figured I'd try. Been years since I spent my days with PHP (don't particularly miss it).

Anyway, skip the whole switch lookup. chr() will take an int and give you its ASCII value. The 97-122 range (inclusive) is a-z (65-90 is A-Z). As for numbers, well, that's what a random function gives you - no need to look that up. See Rene Geuze's comment below; while chr works just fine, range is indeed more readable.

function get_rand($min, $max) {
  mt_srand((double) microtime() * 1000000);
  return mt_rand($min, $max);
}

function get_rand_alphanumeric($length) {
  $alnum = "";
  $range = range("a", "z");
  $limit = count($range) + 9;
  while(strlen($alnum) < $length) {
    $rand = get_rand(0, $limit);
    if( $rand < 10 ) {
      $alnum .= $rand;
    } else {
      $alnum .= $range[$rand - 10];
    }
  }
  return $alnum;
}

function get_rand_numbers($length) {
  if( $length <= 8 ) { // avoid int overflow
    return (string) get_rand(pow(10, $length-1), pow(10, $length));
  } else {
    $numbers = "";
    while(strlen($numbers) < $length) $numbers .= get_rand_numbers(8);
    return substr($numbers, 0, $length);
  }
}

function get_rand_letters($length) {
  $letters = "";
  $range   = range("a", "z");
  $limit   = count($range) - 1;
  while(strlen($letters) < $length) $letters .= $range[get_rand(0, $limit)];
  return $letters;
}

You get results like this

get_rand_alphanumeric(42) => upop8eoome0y0av2qav1j7yn5linyjshiurc8lbjja
get_rand_numbers(42)      => 365818982423371436493339856184748778003731
get_rand_letters(42)      => abfmthyuxdlganhfthebfjaugeeoniqawocgavowpx

Edit I just realized that your original alphanumeric function will pick randomly from your entire list, meaning it's 2.6 times more likely that the pick will be a letter. I edited mine to do the same to keep the results similar.

Also, the calls to mt_srand are by far the most expensive operation. Consider calling it less frequently, if speed is a concern. If I omit it, the functions above are 7.5-10x faster than the original; if I leave it as you see in the code, the functions are still faster, but only very marginally so (1.1x to 1.4x). In either case, get_rand_numbers obviously sees the biggest speedup since there's not lookup going on.

Edit 2 Replaced chr usage with a range array, re: the comments