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mt_rand

(PHP 4, PHP 5, PHP 7)

mt_randGerador melhorado de números aleatórios

Descrição

mt_rand ( void ) : int
mt_rand ( int $min , int $max ) : int

Muitos geradores de números aleatórios das libcs antigas são duvidosos ou com características duvidosas e lentos. Por default, o PHP utiliza o gerador de números aleatórios da libc para a função rand(). A função mt_rand() é um bom substituto para a primeira. Ela utiliza um gerador de números aleatórios com características conhecidas, o »  Mersenne Twister, que produzirá números randômicos quatro vezes mais rápido que média provida pela rand() da libc.

Se chamada sem os argumentos opcionais min e max, mt_rand() retorna um pseudo número aletório enrte 0 e mt_getrandmax(). se você precisa de um número randômico entre 5 e 15 (inclusive), por exemplo, utilize mt_rand (5, 15).

Nota: A partir do PHP 4.2.0, não é mais necessário semear o gerador de números aleatórios com srand() ou mt_srand() pois isso é feito automaticamente.

Parâmetros

min

Menor valor opcional para ser retornado (padrão: 0)

max

Maior valor opcional para ser retornado (padrão: mt_getrandmax())

Valor Retornado

Um valor inteiro entre min (ou 0) e max (ou mt_getrandmax(), inclusive)

Changelog

Versão Descrição
Desde o 3.0.7 Em versões anteriores a 3.0.7, o significado do parâmetro max era range. Assim, para obter os mesmos resultados do exemplo acima, você precisaria utilizar rand (5, 11) para obter um número aleatório entre 5 e 15.

Exemplos

Exemplo #1 mt_rand() example

<?php
echo mt_rand() . "\n";
echo 
mt_rand() . "\n";

echo 
mt_rand(515);
?>

O exemplo acima irá imprimir algo similar à:

1604716014
1478613278
6

Veja Também

  • mt_srand() - Semeia o gerador melhorado de números aleatórios
  • mt_getrandmax() - Retorna o maior valor aletório possível
  • rand() - Gera um inteiro aleatório

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User Contributed Notes 24 notes

up
102
cyrax21 at gmail dot com
8 years ago
i wanted to spot out the big difference between rand and mt_rand when producing images using randomness as noise.

for example this is a comparation between rand and mt_rand on a 400x400 pixel png: http://oi43.tinypic.com/vwtppl.jpg

code:
<?php
header
("Content-type: image/png");
$sizex=800;
$sizey=400;

$img = imagecreatetruecolor($sizex,$sizey);
$ink = imagecolorallocate($img,255,255,255);

for(
$i=0;$i<$sizex/2;$i++) {
  for(
$j=0;$j<$sizey;$j++) {
 
imagesetpixel($img, rand(1,$sizex/2), rand(1,$sizey), $ink);
  }
}

for(
$i=$sizex/2;$i<$sizex;$i++) {
  for(
$j=0;$j<$sizey;$j++) {
 
imagesetpixel($img, mt_rand($sizex/2,$sizex), mt_rand(1,$sizey), $ink);
  }
}

imagepng($img);
imagedestroy($img);
?>

the differences reduce when reducing the pixels of the image.. infact for a 100x100 pixel image the noise produced from the rand function is much more realistic than how it is for a 400x400 image: http://oi39.tinypic.com/5k0row.jpg

(rand is on the left, mt_rand on the right)
up
3
Anonymous
16 years ago
Here is a example of a very small, compact, quite random-random string generator. It will make a string with uppercase & lowercase letters, with numbers. You simply need to set $len in the for() structure, and then the string will be in $r.  It has been designed for size, while it's still quite fast.  Mind the wrapping, it should be 1 line.

<?php
for($len=8,$r='';strlen($r)<$len;$r.=chr(!mt_rand(0,2)?
mt_rand(48,57):(!mt_rand(0,1)?mt_rand(65,90):mt_rand
(97,122))));
?>

Armond Carroll
up
2
nospamremove dot francois dot gannaz at silecs dot info
3 years ago
rand() comes from the libc, and mt_rand() is internal to PHP. So the differences vary with their respective versions.

On a 64b Debian Stretch with PHP 5.6.21, there is no visible difference: http://oi64.tinypic.com/2nkqas6.jpg

This image compares the two functions. In the top half with random points, in the lower half with random intensity on each point. This is totally different from what was obtained 4 years ago in another note, with an unknown environment.

Here is the code for this visual comparison.
<?php
$sizex
= 400;
$sizey = 400;

$img = imagecreatetruecolor(2 * $sizex, 2 * $sizey);
$white = imagecolorallocate($img, 255, 255, 255);
$inks = array_map(
    function(
$r) use($img) { return imagecolorallocate($img, $r, $r, $r); },
   
range(0, 255)
);
for (
$x = 0 ; $x < $sizex ; $x++) {
    for (
$y = 0 ; $y < $sizey ; $y++) {
       
// placing ($sizex x $sizey) white points at random in the top half
       
imagesetpixel($img, rand(0, $sizex - 1), rand(0, $sizey - 1), $white);
       
imagesetpixel($img, $sizex + mt_rand(0, $sizex - 1), mt_rand(0, $sizey - 1), $white);
       
// random intensity for each point in the lower half
       
imagesetpixel($img, $x, $sizey + $y, $inks[rand(0, 255)]);
       
imagesetpixel($img, $sizex + $x, $sizey + $y, $inks[mt_rand(0, 255)]);
    }
}
header("Content-type: image/png");
imagepng($img);
?>
up
10
contact at sheyd dot fr
8 years ago
To quickly build a human-readable random string for a captcha per example :

<?php

function random($length = 8)
{     
   
$chars = 'bcdfghjklmnprstvwxzaeiou';
   
    for (
$p = 0; $p < $length; $p++)
    {
       
$result .= ($p%2) ? $chars[mt_rand(19, 23)] : $chars[mt_rand(0, 18)];
    }
   
    return
$result;
}

?>

Note that I have removed q and y from $chars to avoid readability problems.
up
3
nowhere at where dot net
15 years ago
Allows characters 0-9, a-z
Weighted (and tested) ok.

<?php
function generate_string ($length = 20)
{
   
$nps = "";
    for(
$i=0;$i<$length;$i++)
    {
       
$nps .= chr( (mt_rand(1, 36) <= 26) ? mt_rand(97, 122) : mt_rand(48, 57 ));
    }
    return
$nps;
}
?>
up
5
chagenbu at php dot net
13 years ago
The algorithm used by mt_rand() changed in PHP 5.2.1. If you are relying on getting the same sequence from mt_rand() after calling mt_srand() with a known seed, upgrading to PHP 5.2.1 will break your code. See http://bugs.php.net/bug.php?id=40724 for something of an explanation; there is no workaround.
up
1
demogracia at metropoliglobal dot com
18 years ago
<?php
//
// Generates a random string with the specified length
// Chars are chosen from the provided [optional] list
//
function simpleRandString($length=16, $list="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"){
   
mt_srand((double)microtime()*1000000);
   
$newstring="";

    if(
$length>0){
        while(
strlen($newstring)<$length){
           
$newstring.=$list[mt_rand(0, strlen($list)-1)];
        }
    }
    return
$newstring;
}

//
// Generates a random string with the specified length
// Includes: a-z, A-Z y 0-9
//
function randString($length=16) {
  
$newstring="";
   if(
$length>0) {
       while(
strlen($newstring)<$length) {
          
$randnum = mt_rand(0,61);
           if (
$randnum < 10) {
              
$newstring.=chr($randnum+48);
           } elseif (
$randnum < 36) {
              
$newstring.=chr($randnum+55);
           } else {
              
$newstring.=chr($randnum+61);
           }
       }
   }
   return
$newstring;
}
?>
up
6
zolaar at nothanks dot com
13 years ago
a better (and likely faster) way to generate a random 6-digit hex string:

<?php
$num
= mt_rand ( 0, 0xffffff ); // trust the library, love the library...
$output = sprintf ( "%06x" , $num ); // muchas smoochas to you, PHP!
return $output;
?>

The mt_rand function won't give you a number outside the bounds you asked for -- no need to and-off the top bits -- and the sprintf function has params for length-padding & hexidecimal output.  It's likely faster because most of the work is being done by the wicked fast C functions that PHP sits on top of, though YMMV in that dept.
up
4
Hayley Watson
7 years ago
Another graphical comparison of rand() and mt_rand(). It effectively draws a graph showing how the last generated number affects the next by plotting the numbers in consecutive pairs against each other.

<?php
header
("Content-type: image/png");
$sizex=800;
$sizey=800;

$img = imagecreatetruecolor(2 * $sizex,$sizey);
$black = imagecolorallocate($img,0, 0, 0);
imagefilledrectangle($img, 0, 0, 2 * $sizex, $sizey, imagecolorallocate($img, 255, 255, 255));

$p = 0;
for(
$i=0; $i < 100000; $i++) {
   
$np = rand(0,$sizex);
   
imagesetpixel($img, $p, $np, $black);
   
$p = $np;
}

$p = 0;
for(
$i=0; $i < 100000; $i++) {
   
$np = mt_rand(0,$sizex);
   
imagesetpixel($img, $p + $sizex, $np, $black);
   
$p = $np;
}

imagepng($img);
imagedestroy($img);
?>
up
4
geompse at yopmail dot com
12 years ago
mt_rand() is not faster than rand() !

Tested over 100'000 iterations, with none/various/random arguments, mt_rand is always 3% slower than rand().
up
3
mark omohundro, ajamyajax dot com
11 years ago
just another example: both of these routines return a random decimal number between -1 and 1... since rand() only returns a max 'integer' value while mt_rand() return a max 'long' value -- at least on some platforms -- mt_rand() could be the better precision choice for some on any variation to this routine (but i don't think it matters here):

<?php
echo "-1 to 1 rand() value: ". (rand(getrandmax()*-1,getrandmax())/getrandmax()). "\n<br/>";
echo
"-1 to 1 mt_rand() value: ". (mt_rand(mt_getrandmax()*-1,mt_getrandmax())/mt_getrandmax()). "\n";
?>
up
1
nilesh at itech7 dot com
10 years ago
A class to generate 99.5% unqiue strings. I found that there is only one or two characters common between two subsequent strings.

<?php

class Local_RandomString {
 
  protected
$_length;
  protected
$_prevRand;

  public function
__construct($length = 15) {
   
   
$this->_length = $length;

  }

  public function
getRand() {
   
   
$randStr = null;   
   
$args[] = 'N' . $this->_length;

    for(
$i = 0; $i < $this->_length; $i++) {
     
$args[] = mt_rand();     
    }
   
   
$randStr = substr(base64_encode((call_user_func_array('pack', $args))), 1, $this->_length);
   
$this->_prevRand = $randStr;
    return
$randStr;

  }

  public function
setLength($l) {
   
   
$this->_length = (int) $l;
   
    if(
$this->_length <= 0) {
      throw new
Exception('Invalid random string length');
    }

  }

  public function
getPrevRand() {
   
    return
$this->_prevRand;

  }

}
?>
up
4
Mark Seecof
11 years ago
If you need some pseudorandom bits for security or cryptographic purposes (e.g.g., random IV for block cipher, random salt for password hash) mt_rand() is a poor source.  On most Unix/Linux and/or MS-Windows platforms you can get a better grade of pseudorandom bits from the OS or system library, like this:

<?php
// get 128 pseudorandom bits in a string of 16 bytes

$pr_bits = '';

// Unix/Linux platform?
$fp = @fopen('/dev/urandom','rb');
if (
$fp !== FALSE) {
   
$pr_bits .= @fread($fp,16);
    @
fclose($fp);
}

// MS-Windows platform?
if (@class_exists('COM')) {
   
// http://msdn.microsoft.com/en-us/library/aa388176(VS.85).aspx
   
try {
       
$CAPI_Util = new COM('CAPICOM.Utilities.1');
       
$pr_bits .= $CAPI_Util->GetRandom(16,0);

       
// if we ask for binary data PHP munges it, so we
        // request base64 return value.  We squeeze out the
        // redundancy and useless ==CRLF by hashing...
       
if ($pr_bits) { $pr_bits = md5($pr_bits,TRUE); }
    } catch (
Exception $ex) {
       
// echo 'Exception: ' . $ex->getMessage();
   
}
}

if (
strlen($pr_bits) < 16) {
   
// do something to warn system owner that
    // pseudorandom generator is missing
}
?>

NB: it is generally safe to leave both the attempt to read /dev/urandom and the attempt to access CAPICOM in your code, though each will fail silently on the other's platform.  Leave them both there so your code will be more portable.
up
1
Jeff Cours
5 years ago
With PHP 5.3.3, we're seeing odd behavior on 32 bit Linux.

This works fine on 64 bit Linux:
<?php
printf
("%08x\n", mt_rand (0, 0xFFFFFFFF));
?>
but on our 32 bit Linux development server, it's always yielding "00000000".

On that same machine, this:
<?php
printf
("%08x\n", mt_rand (0, 0xFFFFFFF0));
?>
seems to always yield either 00000000 or a number in the range fffffff2 to ffffffff. This:
<?php
printf
("%08x\n", mt_rand (0, 0xFFFFFF00));
?>
gives numbers where the last two digits vary, and so on through at least 0xF0000000.

However, this:
<?php
printf
("%08x\n", mt_rand (0, 0x7FFFFFFF));
?>
seems to be well-behaved.

The moral? On 32 bit systems, be careful about crossing the signed number boundary, 0x7FFFFFFF.
up
1
Pawe Krawczyk
6 years ago
To reiterate the message about *not* using mt_rand() for anything security related, here's a new tool that has been just posted that recovers the seed value given a single mt_rand() output:

http://www.openwall.com/php_mt_seed/README
up
1
j dot s dot shiuan at gmail dot com
8 years ago
Another good way to get a random float is to divide the result of mt_rand.
Let's say we want a float between 0.75 and 1.25.

<?php

$i
= mt_rand(75,125) / 100;

?>
up
3
Miller
6 years ago
Another generic random string function, but very small and fast.

<?php
function mt_rand_str ($l, $c = 'abcdefghijklmnopqrstuvwxyz1234567890') {
    for (
$s = '', $cl = strlen($c)-1, $i = 0; $i < $l; $s .= $c[mt_rand(0, $cl)], ++$i);
    return
$s;
}

echo
mt_rand_str(8); // Something like mp2tmpsw
echo mt_rand_str(6, '0123456789ABCDEF'); // Something like B9CD0F
?>
up
1
falkartis at gmail dot com
5 years ago
I wrote another function to get a random float, if its not precise enougth jut add some '0' to the $mul parameter.

<?php

function f_rand($min=0,$max=1,$mul=1000000){
    if (
$min>$max) return false;
    return
mt_rand($min*$mul,$max*$mul)/$mul;
}
?>
I made following  tests:
<?php
   
echo f_rand()."<br>";              //0.497153
   
echo f_rand(0.5)."<br>";           //0.857822
   
echo f_rand(0.5,0.6)."<br>";       //0.599956
   
echo f_rand(0,10)."<br>";          //5.801949
   
echo f_rand(0,2,2)."<br>";         //1.5
   
echo f_rand(0,2,10)."<br>";        //1.7
?>
up
0
Nikhil S.
3 months ago
A nifty function to generate pretty coupon codes. This will generate unique coupon codes and you don't have to do a database check whether the code already exists.

<?php
function giftCodes(){
   
$a='';
    for(
$j=0; $j<=4; $j++){
       
$c = 4;
        if(
$j == 1) { $a .= date("d"); $c=2; }
        if(
$j == 3) { $a .= date("m"); $c=2; }
        if(
$j == 4) { $a .= date("y"); $c=2; }
        for(
$i=0; $i<= $c; $i++)
           
$a .= chr(!mt_rand(0,2)?mt_rand(48,57):mt_rand(65,90));
       
$a .='-';
    }
    return
substr($a, 0, -1);
}
?>
up
0
fabiovh on gmail
12 years ago
performance: for a repetitive task, it's much faster not to use the limit parameters, as shown below. just use the % operator.

$t=microtime(true);
for($i=0;$i<1000000;$i++)
mt_rand()%3;
echo microtime(true)-$t;

echo '|';

$t=microtime(true);
for($i=0;$i<1000000;$i++)
mt_rand(0,2);
echo microtime(true)-$t;

echo '|';

$t=microtime(true);
for($i=0;$i<1000000;$i++)
mt_rand();
echo microtime(true)-$t;

output: 0.316797971725|0.442242145538|0.253082036972
up
0
rok dot kralj at gmail dot com
12 years ago
mt_rand function returns just a whole numbers. If you want a random float, then here's an elegant way:

<?php
function random_float ($min,$max) {
   return (
$min+lcg_value()*(abs($max-$min)));
}
?>
up
-1
Robin Leffmann
9 years ago
Fast, pseudo-random binary data generation using mt_rand():

<?php

function rnd_bin( $length )
{
    while( @
$c++ * 16 < $length )
        @
$tmp .= md5( mt_rand(), true );
    return
substr( $tmp, 0, $length );
}

?>
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