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Last updated: Fri, 27 Jun 2008

rand

(PHP 4, PHP 5)

rand — Generate a random integer

Description

int rand ( void )

int rand ( int $min , int $max )

If called without the optional min , max arguments rand() returns a pseudo-random integer between 0 and RAND_MAX. If you want a random number between 5 and 15 (inclusive), for example, use rand (5, 15).

Note: On some platforms (such as Windows) RAND_MAX is only 32768. If you require a range larger than 32768, specifying min and max will allow you to create a range larger than RAND_MAX, or consider using mt_rand() instead.

Note: As of PHP 4.2.0, there is no need to seed the random number generator with srand() or mt_srand() as this is now done automatically.

Parameters

min: The lowest value to return (default: 0)
max: The highest value to return (default: RAND_MAX)

Return Values

A pseudo random value between min (or 0) and max (or RAND_MAX, inclusive).

ChangeLog

Version	Description
Since 3.0.7	In versions before 3.0.7 the meaning of `max` was `range` . To get the same results in these versions the short example should be rand (5, 11) to get a random number between 5 and 15.

Examples

Example #1 rand() example


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

echo rand(5, 15);
?>

The above example will output something similar to:

7771
22264
11


For some people who would need to generate random string:





<?php





function random_letters ($numofletters) {


    if (!isset($numofletters)) $numofletters = 10; // if $numofletters is not specified sets to 10 letters


    $literki = array('A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'R', 'S', 'T', 'U', 'W');


    $ilosc_literek = count($literki);


    for ($licz = 0; $licz < $numofletters; $licz++) {


    $rand = rand(0, $ilosc_literek-1);


    $vercode = $vercode.$literki[$rand];


    }


}





?>

rammichael@home
24-May-2008 02:11


I always thought the RAND_MAX on windows is 0x7FFF (=32767)

davidsteinsland [at] gmail [dot] com
26-Apr-2008 09:27


emad_ramahi at hotmail dot com:

I've actually noticed that with a large dataset (100k rows), the query dramatically slows down the server and performance is way too bad.



The way I see it, you have to workable solutions:



Using PHP:



<?php

//$Table holds the name of the table we're getting the random row from

//$Rows specifies how many rows we need to fetch 

function mysql_rand ($Query, $Rows = 1) {

    //getting the table name from $query

    //what you can do, is replace the $query argument with $table,

    //this way to dont have to search for the table's name

    $SQL = sprintf ('SELECT COUNT(*) FROM %s', substr ($query, stripos ($query, 'from')+5, strpos ($query, ' ')));

    

    $Max = mysql_result (mysql_query ($SQL), 0);

    $Random = rand (0, $Max);

    

    //If the random number is 99, and the database only has 100 rows

    //We'll subtract the random number, so we don't exceed 100, thus preventing a MySQL error message

    return $Query . sprintf ('LIMIT %d, %d', ($Max < $Rows) ? 0 : (($Random > $Max - $Rows) ? $Max - $Rows : $Random), $Rows);

}



//Instead of using MySQL's RAND(), we use LIMIT to fetch rows

//E.g. LIMIT 5,9 fetches from row 5, and the subsequent 9

$SQL = mysql_rand ('SELECT row FROM table'); //SELECT row FROM table LIMIT x, y

?>



or SQL:

SELECT * FROM Table T JOIN (SELECT FLOOR(MAX(ID)*RAND()) AS ID FROM Table) AS x ON T.ID >= x.ID LIMIT 1;

emad_ramahi at hotmail dot com
09-Mar-2008 10:16


Hi All,

For those whom wants to get a random value from MySQL:

select coumnName from TableName order by rand()

Anonymous
25-Feb-2008 09:37


Using the script below with rand() instead of mt_rand(), 

I was suprised to find many "words" and part of "phrases" in common.



mt_rand() is highly recomanded when you want to build test cases.



 $ressbdd = mysql_connect("localhost", "usr", "pass") 

  or  die("Could not connect: " . mysql_error()); 

 for($i=0;$i<100000;$i++){

  $xx1=mt_rand(15,80);

  $texte = phrase($xx1);

  $sql = 'INSERT into phrases.valeurs set valeur = \'' . $texte . '\'';

  mysql_query( $sql , $ressbdd );

 }

//=============================================

function phrase($n){

 $phrase='';

 for($i=0;$i<$n;$i++){

  $xx2=mt_rand(2,20);

  $phrase.=mot($xx2) . ' ';

 }

 return(substr($phrase,0,-1).'.'); // phrase ends with a .

}

//=============================================

function mot($n){

 if($n<=0)  return 'hello';

 if($n>=31) return 'world';

 $voyelle =Array('a','e','i','o','u','y');

 $consonne=Array(

'b','c','d','f','g','h','j','k','l','m',

'n','p','q','r','s','t','v','w','x','z');

 $mot='';

 for($i=0;$i<$n;$i++){

  if($i%2==0){

   $xx3=mt_rand(0,19);

   $mot.=$consonne[$xx3];

  }else{

   $xx4=mt_rand(0,5);

   $mot.=$voyelle[$xx4];

  }

 }

 return $mot;

}

Anonymous
06-Nov-2007 10:24


Note, the function used by PHP to constrain a random number between (min, max) is the following:



$number = $min + (($max - $min + 1) * ($number / ($rand_max + 1));



where the following:

$number - the initially generated random number

$min - the minimum in the range

$max - the maximum in the range

$rand_max - the maximum possible random value



What this algorithm does is constrain the generated number to a 0-1 range, then multiply it against your range, mapping the two to each other. 



In practice you'll see the following results:

For a generated $number of 16384 and a $rand_max of 32768:

rand(0, 10) = 5

rand(0, 100) = 50

rand(0, 1000) = 500



with the additional property that if the range you're asking for is larger than $rand_max, random numbers will be in a multiple of $max/$rand_max.

Ishtar
10-Sep-2007 03:18


A small comment on phpdev-dunnbypauls conclusion that rand() only generates numbers that are a multiply of 3. 

<?php

$n = rand(0,100000); // with MAX_RAND=32768

?>

Since, 100000/32768=3.05 you get multiples of 3. The random integer will be multiplied by 3.05 to fit between 0 and 100000. rand() works fine, if you don't ask for bigger numbers then RAND_MAX.

rok dot kralj at gmail dot com
16-Jun-2007 09:43


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)));

}

?>

bozo_z_clown at yahoo dot com
24-May-2007 02:36


Note that the automatic seeding seems to be done with the current number of seconds which means you can get the same results for several runs on a fast server.  Either call srand() yourself with a more frequently changing seed or use mt_rand() which doesn't appear to suffer from the problem.

jont at live dot co dot uk
05-Apr-2007 12:42


isn't this just a simpler way of making a random id for somthing? I mean i know that there is a very slight chance that a duplicate could be made but its a very, very, very small chance, nearly impossible.



$rand = mt_rand(0, 32);

$code = md5($rand . time());

echo "$code";



and if you don't want it the md5 can be removed, I've just added it as a prefer it there :)



Jon

phpdev at dunnbypaul dot net
08-Mar-2007 05:51


Here's an interesting note about the inferiority of the rand() function. Try, for example, the following code...



<?php

$r = array(0,0,0,0,0,0,0,0,0,0,0);

for ($i=0;$i<1000000;$i++) {

  $n = rand(0,100000);

  if ($n<=10) {

    $r[$n]++;

  }

}

print_r($r); 

?>



which produces something similar to the following output (on my windows box, where RAND_MAX is 32768):



Array

(

    [0] => 31

    [1] => 0

    [2] => 0

    [3] => 31

    [4] => 0

    [5] => 0

    [6] => 30

    [7] => 0

    [8] => 0

    [9] => 31

    [10] => 0

)



Within this range only multiples of 3 are being selected. Also note that values that are filled are always 30 or 31 (no other values! really!) 



Now replace rand() with mt_rand() and see the difference...



Array

(

    [0] => 8

    [1] => 8

    [2] => 14

    [3] => 16

    [4] => 9

    [5] => 11

    [6] => 8

    [7] => 9

    [8] => 7

    [9] => 7

    [10] => 9

)



Much more randomly distributed!



Conclusion: mt_rand() is not just faster, it is a far superior algorithm.

Patrick Daryll G.
02-Mar-2007 03:51


Using rand()%x is faster than rand(0,x) yes, but it is wrong.

Consider the following example:

RAND_MAX is 32768 (like on Windows for example)

You use rand()%30000

Imagine rand() returns a value between 30000 and 32768.

Modulo could make any value between 0 and 2768, but not any between 2769 and 29999 (except the value is below 29999).

This would double the chance of getting a number between 0 and 2768, which is speaking against the principles of randomness.

ludicruz at yahoo dot com
27-Sep-2006 02:42


frank, nick at nerdynick dot com, and kniht

this is now O(n) instead of O(n^2) ish...



<?php

function rand_permute($size, $min, $max)

{

    $retval = array();

    //initialize an array of integers from $min to $max

    for($i = $min;$i <= $max;$i++)

    {

        $retval[$i] = $i;

    }

    //start with the the first index ($min).

    //randomly swap this number with any other number in the array.

    //this way we guarantee all numbers are permuted in the array,

    //and we assure no number is used more than once (technically reiterating prev line).

    //therefore we don't have to do the random checking each time we put something into the array.

    for($i=$min; $i < $size; $i++)

    {

        $tmp = $retval[$i];

        $retval[$i] = $retval[$tmpkey = rand($min, $max)];

        $retval[$tmpkey] = $tmp;

    }

    return array_slice($retval, 0, $size);

}

?>

umpalump at poczta dot neostrada dot pl
13-Jun-2005 02:32


Random numbers with Gauss distribution (normal distribution).

A correct alghoritm. Without aproximations, like Smaaps'

It is specially usefull for simulations in physics.

Check yourself, and have a fun.



<?php



function gauss()

{   // N(0,1)

    // returns random number with normal distribution:

    //   mean=0

    //   std dev=1

    

    // auxilary vars

    $x=random_0_1();

    $y=random_0_1();

    

    // two independent variables with normal distribution N(0,1)

    $u=sqrt(-2*log($x))*cos(2*pi()*$y);

    $v=sqrt(-2*log($x))*sin(2*pi()*$y);

    

    // i will return only one, couse only one needed

    return $u;

}



function gauss_ms($m=0.0,$s=1.0)

{   // N(m,s)

    // returns random number with normal distribution:

    //   mean=m

    //   std dev=s

    

    return gauss()*$s+$m;

}



function random_0_1()

{   // auxiliary function

    // returns random number with flat distribution from 0 to 1

    return (float)rand()/(float)getrandmax();

}



?>



JanS

student of astronomy

on Warsaw University

smaaps at kaldamar dot de
07-Jun-2005 07:44


Lately I needed some random numbers with a gaussian (normal) distribution, not evenly distributed as the numbers generated by rand(). After googling a while, I found out that there is no perfect algrorithm that creates such numbers out of evenly distruted random numbers but a few methods that have similar effect. The following function implements all three algorithms I found- The the last two methods create numbers where you can find a lower and upper boundary and the first one will create a number from time to time (such as one in every 10000) that may be very far from the average value. Have fun testing and using it.



function gauss($algorithm = "polar") {

    $randmax = 9999;

    

    switch($algorithm) {

        

        //polar-methode by marsaglia

        case "polar": 

            $v = 2;

            while ($v > 1) {

                $u1 = rand(0, $randmax) / $randmax;

                $u2 = rand(0, $randmax) / $randmax;



                $v = (2 * $u1 - 1) * (2 * $u1 - 1) + (2 * $u2 - 1) * (2 * $u2 - 1);

            }

            

            return (2* $u1 - 1) * (( -2 * log($v) / $v) ^ 0.5);

        

        // box-muller-method

        case "boxmuller": 

            do {

                $u1 = rand(0, $randmax) / $randmax;

                $u2 = rand(0, $randmax) / $randmax;                    

                

                $x = sqrt(-2 * log($u1)) * cos(2 * pi() * $u2);

            } while (strval($x) == "1.#INF" or strval($x) == "-1.#INF");

            

            // the check has to be done cause sometimes (1:10000)

            // values such as "1.#INF" occur and i dont know why

            

            return $x;



        // twelve random numbers   

        case "zwoelfer":

            $sum = 0;

            for ($i = 0; $i < 12; $i++) {

                $sum += rand(0, $randmax) / $randmax;

            }

            return $sum;

     }       

}

relsqui at armory dot com
21-Jan-2005 11:23


Don't forget, it's faster to use bitwise operations when you need a random number that's less than some power of two. For example,



<?php

rand()&1;

// instead of

rand(0,1);

// for generating 0 or 1,



rand()&3;

// instead of

rand(0,3);

// for generating 0, 1, 2, or 3,



rand()&7;

// instead of

rand(0,7)

// for generating 0, 1, 2, 3, 4, 5, 6, or 7,

?>



and so on. All you're doing there is generating a default random number (so PHP doesn't have to parse any arguments) and chopping off the piece that's useful to you (using a bitwise operation which is faster than even basic math).

onno at onnovanbraam dot com
12-Sep-2004 03:58


>>Note: As of PHP 4.2.0, there is no need to seed the random number generator with srand() or mt_srand() as this is now done automatically.<<



Do realize that you don't HAVE to call srand, but if you don't you get the same results every time... Took me quite a while to get to that conclusion, since I couldn't figure it out:



foreach($Unit AS $index => $value) {

    srand() ;

    $rand_pos_x = rand(0, $EVAC['x_max']) ;

    $rand_pos_y = rand(0, $EVAC['y_max']) ;

}



works, but:



foreach($Unit AS $index => $value) {

    $rand_pos_x = rand(0, $EVAC['x_max']) ;

    $rand_pos_y = rand(0, $EVAC['y_max']) ;

}



doesnt even though they say it should.



I'm using Windows XP Pro (Windows NT EVOLUTION 5.1 build 2600), PHP Version 4.3.3, CGI/FastCGI.

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