Looks like this function has a strange randomness.
If you take any number of elements in an array which has 40..100 elements, the 31st one is always by far the less occuring (by about 10% less than others).
I tried this piece of code at home (PHP Version 5.3.2-1ubuntu4.9) and on my server (PHP Version 5.2.17), unfortunately i haven't any server with the last version here :
<?php
$valeurs = range(1, 40);
$proba = array_fill(1, 40, 0);
for ($i = 0; $i < 10000; ++$i)
{
$tirage_tab = array_rand($valeurs, 10);
foreach($tirage_tab as $key => $value)
{
$proba[$valeurs[$value]]++;
}
}
asort($proba);
echo "Proba : <br/>\n";
foreach($proba as $key => $value)
{
echo "$key : $value<br/>\n";
}
?>
In every try, the number of occurrences change a bit but the 31 is always far less (around 2200) than the others (2400-2600). I tried with 50 and 100 elements, no change. I tried with more or less elements to pick (second parameter to array_rand), same result. If you pick only one element it's even worse : 31 has half the result of the others.
For this particular case, i recommend shuffling the array and taking the nth first elements, in this test it's 60% faster and the statistics are ok.
array_rand
(PHP 4, PHP 5)
array_rand — Retorna um ou mais elementos aleatórios de um array
Descrição
array_rand() é bastante útil quando se quer conseguir aleatoriamente um ou mais elementos de um array.
Parâmetros
-
input -
O array de entrada.
-
num_req -
Especifica quantos elementos deseja obter - se não especificado, o padrão é 1.
Valor Retornado
Se estiver pegando apenas um elemento, array_rand() retorna a chave para este elemento aleatório. De outra forma, ele retorna um array com as chaves desses elementos aleatórios. Assim é possível conseguir chaves e valores aleatórios a partir da mesma função.
Exemplos
Exemplo #1 Exemplo de array_rand()
<?php
srand((float) microtime() * 10000000);
$input = array("Neo", "Morpheus", "Trinity", "Cypher", "Tank");
$rand_keys = array_rand($input, 2);
echo $input[$rand_keys[0]] . "\n";
echo $input[$rand_keys[1]] . "\n";
?>
Notas
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.
add a note
User Contributed Notes 25 notes
Sebmil ¶
2 years ago
qeremy ¶
2 years ago
An example for getting random value from arrays;
<?php
function array_random($arr, $num = 1) {
shuffle($arr);
$r = array();
for ($i = 0; $i < $num; $i++) {
$r[] = $arr[$i];
}
return $num == 1 ? $r[0] : $r;
}
$a = array("apple", "banana", "cherry");
print_r(array_random($a));
print_r(array_random($a, 2));
?>
cherry
Array
(
[0] => banana
[1] => apple
)
And example for getting random value from assoc arrays;
<?php
function array_random_assoc($arr, $num = 1) {
$keys = array_keys($arr);
shuffle($keys);
$r = array();
for ($i = 0; $i < $num; $i++) {
$r[$keys[$i]] = $arr[$keys[$i]];
}
return $r;
}
$a = array("a" => "apple", "b" => "banana", "c" => "cherry");
print_r(array_random_assoc($a));
print_r(array_random_assoc($a, 2));
?>
Array
(
[c] => cherry
)
Array
(
[a] => apple
[b] => banana
)
uvm at sun dot he dot net ¶
12 years ago
If you're just trying to draw a random subset of n elements from an array, it seems more effecient to do something like this:
<?php
function draw_rand_array($array,$draws)
{
$lastIndex = count($array) - 1;
$returnArr = array();
while($draws > 1)
{
$rndIndex = rand(0,$lastIndex);
array_push($returnArr,array_splice($array,$rndIndex,1));
$draws--;
$lastIndex--;
}
return $returnArr;
}
?>
No messing with indexes when you're done... you just have an array with the elements you're looking for in it.
herodesh [-at_] gmail [-dot-] com ¶
6 years ago
this is to generate a random selection from an array with array_rand preety nice, can be used to generate random passwords or anything:
$my_array = array("a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "0", "1", "2", "3", "4", "5");
for ($i=0; $i<=10; $i++)
{
$random = array_rand($my_array);
//this generates the random number from the array
$parola .= $my_array[$random];
//here we will display the exact charachter from the array
}
echo $parola; // printing result
Anonymous ¶
1 year ago
It doesn't explicitly say it in the documentation, but PHP won't pick the same key twice in one call.
dragonfly at dragonflyeye dot net ¶
7 years ago
Well, this is interesting. I don't see anyone else commenting on this, so just in case you were planning to use this function like I was, be prepared: array_rand does not handle multidimensional arrays. It just ends up returning a list of the X-axis values without the Y-axis arrays. Bummer. I'm going to have to find another way to do what I wanted.
Jordan Doyle ¶
10 months ago
Note: array_rand uses the libc generator, which is slower and less-random than Mersenne Twister.
<?php
$a = ['http://php.net/', 'http://google.com/', 'http://bbc.co.uk/'];
$website = $a[mt_rand(0, count($a) - 1)];
?>
This is a better alternative.
jpinedo ¶
10 years ago
An array of arrays example:
<?php
$banners[0]['imagen']="imagen0.gif";
$banners[0]['url']="www.nosenada.tal";
$banners[1]['imagen']="imagen1.gif";
$banners[1]['url']="www.nose.tal";
$banners[2]['imagen']="imagen2.gif";
$banners[2]['url']="pagina.html";
$banners[3]['imagen']="imagen3.jpg";
$banners[3]['url']="../pagina.php";
$id_banner = array_rand($banners);
echo "Archivo:--".$banners[$id_banner]['imagen']. "<br />\n";
echo "URL:-----".$banners[$id_banner]['url']. "<br />\n";
?>
yhoko at yhoko dot com ¶
9 years ago
According to office at at universalmetropolis dot com I have to say that the example is wrong.
<?php
// retrieve one of the options at random from the array
$teamcolours = $teamcolours[rand(0,count($teamcolours))];
?>
The count() function will return the number of items in the array, that's the last index + 1. So if there's 2 items in the array, count() will return 2 but the indices are 0 and 1. Now since rand(x,y) randomizes between x and y inclusively the index from the above example may be out of bounds. Thus you have to subtract 1 from the count:
<?php
// Get random item
$teamcolours = $teamcolours[rand(0,count($teamcolours)-1)];
?>
Anonymous ¶
4 years ago
If the array elements are unique, and are all integers or strings, here is a simple way to pick $n random *values* (not keys) from an array $array:
<?php array_rand(array_flip($array), $n); ?>
nospamplease at mail dot com ¶
8 months ago
Random choice with a closure.
$randomChoice = function($array) {return $array[array_rand($array)];};
$names = ['Dexter', 'Esther', 'David', 'Richard', 'Rachel', 'Belinda'];
echo $randomChoice($names);
info at pavliga dot com ¶
5 years ago
If you want get unique range:
<?php
$n = 15;
$data = range(1, 20);
$rand = array_rand($data,$n);
for($i=0; $i<$n; $i++)
{
echo $rand[$i]."<br>";
}
?>
admin at jeremyzaretski dot com ¶
5 years ago
<?php
// a foreach friendly version of array_rand
function Select_Random_Indices($source_array, $count = 1)
{
if($count > 0)
{
if($count == 1)
{
$result = array(array_rand($source_array, $count));
}
else
{
$result = array_rand($source_array, $count);
}
}
else
{
$result = array();
}
return $result;
}
// using the above function to pick random values instead of entries
function Select_Random_Entries($source_array, $count = 1)
{
$result = array();
$index_array = Select_Random_Indices($source_array, $count);
foreach($index_array as $index)
{
$result[$index] = $source_array[$index];
}
return $result;
}
?>
Frederick Lemasson aka djassper at gmail ¶
7 years ago
To select a random Value (not a Key) from a Multi-Dimentionnal array I made a recursive function : array_multi_rand()
the following exemple randomly selects an url from a multidimentionnal array :
<?
$Expos['Google']['Science']='news.google.fr/news?topic=t';
$Expos['Google']['Economie']='news.google.fr/news?topic=b';
$Expos['Google']['Sante']='news.google.fr/news?topic=m';
$Expos['Yahoo']='fr.news.yahoo.com';
$Expos['Events']['LogicielLibre']='agendadulibre.org';
$Expos['MyBlog']='www.kik-it.com';
function array_multi_rand($Zoo){
$Boo=array_rand($Zoo);
if(is_array($Zoo[$Boo])){
return array_multi_rand($Zoo[$Boo]);
}else{
return $Zoo[$Boo];
}
}
echo(array_multi_rand($Expos));
?>
Will-ster ¶
8 years ago
This is something I have been playing with for quite awhile. I'm very new to php, but i finally got it to work. it's a function that will take and array[$arrquo] and find a particular keyword[$find] in the different elements of the array then take those elements that posess that keyword and display them at random
<?php
function popbyword($arrquo,$find)
{
$newarr = array('');
foreach($arrquo as $line)
{
if( strstr( $line, $find ) )
{
array_push($newarr, $line);
}
}
srand((double)microtime()*1000000);
$rquote = array_rand($newarr);
echo $newarr[$rquote];
}
popbyword($images, 'Albert');
?>
In my case I had this huge array of quotes with 90 some elements. I was able to find certain keywords in those elements then ONLY display the elements that had those keywords. NEAT! Maybe only because I'm new.
alexkropivko at(dog) yandex dot ru ¶
8 years ago
There was a mistake at "Paul Hodel (paul at ue dot com dot br) 17-Apr-2003 04:40":
String
echo $new_input = $input[$v];
have to be:
echo $new_input[] = $input[$v];
maxnamara at yahoo dot com ¶
9 years ago
<?php
$input = array("Neo", "Morpheus", "Trinity", "Cypher", "Tank");
function my_array_rand($input,$i=2){
srand((float) microtime() * 10000000);
$rand_keys = array_rand($input, $i);
/*
print $input[$rand_keys[0]] . "\n";
print $input[$rand_keys[1]] . "\n";
*/
$res = array();
if($i > 1){
for($a=0;$a<$i;$a++){
$res[] = $input[$rand_keys[$a]];
}
}
else{
$res[] = $input[$rand_keys];
}
return $res;
}
$a = my_array_rand($input,3);
echo "<pre>";
print_r($a);
echo "</pre>";
?>
asarnoNOSPAM@interbaun DOT com ¶
10 years ago
It is correct that using array_rand() with num_req=1 will return an integer and not an array, but why get so complicated with getting just the one value. The K.I.S.S. method would suggest to do it this way:
<?
srand((double)microtime() * 10000000);
$originalArray = array("red", "blue", "green", "brown",
"cyan", "magenta", "purle", "cheezy");
$pickOne = array_rand($originalArray, 1);
$aRandomSelection = $originalArray[$pickOne ];
echo "$aRandomSelection was the random selection made";
?>
You only need to use the foreach if the num_req >=2. In those cases the array_rand() function will return an array of random elements which are a subset of the original array. When num_req = 1, the array_rand() function returns an integer that signifies a randomly picked key of the original array. Hope this clarifies things ... it works for me.
Paul Hodel (paul at ue dot com dot br) ¶
11 years ago
If you trying to get a randon array just use that... it's easier! And you have no repeats...
<?
srand ((float) microtime() * 10000000);
$input = array ("Neo", "Morpheus", "Trinity", "Cypher", "Tank");
$keys = array_rand ($input, sizeof($input));
while (list($k, $v) = each($keys))
{
echo $new_input = $input[$v];
}
?>
scandar at home dot se ¶
11 years ago
Note that the int num_req parameter is the required number of element to randomly select. So if your array has 3 element and num_req=4 then array_rand() will not return anything since it is impossible to select 4 random elements out of an array that only contains 3 elements. Many people think that they will get 3 elements returned but that is of course not the case.
mickoz[at]parodius[dot]com ¶
11 years ago
For those of you thinking that it does not work for num_req = 1, it is because it return a variable and not an array. This mainly cause some problem with people using foreach.
The correct way to handle this is explained by that example:
<?php
$some_array = array("blah","bleh","foo","lele");
$nb_value = 1;
srand ((float) microtime() * 10000000);
$rand_keys = array_rand($some_array, $nb_value);
if(!is_array($rand_keys))
{
$rand_keys = array($rand_keys);
}
print_r($rand_keys); // verify here the array of keys
echo "\n<BR>";
?>
// You can then correctly use the foreach, as it require an array to work
// If you use foreach with one element, it won't work.
<?php
$random_array = array();
foreach($rand_keys as $value)
{
array_push($random_array, $some_array[$value]);
}
print_r($random_array);
?>
josh at 3io dot com ¶
11 years ago
I modified fake_array_rand to always only return 1 element, and did some benchmarks against calling array_rand with the second parameter as 1. I ran 100 samples for each function for each number of elements and took the average result. While the internal array_rand is faster for a small number of elements, it scales very poorly.
1 elements: 2.0619630813599E-05 sec. for array_rand,8.4352493286133E-05 sec. for fake_array_rand
10 elements: 2.1675825119019E-05 sec. for array_rand,8.427619934082E-05 sec. for fake_array_rand
100 elements: 2.9319524765015E-05 sec. for array_rand,8.4599256515503E-05 sec. for fake_array_rand
1000 elements: 0.0001157283782959 sec. for array_rand,8.5572004318237E-05 sec. for fake_array_rand
10000 elements: 0.0016669762134552 sec. for array_rand,8.5201263427734E-05 sec. for fake_array_rand
100000 elements: 0.015599734783173 sec. for array_rand,8.5580348968506E-05 sec. for fake_array_rand
1000000 elements: 0.18011983394623 sec. for array_rand,8.6690187454224E-05 sec. for fake_array_rand
<?php
function fake_array_rand ($array)
{
$count = count ($array);
# Help keep the number generator random :)
$randval and usleep ("0.$randval");
# Seed the random number generator
# Generate a random number
srand ((double) microtime() * 10000000);
$randval = rand();
# Use the random value to 'pick' an entry from the array
# Count the number of times that the entry is picked
++$index[$randval % $count];
return $array[$randval % $count];
}
?>
JS ¶
7 years ago
I wanted to write something that picks a random entry from a 1column-MySQL database - simply Post Of The Moment (potm). I know there surly are many better ways to do it, but I`m rather new to PHP :) Anyway, it`s simple and no-problem working code.
Of course I assume your DB exists and you always have something in it.
@$link = MySQL_Connect("localhost", "username", "password"); //connect to mysql
mySQL_Select_DB("database"); //..to DB
@$potms = MySQL_Query("SELECT * FROM potm"); //now we get all from our table and store it
MySQL_Close($link); //there`s no need for connection, so we should close it
$potm_array = ''; //sets variables to "zero" values
$i = 0;
while ($entry = MySQL_Fetch_Array($potms)) //now we go through our DB
{
$potm_array[$i] = $entry; //our temporary array from which we will random pick a field key
$i++; //now we increment our field key
}
$potm_id = array_rand($potw_array); //picks a random key from array
$potm = $potm_array[$potm_id]['name_of_the_field']; //now we have stored our Post Of The Moment in $potm
..hope this helps
bjcffnet at gmail dot com ¶
8 years ago
As wazaawazaa600 at msn dot com pointed out, a multi-dimensional array doesn't work with this function. So, I hope I can help someone with this :)
<?php
/**
* Returns a number of random elements from an array.
*
* It returns the number (specified in $limit) of elements from
* $array. The elements are returned in a random order, exactly
* as it was passed to the function. (So, it's safe for multi-
* dimensional arrays, aswell as array's where you need to keep
* the keys)
*
* @author Brendan Caffrey <bjcffnet at gmail dot com>
* @param array $array The array to return the elements from
* @param int $limit The number of elements to return from
* the array
* @return array The randomized array
*/
function array_rand_keys($array, $limit = 1) {
$count = @count($array)-1;
// Sanity checks
if ($limit == 0 || !is_array($array) || $limit > $count) return array();
if ($count == 1) return $array;
// Loop through and get the random numbers
for ($x = 0; $x < $limit; $x++) {
$rand = rand(0, $count);
// Can't have double randoms, right?
while (isset($rands[$rand])) $rand = rand(0, $count);
$rands[$rand] = $rand;
}
$return = array();
$curr = current($rands);
// I think it's better to return the elements in a random
// order, which is why I'm not just using a foreach loop to
// loop through the random numbers
while (count($return) != $limit) {
$cur = 0;
foreach ($array as $key => $val) {
if ($cur == $curr) {
$return[$key] = $val;
// Next...
$curr = next($rands);
continue 2;
} else {
$cur++;
}
}
}
return $return;
}
?>
trukin at gmail dot com ¶
8 years ago
Modify of last note:
<?php
if (!function_exists('array_rand')) {
function array_rand($array, $lim=1) {
mt_srand((double) microtime() * 1000000);
for($a=0; $a<=$lim; $a++){
$num[] = mt_srand(0, count($array)-1);
}
return @$num;
}
}
?>
mt_rand generates a better random number, and with the limit.
