PHP: Non-Repeating Selection From an Array

Question

In my php script, I have a variable $data that contains an array that may have various number of elements.

The script picks one of the array elements at random and outputs it to the browser:

# count number of elements in $data
$n = count($data);

# pick a random number out of the number of elements in $data
$rand = rand(0, ($n - 1));

# output a random element
echo '<p> . trim($data[$rand]) . '</p>';

QUESTION: I want to improve that script, so that it doesn't output the same array element again until it runs out of array elements. For example, if an array contains elements numbered 0 to 9, and the script picked an array element #4, I want it to remember that, and next time the script runs, to exclude the element that was #4.

It could probably be done in many different ways, but I'm looking for the simplest and most elegant solution, and would be grateful for help from a PHP expert.

nickb · Accepted Answer · 2013-01-21 21:59:39Z

2

Save the numbers that were already picked in the user's session.

session_start();
$n = count($data); 

// If the array isn't initialized, or we used all the numbers, reset the array
if( !isset( $_SESSION['used_nums']) || count( $_SESSION['used_nums']) == $n) {
    $_SESSION['used_nums'] = array();
}

do{ 
    $rand = rand(0, ($n - 1));
} while( isset( $_SESSION['used_nums'][$rand]));

echo '<p>' . trim($data[$rand]) . '</p>';    
$_SESSION['used_nums'][$rand] = 1;

Or, perhaps a more clever way, using array_intersect_key and array_rand:

session_start();
$n = count($data); 

// If the array isn't initialized, or we used all the numbers, reset the array
if( !isset( $_SESSION['used_nums']) || count( $_SESSION['used_nums']) == $n) {
    $_SESSION['used_nums'] = array();
}

$unused = array_intersect_key( $data, $_SESSION['used_nums'];
$rand = array_rand( $unused);

echo '<p>' . trim($unused[$rand]) . '</p>';    
$_SESSION['used_nums'][$rand] = 1;

edited Jan 21, 2013 at 21:59

answered Jan 21, 2013 at 5:24

nickb

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1 Comment

Austin Brunkhorst Over a year ago

For the second example, you would need to echo trim($unused[$rand])

Gumbo · Accepted Answer · 2013-01-21 06:12:26Z

2

You could shuffle the array and then simply iterate over it:

shuffle($data);
foreach ($data as $elem) {
    // …
}

If you don’t want to alter the array order, you could simply shuffle the array’s keys:

$keys = array_keys($data);
shuffle($keys);
foreach ($keys as $key) {
    // $data[$key]
}

answered Jan 21, 2013 at 6:12

Gumbo

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7 Comments

Austin Brunkhorst Over a year ago

This still leaves the possibility of having the same index twice.

Gumbo Over a year ago

@AustinBrunkhorst Why do you think so?

Austin Brunkhorst Over a year ago

Because OP wants a random, unique index selected each page load.

Dimitri Vorontzov Over a year ago

Probably there's a way not only to shuffle the order, but also unset the one that has been chosen? Would that work?

Gumbo Over a year ago

@DimitriVorontzov You could do the same with the shuffled array keys, store it in a more persistent storage, and remove the key from the array when displayed.

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Austin Brunkhorst · Accepted Answer · 2013-01-21 06:43:22Z

1

You can use sessions to store the indexes you've used so far. Try this.

session_start();

$used = &$_SESSION['usedIndexes'];

// used all of our array indexes
if(count($used) > count($data)) 
    $used = array();

// remove the used indexes from data
foreach($used as $index)
    unset($data[$index]);

$random = array_rand($data);

// append our new index to used indexes
$used[] = $random;

echo '<p>', trim($data[$random]) ,'</p>';

edited Jan 21, 2013 at 6:43

answered Jan 21, 2013 at 5:24

Austin Brunkhorst

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Comments

Ricardo Ortega Magaña · Accepted Answer · 2013-01-21 05:22:23Z

0

$selection=$x[$randomNumber];
unset($x[$randomNumber]);

answered Jan 21, 2013 at 5:22

Ricardo Ortega Magaña

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