PHP 5.6.0beta1 released

Arrays

An array in PHP is actually an ordered map. A map is a type that associates values to keys. This type is optimized for several different uses; it can be treated as an array, list (vector), hash table (an implementation of a map), dictionary, collection, stack, queue, and probably more. As array values can be other arrays, trees and multidimensional arrays are also possible.

Explanation of those data structures is beyond the scope of this manual, but at least one example is provided for each of them. For more information, look towards the considerable literature that exists about this broad topic.

Syntax

Specifying with array()

An array can be created by the array() language construct. It takes as parameters any number of comma-separated key => value pairs.

array(  key =>  value
     , ...
     )
// key may only be an integer or string
// value may be any value of any type
<?php
$arr 
= array("foo" => "bar"12 => true);

echo 
$arr["foo"]; // bar
echo $arr[12];    // 1
?>

A key may be either an integer or a string. If a key is the standard representation of an integer, it will be interpreted as such (i.e. "8" will be interpreted as 8, while "08" will be interpreted as "08"). Floats in key are truncated to integer. The indexed and associative array types are the same type in PHP, which can both contain integer and string indices.

A value can be any PHP type.

<?php
$arr 
= array("somearray" => array(=> 513 => 9"a" => 42));

echo 
$arr["somearray"][6];    // 5
echo $arr["somearray"][13];   // 9
echo $arr["somearray"]["a"];  // 42
?>

If a key is not specified for a value, the maximum of the integer indices is taken and the new key will be that value plus 1. If a key that already has an assigned value is specified, that value will be overwritten.

<?php
// This array is the same as ...
array(=> 433256"b" => 12);

// ...this array
array(=> 43=> 32=> 56"b" => 12);
?>
Warnung

Before PHP 4.3.0, appending to an array in which the current maximum key was negative would create a new key as described above. Since PHP 4.3.0, the new key will be 0.

Using TRUE as key will evaluate to integer 1 as a key. Using FALSE as key will evaluate to integer 0 as a key. Using NULL as a key will evaluate to the empty string. Using the empty string as a key will create (or overwrite) a key with the empty string and its value; it is not the same as using empty brackets.

Arrays and objects can not be used as keys. Doing so will result in a warning: Illegal offset type.

Creating/modifying with square bracket syntax

An existing array can be modified by explicitly setting values in it.

This is done by assigning values to the array, specifying the key in brackets. The key can also be omitted, resulting in an empty pair of brackets ([]).

$arr[key] = value;
$arr[] = value;
// key may be an integer or string
// value may be any value of any type

If $arr doesn't exist yet, it will be created, so this is also an alternative way to create an array. To change a certain value, assign a new value to that element using its key. To remove a key/value pair, call the unset() function on it.

<?php
$arr 
= array(=> 112 => 2);

$arr[] = 56;    // This is the same as $arr[13] = 56;
                // at this point of the script

$arr["x"] = 42// This adds a new element to
                // the array with key "x"
                
unset($arr[5]); // This removes the element from the array

unset($arr);    // This deletes the whole array
?>

Hinweis:

As mentioned above, if no key is specified, the maximum of the existing integer indices is taken, and the new key will be that maximum value plus 1. If no integer indices exist yet, the key will be 0 (zero). If a key that already has a value is specified, that value will be overwritten.

Note that the maximum integer key used for this need not currently exist in the array. It need only have existed in the array at some time since the last time the array was re-indexed. The following example illustrates:

<?php
// Create a simple array.
$array = array(12345);
print_r($array);

// Now delete every item, but leave the array itself intact:
foreach ($array as $i => $value) {
    unset(
$array[$i]);
}
print_r($array);

// Append an item (note that the new key is 5, instead of 0).
$array[] = 6;
print_r($array);

// Re-index:
$array array_values($array);
$array[] = 7;
print_r($array);
?>

Das oben gezeigte Beispiel erzeugt folgende Ausgabe:

Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => 4
    [4] => 5
)
Array
(
)
Array
(
    [5] => 6
)
Array
(
    [0] => 6
    [1] => 7
)

Useful functions

There are quite a few useful functions for working with arrays. See the array functions section.

Hinweis:

The unset() function allows removing keys from an array. Be aware that the array will not be reindexed. If a true "remove and shift" behavior is desired, the array can be reindexed using the array_values() function.

<?php
$a 
= array(=> 'one'=> 'two'=> 'three');
unset(
$a[2]);
/* will produce an array that would have been defined as
   $a = array(1 => 'one', 3 => 'three');
   and NOT
   $a = array(1 => 'one', 2 =>'three');
*/

$b array_values($a);
// Now $b is array(0 => 'one', 1 =>'three')
?>

The foreach control structure exists specifically for arrays. It provides an easy way to traverse an array.

Array do's and don'ts

Why is $foo[bar] wrong?

Always use quotes around a string literal array index. For example, $foo['bar'] is correct, while $foo[bar] is not. But why? It is common to encounter this kind of syntax in old scripts:

<?php
$foo
[bar] = 'enemy';
echo 
$foo[bar];
// etc
?>

This is wrong, but it works. The reason is that this code has an undefined constant (bar) rather than a string ('bar' - notice the quotes). PHP may in future define constants which, unfortunately for such code, have the same name. It works because PHP automatically converts a bare string (an unquoted string which does not correspond to any known symbol) into a string which contains the bare string. For instance, if there is no defined constant named bar, then PHP will substitute in the string 'bar' and use that.

Hinweis: This does not mean to always quote the key. Do not quote keys which are constants or variables, as this will prevent PHP from interpreting them.

<?php
error_reporting
(E_ALL);
ini_set('display_errors'true);
ini_set('html_errors'false);
// Simple array:
$array = array(12);
$count count($array);
for (
$i 0$i $count$i++) {
    echo 
"\nChecking $i: \n";
    echo 
"Bad: " $array['$i'] . "\n";
    echo 
"Good: " $array[$i] . "\n";
    echo 
"Bad: {$array['$i']}\n";
    echo 
"Good: {$array[$i]}\n";
}
?>

Das oben gezeigte Beispiel erzeugt folgende Ausgabe:

Checking 0: 
Notice: Undefined index:  $i in /path/to/script.html on line 9
Bad: 
Good: 1
Notice: Undefined index:  $i in /path/to/script.html on line 11
Bad: 
Good: 1

Checking 1: 
Notice: Undefined index:  $i in /path/to/script.html on line 9
Bad: 
Good: 2
Notice: Undefined index:  $i in /path/to/script.html on line 11
Bad: 
Good: 2

More examples to demonstrate this behaviour:

<?php
// Show all errors
error_reporting(E_ALL);

$arr = array('fruit' => 'apple''veggie' => 'carrot');

// Correct
print $arr['fruit'];  // apple
print $arr['veggie']; // carrot

// Incorrect.  This works but also throws a PHP error of level E_NOTICE because
// of an undefined constant named fruit
// 
// Notice: Use of undefined constant fruit - assumed 'fruit' in...
print $arr[fruit];    // apple

// This defines a constant to demonstrate what's going on.  The value 'veggie'
// is assigned to a constant named fruit.
define('fruit''veggie');

// Notice the difference now
print $arr['fruit'];  // apple
print $arr[fruit];    // carrot

// The following is okay, as it's inside a string. Constants are not looked for
// within strings, so no E_NOTICE occurs here
print "Hello $arr[fruit]";      // Hello apple

// With one exception: braces surrounding arrays within strings allows constants
// to be interpreted
print "Hello {$arr[fruit]}";    // Hello carrot
print "Hello {$arr['fruit']}";  // Hello apple

// This will not work, and will result in a parse error, such as:
// Parse error: parse error, expecting T_STRING' or T_VARIABLE' or T_NUM_STRING'
// This of course applies to using superglobals in strings as well
print "Hello $arr['fruit']";
print 
"Hello $_GET['foo']";

// Concatenation is another option
print "Hello " $arr['fruit']; // Hello apple
?>

When error_reporting is set to show E_NOTICE level errors (by setting it to E_ALL, for example), such uses will become immediately visible. By default, error_reporting is set not to show notices.

As stated in the syntax section, what's inside the square brackets ('[' and ']') must be an expression. This means that code like this works:

<?php
echo $arr[somefunc($bar)];
?>

This is an example of using a function return value as the array index. PHP also knows about constants:

<?php
$error_descriptions
[E_ERROR]   = "A fatal error has occured";
$error_descriptions[E_WARNING] = "PHP issued a warning";
$error_descriptions[E_NOTICE]  = "This is just an informal notice";
?>

Note that E_ERROR is also a valid identifier, just like bar in the first example. But the last example is in fact the same as writing:

<?php
$error_descriptions
[1] = "A fatal error has occured";
$error_descriptions[2] = "PHP issued a warning";
$error_descriptions[8] = "This is just an informal notice";
?>

because E_ERROR equals 1, etc.

So why is it bad then?

At some point in the future, the PHP team might want to add another constant or keyword, or a constant in other code may interfere. For example, it is already wrong to use the words empty and default this way, since they are reserved keywords.

Hinweis: To reiterate, inside a double-quoted string, it's valid to not surround array indexes with quotes so "$foo[bar]" is valid. See the above examples for details on why as well as the section on variable parsing in strings.

Converting to array

For any of the types: integer, float, string, boolean and resource, converting a value to an array results in an array with a single element with index zero and the value of the scalar which was converted. In other words, (array)$scalarValue is exactly the same as array($scalarValue).

If an object is converted to an array, the result is an array whose elements are the object's properties. The keys are the member variable names, with a few notable exceptions: private variables have the class name prepended to the variable name; protected variables have a '*' prepended to the variable name. These prepended values have null bytes on either side. This can result in some unexpected behaviour:

<?php

class {
    private 
$A// This will become '\0A\0A'
}

class 
extends {
    private 
$A// This will become '\0B\0A'
    
public $AA// This will become 'AA'
}

var_dump((array) new B());
?>

The above will appear to have two keys named 'AA', although one of them is actually named '\0A\0A'.

Converting NULL to an array results in an empty array.

Comparing

It is possible to compare arrays with the array_diff() function and with array operators.

Examples

The array type in PHP is very versatile. Here are some examples:

<?php
// this
$a = array( 'color' => 'red',
            
'taste' => 'sweet',
            
'shape' => 'round',
            
'name'  => 'apple',
                       
4        // key will be 0
          
);

// is completely equivalent with
$a['color'] = 'red';
$a['taste'] = 'sweet';
$a['shape'] = 'round';
$a['name']  = 'apple';
$a[]        = 4;        // key will be 0

$b[] = 'a';
$b[] = 'b';
$b[] = 'c';
// will result in the array array(0 => 'a' , 1 => 'b' , 2 => 'c'),
// or simply array('a', 'b', 'c')
?>

Beispiel #1 Using array()

<?php
// Array as (property-)map
$map = array( 'version'    => 4,
              
'OS'         => 'Linux',
              
'lang'       => 'english',
              
'short_tags' => true
            
);
            
// strictly numerical keys
$array = array( 7,
                
8,
                
0,
                
156,
                -
10
              
);
// this is the same as array(0 => 7, 1 => 8, ...)

$switching = array(         10// key = 0
                    
5    =>  6,
                    
3    =>  7
                    
'a'  =>  4,
                            
11// key = 6 (maximum of integer-indices was 5)
                    
'8'  =>  2// key = 8 (integer!)
                    
'02' => 77// key = '02'
                    
0    => 12  // the value 10 will be overwritten by 12
                  
);
                  
// empty array
$empty = array();         
?>

Beispiel #2 Collection

<?php
$colors 
= array('red''blue''green''yellow');

foreach (
$colors as $color) {
    echo 
"Do you like $color?\n";
}

?>

Das oben gezeigte Beispiel erzeugt folgende Ausgabe:

Do you like red?
Do you like blue?
Do you like green?
Do you like yellow?

Changing the values of the array directly is possible since PHP 5 by passing them by reference. Before that, a workaround is necessary:

Beispiel #3 Collection

<?php
// PHP 5
foreach ($colors as &$color) {
    
$color strtoupper($color);
}
unset(
$color); /* ensure that following writes to
$color will not modify the last array element */

// Workaround for older versions
foreach ($colors as $key => $color) {
    
$colors[$key] = strtoupper($color);
}

print_r($colors);
?>

Das oben gezeigte Beispiel erzeugt folgende Ausgabe:

Array
(
    [0] => RED
    [1] => BLUE
    [2] => GREEN
    [3] => YELLOW
)

This example creates a one-based array.

Beispiel #4 One-based index

<?php
$firstquarter  
= array(=> 'January''February''March');
print_r($firstquarter);
?>

Das oben gezeigte Beispiel erzeugt folgende Ausgabe:

Array 
(
    [1] => 'January'
    [2] => 'February'
    [3] => 'March'
)

Beispiel #5 Filling an array

<?php
// fill an array with all items from a directory
$handle opendir('.');
while (
false !== ($file readdir($handle))) {
    
$files[] = $file;
}
closedir($handle); 
?>

Arrays are ordered. The order can be changed using various sorting functions. See the array functions section for more information. The count() function can be used to count the number of items in an array.

Beispiel #6 Sorting an array

<?php
sort
($files);
print_r($files);
?>

Because the value of an array can be anything, it can also be another array. This enables the creation of recursive and multi-dimensional arrays.

Beispiel #7 Recursive and multi-dimensional arrays

<?php
$fruits 
= array ( "fruits"  => array ( "a" => "orange",
                                       
"b" => "banana",
                                       
"c" => "apple"
                                     
),
                  
"numbers" => array ( 1,
                                       
2,
                                       
3,
                                       
4,
                                       
5,
                                       
6
                                     
),
                  
"holes"   => array (      "first",
                                       
=> "second",
                                            
"third"
                                     
)
                );

// Some examples to address values in the array above 
echo $fruits["holes"][5];    // prints "second"
echo $fruits["fruits"]["a"]; // prints "orange"
unset($fruits["holes"][0]);  // remove "first"

// Create a new multi-dimensional array
$juices["apple"]["green"] = "good"
?>

Array assignment always involves value copying. It also means that the internal array pointer used by current() and similar functions is reset. Use the reference operator to copy an array by reference.

<?php
$arr1 
= array(23);
$arr2 $arr1;
$arr2[] = 4// $arr2 is changed,
             // $arr1 is still array(2, 3)
             
$arr3 = &$arr1;
$arr3[] = 4// now $arr1 and $arr3 are the same
?>
add a note add a note

User Contributed Notes 20 notes

up
61
mlvljr
2 years ago
please note that when arrays are copied, the "reference status" of their members is preserved (http://www.php.net/manual/en/language.references.whatdo.php).
up
36
chris at ocportal dot com
10 months ago
Note that array value buckets are reference-safe, even through serialization.

<?php
$x
='initial';
$test=array('A'=>&$x,'B'=>&$x);
$test=unserialize(serialize($test));
$test['A']='changed';
echo
$test['B']; // Outputs "changed"
?>

This can be useful in some cases, for example saving RAM within complex structures.
up
43
ken underscore yap atsign email dot com
6 years ago
"If you convert a NULL value to an array, you get an empty array."

This turns out to be a useful property. Say you have a search function that returns an array of values on success or NULL if nothing found.

<?php $values = search(...); ?>

Now you want to merge the array with another array. What do we do if $values is NULL? No problem:

<?php $combined = array_merge((array)$values, $other); ?>

Voila.
up
1
brta dot akos at gmail dot com
3 months ago
Why not to user one-based arrays:

<?php
$a 
= array(1 => 'a', 'b', 'd');
print_r($a);
array_splice($a,2,0,'c');
print_r($a);
?>

output:
Array ( [1] => a [2] => b [3] => d ) Array ( [0] => a [1] => b [2] => c [3] => d )
up
37
jeff splat codedread splot com
8 years ago
Beware that if you're using strings as indices in the $_POST array, that periods are transformed into underscores:

<html>
<body>
<?php
    printf
("POST: "); print_r($_POST); printf("<br/>");
?>
<form method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>">
    <input type="hidden" name="Windows3.1" value="Sux">
    <input type="submit" value="Click" />
</form>
</body>
</html>

Once you click on the button, the page displays the following:

POST: Array ( [Windows3_1] => Sux )
up
16
ivegner at yandex dot ru
7 months ago
Note that objects of classes extending ArrayObject SPL class are treated as arrays, and not as objects when converting to array.

<?php
class ArrayObjectExtended extends ArrayObject
{
    private
$private = 'private';
    public
$hello = 'world';
}

$object = new ArrayObjectExtended();
$array = (array) $object;

// This will not expose $private and $hello properties of $object,
// but return an empty array instead.
var_export($array);
?>
up
30
lars-phpcomments at ukmix dot net
9 years ago
Used to creating arrays like this in Perl?

@array = ("All", "A".."Z");

Looks like we need the range() function in PHP:

<?php
$array
= array_merge(array('All'), range('A', 'Z'));
?>

You don't need to array_merge if it's just one range:

<?php
$array
= range('A', 'Z');
?>
up
27
ia [AT] zoznam [DOT] sk
8 years ago
Regarding the previous comment, beware of the fact that reference to the last value of the array remains stored in $value after the foreach:

<?php
foreach ( $arr as $key => &$value )
{
   
$value = 1;
}

// without next line you can get bad results...
//unset( $value );

$value = 159;
?>

Now the last element of $arr has the value of '159'. If we remove the comment in the unset() line, everything works as expected ($arr has all values of '1').

Bad results can also appear in nested foreach loops (the same reason as above).

So either unset $value after each foreach or better use the longer form:

<?php
foreach ( $arr as $key => $value )
{
   
$arr[ $key ] = 1;
}
?>
up
20
caifara aaaat im dooaat be
8 years ago
[Editor's note: You can achieve what you're looking for by referencing $single, rather than copying it by value in your foreach statement. See http://php.net/foreach for more details.]

Don't know if this is known or not, but it did eat some of my time and maybe it won't eat your time now...

I tried to add something to a multidimensional array, but that didn't work at first, look at the code below to see what I mean:

<?php

$a1
= array( "a" => 0, "b" => 1 );
$a2 = array( "aa" => 00, "bb" => 11 );

$together = array( $a1, $a2 );

foreach(
$together as $single ) {
   
$single[ "c" ] = 3 ;
}

print_r( $together );
/* nothing changed result is:
Array
(
    [0] => Array
        (
            [a] => 0
            [b] => 1
        )
    [1] => Array
        (
            [aa] => 0
            [bb] => 11
        )
) */

foreach( $together as $key => $value ) {
   
$together[$key]["c"] = 3 ;
}

print_r( $together );

/* now it works, this prints
Array
(
    [0] => Array
        (
            [a] => 0
            [b] => 1
            [c] => 3
        )
    [1] => Array
        (
            [aa] => 0
            [bb] => 11
            [c] => 3
        )
)
*/

?>
up
0
ffe
2 months ago
<?php

error_reporting
(E_ALL);

$res=42;

echo(
"+".$res[0]."+".$res[10]."+");

$res[0]=1;

?>

The last line correctly states "Warning: Cannot use a scalar value as an array", but the echo produces +++ as output with no warning. This seems strange to me.
up
0
note dot php dot lorriman at spamgourmet dot org
3 months ago
There is another kind of array (php>=  5.3.0) produced by

$array = new SplFixedArray(5);

Standard arrays, as documented here, are marvellously flexible and, due to the underlying hashtable, extremely fast for certain kinds of lookup operation.

Supposing a large string-keyed array

$arr=['string1'=>$data1, 'string2'=>$data2 etc....]

when getting the keyed data with

$data=$arr['string1'];

php does *not* have to search through the array comparing each key string to the given key ('string1') one by one, which could take a long time with a large array. Instead the hashtable means that php takes the given key string and computes from it the memory location of the keyed data, and then instantly retrieves the data. Marvellous! And so quick. And no need to know anything about hashtables as it's all hidden away.

However, there is a lot of overhead in that. It uses lots of memory, as hashtables tend to (also nearly doubling on a 64bit server), and should be significantly slower for integer keyed arrays than old-fashioned (non-hashtable) integer-keyed arrays. For that see more on SplFixedArray :

http://uk3.php.net/SplFixedArray

Unlike a standard php (hashtabled) array, if you lookup by integer then the integer itself denotes the memory location of the data, no hashtable computation on the integer key needed. This is much quicker. It's also quicker to build the array compared to the complex operations needed for hashtables. And it uses a lot less memory as there is no hashtable data structure. This is really an optimisation decision, but in some cases of large integer keyed arrays it may significantly reduce server memory and increase performance (including the avoiding of expensive memory deallocation of hashtable arrays at the exiting of the script).
up
-1
martijntje at martijnotto dot nl
2 years ago
Please note that adding the magic __toString() method to your objects will not allow you to seek an array with it, it still throws an Illegal Offset warning.

The solution is to cast it to a string first, like this

$array[(string) $stringableObject]
up
-4
harry at ddtechnologies dot com
9 months ago
PHP array_diff_assoc() Function

You can compare the keys and values of two arrays, and return the differences:

<?php
 $a1
=array("red","green","blue","yellow");
$a2=array("red","green","blue");

$result=array_diff_assoc($a1,$a2);
print_r($result);
?>

http://www.show-ip.org
up
-7
cromney at pointslope dot com
1 year ago
One thing to be careful of is making any assumptions about the underlying implementation with respect to performance. For example, the documentation talks about hash-maps, which might lead you to expect O(1) key lookups.

<?php

function find_val($n) {
 
$t = array();
 
$last = null;
 
  for (
$x = 0; $x < $n; $x++) {
   
$last = "" . $x;
   
$t[] = $last;
  }
 
 
var_dump(in_array($last, $t));


 
function
find_key($n) {
 
$t = array();
 
$last = null;
 
  for (
$x = 0; $x < $n; $x++) {
   
$last = "" . $x;
   
$t[$last] = true;
  }
 
 
var_dump(array_key_exists($last, $t));


$n = 1600000;

find_val($n);
// Time taken: 1123ms

find_key($n);
// Time taken: 803

/*

Additional Timings:

n        find_val(ms)  find_key(ms)
100000   99             82
200000   169            130
400000   301            217
800000   570            416
1600000  1123           803

*/
?>

In my tests, both in_array and array_key_exists exhibited the same order of growth.
up
-4
Anonymous
7 years ago
This page should include details about how associative arrays are implemened inside PHP; e.g. using hash-maps or b-trees.

This has important implictions on the permance characteristics of associative arrays and how they should be used; e.g. b-tree are slow to insert but handle collisions better than hashmaps.  Hashmaps are faster if there are no collisions, but are slower to retrieve when there are collisions.  These factors have implictions on how associative arrays should be used.
up
-8
Spudley
7 years ago
On array recursion...

Given the following code:

<?php
$myarray
= array('test',123);
$myarray[] = &$myarray;
print_r($myarray);
?>

The print_r() will display *RECURSION* when it gets to the third element of the array.

There doesn't appear to be any other way to scan an array for recursive references, so if you need to check for them, you'll have to use print_r() with its second parameter to capture the output and look for the word *RECURSION*.

It's not an elegant solution, but it's the only one I've found, so I hope it helps someone.
up
-11
Walter Tross
3 years ago
It is true that "array assignment always involves value copying", but the copy is a "lazy copy". This means that the data of the two variables occupy the same memory as long as no array element changes.

E.g., if you have to pass an array to a function that only needs to read it, there is no advantage at all in passing it by reference.
up
-16
gtisza at gmail dot com
1 year ago
Be very careful when using a result as an array. <?php echo $a['foo']['bar']['baz'] ?> will throw an error if $a is an object, and throw a warning if $a is an array but does not have the right keys, but it will silently return true if $a is null or boolean or int, and if $a is a string, it will return its first character. (This is true even with E_STRICT set.) This can be a major gotcha with functions which return null or false if they are unsuccessful.
up
-27
azuleta at eycambiental dot com
6 months ago
I think there's a mistake in the las example:
...'<?php
$arr1
= array(2, 3);
$arr2 = $arr1;
$arr2[] = 4; // $arr2 ha cambiado,
             // $arr1 sigue siendo array(2, 3)
           
$arr3 = &$arr1;
$arr3[] = 4; // ahora $arr1 y $arr3 son iguales
?>'

I think it should be: ...'ahora $arr2 y $arr3 son iguales'...

Thanks.
up
-51
carl at linkleaf dot com
6 years ago
Its worth noting that there does not appear to be any functional limitations on the length or content of string indexes. The string indexes for your arrays can contain any characters, including new line characters, and can be of any length:

<?php

$key
= "XXXXX";
$test = array($key => "test5");

for (
$x = 0; $x < 500; $x++) {
 
$key .= "X";
 
$value = "test" . strlen($key);
 
$test[$key] = $value;
}

echo
"<pre>";
print_r($test);
echo
"</pre>";

?>

Keep in mind that using extremely long array indexes is not a good practice and could cost you lots of extra CPU time. However, if you have to use a long string as an array index you won't have to worry about the length or content.
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