list

说明

void list ( mixed varname, mixed ... )

像 array() 一样，这不是真正的函数，而是语言结构。list() 用一步操作给一组变量进行赋值。

注意: list() 仅能用于数字索引的数组并假定数字索引从 0 开始。

例 1. list() 例子


<?php

$info = array('coffee', 'brown', 'caffeine');

// Listing all the variables
list($drink, $color, $power) = $info;
echo "$drink is $color and $power makes it special.\n";

// Listing some of them
list($drink, , $power) = $info;
echo "$drink has $power.\n";

// Or let's skip to only the third one
list( , , $power) = $info;
echo "I need $power!\n";

?>

例 2. 使用 list() 的例子


<table>
 <tr>
  <th>Employee name</th>
  <th>Salary</th>
 </tr>

<?php

$result = mysql_query("SELECT id, name, salary FROM employees",$conn);
while (list($id, $name, $salary) = mysql_fetch_row($result)) {
    echo " <tr>\n".
         "  <td><a href=\"info.php?id=$id\">$name</a></td>\n".
         "  <td>$salary</td>\n".
         " </tr>\n";
}

?>

</table>

警告

list() 从最右边一个参数开始赋值。如果你用单纯的变量，不用担心这一点。但是如果你用了具有索引的数组，通常你期望得到的结果和在 list() 中写的一样是从左到右的，但实际上不是。是以相反顺序赋值的。

例 3. 在 list() 中使用数组索引


<?php
$info = array('coffee', 'brown', 'caffeine');
list($a[0], $a[1], $a[2]) = $info;
var_dump($a);
?>

产生如下输出（注意单元顺序和 list() 语法中所写的顺序的比较）：

array(3) {
  [2]=>
  string(8) "caffeine"
  [1]=>
  string(5) "brown"
  [0]=>
  string(6) "coffee"
}

参见 each()，array() 和 extract()。

add a note User Contributed Notes

mick at wireframe dot com
08-Aug-2007 07:08


It's worth noting that, as expected, list() does not have to have as many variables (and/or empty skips) as there are elements in the array. PHP will disregard all elements that there are no variables for. So:



<?php

$Array_Letters = array('A', 'B', 'C', 'D', 'E', 'F');



list($Letter_1, $Letter_2) = $Array_Letters;



echo $Letter_1 . $Letter_2;

?>



Will output: AB



Mick

tobylewis at logogriph dot com
08-May-2007 10:55


The list construct assigns elements from a numbered array starting from element zero.  It does not assign elements from associative arrays.  So



$arr = array();

$arr[1] = 'x';

list($a, $b) = $arr;

var_dump($a); //outputs NULL because there is no element [0]

var_dump($b); //outputs 'x'



and 



$arr = array('red'=>'stop','green'=>'go');

list($a, $b) = $arr;

var_dump($a); //outputs NULL

var_dump($b); //outputs NULL



If there are not enough elements in the array for the variables in the list the excess variables are assigned NULL.



If there are more elements in the array than variables in the list, the extra array elements are ignored without error.



Also the warning above about order of assignment is confusing until you get used to php arrays.  The order in which array elements are stored is the order in which elements are assigned to the array.  So even in a numbered array if you assign $may_arr[2] before you assign $my_array[0] then element [2] will be in the array before [0].  This becomes apparent when using commands like, push, shift or foreach which work with the stored order of the elements.  So the warning only applies when the variables in the list are themselves array elements which have not already been assigned to their array.

ergalvan at bitam dot com
04-May-2006 06:29


With regard to the note written by dolan at teamsapient dot com:



You must take note that list() assigns variables starting from the rightmost one (as stated in the warning). That makes $record having the value "value4" and then $var1, $var2 and $var3 take their values from the "new" $record variable.



It's clear that the behavior stated in the warning wasn't followed by version 5.0.4 (and perhaps previous versions?)

dolan at teamsapient dot com
06-Apr-2006 06:08


I noticed w/ version 5.1.2, the behavior of list() has changed (this occurred at some point between version 5.0.4 and 5.1.2).  When re-using a variable name in list() that list() is being assigned to, instead of the values being assigned all at once, the reused variable gets overwritten before all the values are read.



Here's an example:

** disclaimer: obviously this is sloppy code, but I want to point out the behavior change (in case anyone else comes across similar code) **



<?

$data = array();

$data[] = array("value1", "value2", "value3", "value4");

$data[] = array("value1", "value2", "value3", "value4");

$data[] = array("value1", "value2", "value3", "value4");

$data[] = array("value1", "value2", "value3", "value4");



foreach($data as $record)

{

    list($var1, $var2, $var3, $record) = $record;

    echo "var 1: $var1, var 2: $var2, var 3: $var3, record: $record\\n";

}

?>



OUTPUT on version 5.0.4:

var 1: value1, var 2: value2, var 3: value3, record: value4

var 1: value1, var 2: value2, var 3: value3, record: value4

var 1: value1, var 2: value2, var 3: value3, record: value4

var 1: value1, var 2: value2, var 3: value3, record: value4



OUTPUT on version 5.1.2:

var 1: v, var 2: a, var 3: l, record: value4

var 1: v, var 2: a, var 3: l, record: value4

var 1: v, var 2: a, var 3: l, record: value4

var 1: v, var 2: a, var 3: l, record: value4

mzizka at hotmail dot com
03-Jan-2006 04:49


Elements on the left-hand side that don't have a corresponding element on the right-hand side will be set to NULL. For example,



<?php

$y = 0;

list($x, $y) = array("x");

var_dump($x);

var_dump($y);

?>



Results in:



string(1) "x"

NULL

Nearsighted
25-Jul-2005 02:34


list, coupled with while, makes for a handy way to populate arrays.



while (list($repcnt[], $replnk[], $date[]) = mysql_fetch_row($seek0))

{

// insert what you want to do here.

}



PHP will automatically assign numerical values for the array because of the [] signs after the variable.



From here, you can access their row values by array numbers.



eg.



for ($i=0;$i<$rowcount;$i++)

{

echo "The title number $repcnt[$i] was written on $date[$i].";

}

webmaster at miningstocks dot com
01-Jun-2005 06:05


One way to use the list function with non-numerical keys is to use the array_values() function



<?php

$array = array ("value1" => "one", "value2" => "two");

list ($value1, $value2) = array_values($array);

?>

mortoray at ecircle-ag dot com
16-Feb-2005 09:29


There is no way to do reference assignment using the list function, therefore list assignment is will always be a copy assignment (which is of course not always what you want).



By example, and showing the workaround (which is to just not use list):



    function &pass_refs( &$a ) {

        return array( &$a );

    }



    $a = 1;

    list( $b ) = pass_refs( $a ); //*

    $a = 2;

    print( "$b" ); //prints 1



    $ret = pass_refs( $a );

    $b =& $ret[0];

    $a = 3;

    print( "$b" ); //prints 3



*This is where some syntax like the following would be desired:

   list( &$b ) = pass_refs( $a );

or maybe:

   list( $b ) =& pass_refs( $a );

jennevdmeer at zonnet dot nl
21-Oct-2004 03:29


This is a function simulair to that of 'list' it lists an array with the 'key' as variable name and then those variables contain the value of the key in the array.

This is a bit easier then list in my opinion since you dont have to list up all variable names and it just names them as the key.



<?php

 function lista($a) {

  foreach ($a as $k => $v) {

   $s = "global \$".$k;

   eval($s.";");

   $s = "\$".$k ." = \"". $v."\"";

   eval($s.";");

  }

 }

?>

HW
14-Aug-2004 08:08


The list() construct can be used within other list() constructs (so that it can be used to extract the elements of multidimensional arrays):

<?php

$matrix = array(array(1,2),

                array(3,4));



list(list($tl,$tr),list($bl,$br)) = $matrix;



echo "$tl $tr $bl $br";

?>

Outputs "1 2 3 4".

jeronimo at DELETE_THIS dot transartmedia dot com
29-Jan-2004 03:28


If you want to swap values between variables without using an intermediary, try using the list() and array() language constructs. For instance:



<?



// Initial values.

$biggest = 1;

$smallest = 10;



// Instead of using a temporary variable...

$temp = $biggest;

$biggest = $smallest;

$smallest = $temp;



// ...Just swap the values.

list($biggest, $smallest) = array($smallest, $biggest);



?>



This works with any number of variables; you're not limited to just two.

Cheers,

Jeronimo

rubein at earthlink dot net
29-Dec-2000 01:15


Note: If you have an array full of arrays, you can't use list() in conjunction to foreach() when traversing said array, e.g.





$someArray = array(


  array(1, "one"),


  array(2, "two"),


  array(3, "three")


);





foreach($somearray as list($num, $text)) { ... }








This, however will work





foreach($somearray as $subarray) {


  list($num, $text) = $subarray;


  ...


}

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