What is associative array in php

In PHP, an associative array is an array whose key name is a string or a mixture of strings and numbers; if a key name in an array is not a number, then the array is an associative array. The key name of an associative array can be any integer or string; if the key name is a string, a delimiting modifier must be added to the key name: single quotation mark "''" or double quotation mark """".

The operating environment of this tutorial: windows7 system, PHP version 8.1, DELL G3 computer

Each entity in the array contains two items , respectively key and value. The corresponding array elements can be obtained by key value. These keys can be numeric keys or association keys. If a variable is a container that stores a single value, then an array is a container that stores multiple values.

Array can store different types of data and is a composite data type. The data structure is as follows:

In a PHP array, no matter what type of key name there will be a value corresponding to it, that is, a key/value pair, according to Depending on the data type of the array key name, we can divide PHP arrays into two types:

• Arrays with numbers as key names, that is, indexed arrays (Indexed Array);

• An array whose key name is a string or a mixture of strings and numbers, that is, an Associative Array.

PHP associative array

The subscript (key name) of the associative array is a mixture of numerical values ​​and strings Composition, if a key name in an array is not a number, then the array is an associative array.

<?php
header("Content-type:text/html;charset=utf-8");
echo "索引数组：";
$arr1 = array(&#39;0&#39; => &#39;苹果&#39;,&#39;1&#39; => &#39;香蕉&#39;,&#39;2&#39; => &#39;橘子&#39;,&#39;3&#39; => &#39;李子&#39;,&#39;4&#39; => &#39;草莓&#39;);
var_dump($arr1);

echo "关联数组：";
$arr2 = array(&#39;Apple&#39; => &#39;苹果&#39;,&#39;Banana&#39; => &#39;香蕉&#39;,&#39;Orange&#39; => &#39;橘子&#39;,&#39;Plum&#39; => &#39;李子&#39;,&#39;Strawberry&#39; => &#39;草莓&#39;);
var_dump($arr2);
$arr3 = array(&#39;苹果&#39;,&#39;香蕉&#39;,&#39;橘子&#39;,&#39;Plum&#39; => &#39;李子&#39;,&#39;草莓&#39;);
var_dump($arr3);
?>

The key name of an associative array can be any integer or string. If the key name is a string, add a delimiting modifier to the key name - single quotes '' or double quotes "". For indexed arrays, in order to avoid confusion, it is best to add delimiters.

Note: The key name cannot be NULL.

Extended knowledge: Index arrays and associative arrays can be converted to each other

1. Convert associative arrays to index arrays

The array_values() function is to return the values ​​of all elements in the array

array_values(array)

It is very simple to use. With only one required parameter, you can return an array containing all the values ​​​​in the given array. , but key names are not preserved. The returned array will be in the form of an indexed array, with array indices starting at 0 and increasing by 1.

Simply put, you can use this function to reset the array key name and convert the key name with confusing string or numerical value into a numeric key name starting from 0 and increasing by 1.

array_values() function is particularly suitable for arrays with confusing element subscripts (numeric keys can be reset), or for converting associative arrays into indexed arrays.

<?php
header(&#39;content-type:text/html;charset=utf-8&#39;);  
$arr=array("Peter"=>65,"Harry"=>80,"John"=>78,"Clark"=>90,2,3,4);
echo "原数组：";
var_dump($arr);
$res=array_values($arr);
echo "转为索引数组后：";
var_dump($res);
?>

2. Convert index array to associative array

array_combine() function creates an array by merging two arrays A new array, in which the elements in the $keys array serve as the keys of the new array, and the elements in the $values array serve as the keys of the new array.

array_combine($keys,$values)

But it should be noted that when using the array_combine() function to create an array, the number of elements in the $keys array and the $values array must be consistent. Only in this way can the key name and key value correspond one-to-one, otherwise an error will be reported and FALSE will be returned.

And the $keys array cannot be a multi-dimensional array, otherwise an error will be reported; But the $values array can be a multi-dimensional array.

<?php
header("Content-type:text/html;charset=utf-8");
$keys=array("a","b","c","d");
$values=array("red","green","blue","yellow");
var_dump($keys);
var_dump($values);
echo "使用array_combine()合并数组后:";
var_dump(array_combine($keys,$values));
?>

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