array_reduce()

(PHP 4 >= 4.0.5, PHP 5, PHP 7)

用回调函数迭代地将数组简化为单一的值

说明

array_reduce()将回调函数$callback迭代地作用到$array数组中的每一个单元中，从而将数组简化为单一的值。

参数

输入的 array。

携带上次迭代里的值；如果本次迭代是第一次，那么这个值是$initial。

携带了本次迭代的值。

如果指定了可选参数$initial，该参数将在处理开始前使用，或者当处理结束，数组为空时的最后一个结果。

返回值

返回结果值。

$initial参数，array_reduce()返回NULL。

更新日志

范例

Example #1array_reduce()例子

参见

• array_filter() 用回调函数过滤数组中的单元
• array_map() 为数组的每个元素应用回调函数
• array_unique() 移除数组中重复的值
• array_count_values() 统计数组中所有的值

To make it clearer about what the two parameters of the callback are for, and what "reduce to a single value" actually means (using associative and commutative operators as examples may obscure this).
The first parameter to the callback is an accumulator where the result-in-progress is effectively assembled. If you supply an $initial value the accumulator starts out with that value, otherwise it starts out null.
The second parameter is where each value of the array is passed during each step of the reduction.
The return value of the callback becomes the new value of the accumulator. When the array is exhausted, array_reduce() returns accumulated value.
If you carried out the reduction by hand, you'd get something like the following lines, every one of which therefore producing the same result:

If you made function f($v,$w){return "f($v,$w)";} the last line would be the literal result.
A PHP implementation might therefore look something like this (less details like error checking and so on): 

So, if you were wondering how to use this where key and value are passed in to the function. I've had success with the following (this example generates formatted html attributes from an associative array of attribute => value pairs):

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