如何访问array_reduce中的当前索引？

Question

Having read this SO post, In PHP I am aware that you can get the index under iteration with array_map as such:

array_map(function($item, $index) { ... }, $items, array_keys($items));

How can I get the an $index available to me when I use array_reduce? I have tried:

array_reduce($items, array_keys($items), function($acc, $item, $index) { ... }, array());

array_reduce($items, function($acc, $item, $index) { ... }, array(), array_keys($items));

But I still can't seem to get $index in an array_reduce. Has anyone successfully done this before?

EDIT

Here's some context as to why I am asking this question.

I do not want to use foreach because I would have to mutate an array outside of the foreach in order to create my collection. I would prefer to avoid mutation.

Other languages allow one to use reduce and get access to the current index like in JavaScript and Ruby. I was hoping to get the same feature in PHP. Oh well! Looks like I'm going to have to use a foreach to create my array while also having the current index under iteration.

Answer 1

What you're aware about the getting index in array_reduce doesn't exist. Check documentation.

The question is why?

Look, reduce is being used for the purpose of getting down to one single value, it's aggregation. You have initial value, and next value, show your reduce strategy. There is no point in knowing current item index.

In some other languages, they might provide current index, but not for this array_reduce in PHP. It's language and function designed, there are some constraints in implementation.

But it doesn't mean you can't have it. You can have it, but not through callback!

$i = 0;
$data = [1, 2, 3, 4];

// (1) 
$result = array_reduce($data, function ($c, $i) { return $c + $i; }, 0);

// (2)
array_reduce($data, function ($c, $i) { echo "$i
"; $i++; return $c + $i; }, 0);

The thing is, the function is designed to do this. Or you're trying to use a wrong function. Some other functions might be what you're looking for.

Or you can write a function that do it, too :)