[12.x] Add Arr::reduce to reduce arrays to a single value

[moe-mizrak]

Adds Arr::reduce(array $array, callable $callback, mixed $initial = null) a small, non-breaking helper that iteratively reduces an array to a single value (single accumulated result).

Sample usages:

// Count occurrences
$fruits = ['apple','banana','apple'];
$counts = Arr::reduce($fruits, function ($carry, $item) {
    $carry[$item] = ($carry[$item] ?? 0) + 1;
    return $carry;
}, []); 
// ['apple' => 2, 'banana' => 1]

// Preserve and transform keys
$items = ['x' => 10, 'y' => 20];
$double = Arr::reduce($items, fn($carry, $value, $key) => ($carry[$key] = $value * 2) && $carry, []); 
// ['x'=>20,'y'=>40]

// Sum
$total = Arr::reduce([1,2,3,4], fn($carry, $v) => $carry + $v, 0);
// 10

[browner12]

Is there a reason you're not using the built in PHP function array_reduce for this?

https://www.php.net/manual/en/function.array-reduce.php

[shaedrich]

I was wondering about the same thing until it hit me: You natively only have the value, not the key and would have to have a workaround for this:

        return array_reduce(
            array_keys($array),
            fn ($carry, string|int $key) => $callback($carry, $array[$key], $key),
            $initial,
        );

[moe-mizrak]

Is there a reason you're not using the built in PHP function array_reduce for this?

https://www.php.net/manual/en/function.array-reduce.php

As @shaedrich stated, key is missing in array_reduce. let me give some more examples where having key makes sense.

$settings = ['cache' => true, 'debug' => false];

$flipped = Arr::reduce($settings, function ($carry, $val, $key) {
    if ($key === 'debug') {
        $carry[$key] = !$val;
    } else {
        $carry[$key] = $val;
    }
    return $carry;
}, []);
// Result: ['cache' => true, 'debug' => true]

$arr1 = ['x' => 1, 'y' => 2];
$arr2 = ['y' => 3, 'z' => 4];

$merged = Arr::reduce($arr2, function ($carry, $val, $key) {
    $carry[$key] = $carry[$key] ?? $val; // preserve existing keys
    return $carry;
}, $arr1);
// Result: ['x' => 1, 'y' => 2, 'z' => 4]

$items = ['a' => 'apple', 'b' => 'banana', 'c' => 'apple'];

$counts = Arr::reduce($items, function ($carry, $item, $key) {
    $carry[$item][] = $key; // track which keys had this value
    return $carry;
}, []);
// Result: ['apple' => ['a','c'], 'banana' => ['b']]

[taylorotwell]

All of the key examples could be accomplished with mapWithKeys.

[rodrigopedra]

Some of the given examples are best suited for array_map() or array_merge().

The sum one, from the opening message, is an actual array_reduce() that only depends on the value, so using array_reduce() or the added method is irrelevant.

The other examples, where a reduction is actually done, grouping by values or counting value occurrences, can be done by reducing from an array's keys:

<?php

// Count occurrences
$fruits = ['apple','banana','apple'];

$counts = array_reduce(
    array_keys($fruits),
    function ($carry, $key) use ($fruits) {
        $carry[$fruits[$key]] = ($carry[$fruits[$key]] ?? 0) + 1;

        return $carry;
    },
    []
);

var_dump($counts);


// track which keys had this value
$items = ['a' => 'apple', 'b' => 'banana', 'c' => 'apple'];

$groups = array_reduce(
    array_keys($items),
    function ($carry, $key) use ($items) {
        $carry[$items[$key]][] = $key;

        return $carry;
    },
    []
);

var_dump($groups);

Anyway, if a helper that takes a callback which accepts both the key and value as parameters is acceptable, I think it is best to adopt @shaedrich's implementation, which leverages the native array_reduce() function and might perform better than iterating manually over the items.