Reduces the map to a single value using a callback function

# Ds\Map::reduce

(PECL ds >= 1.0.0)

Ds\Map::reduce — Reduces the map to a single value using a callback function

### 説明

public function Ds\Map::reduce(callable `$callback`, mixed `$initial` = ?): mixed

Reduces the map to a single value using a callback function.

### パラメータ

`callback`
  function callback(mixed `$carry`, mixed `$key`, mixed `$value`): mixed
  `carry`

    The return value of the previous callback, or `initial` if it's the first iteration.

  `key`

    The key of the current iteration.

  `value`

    The value of the current iteration.

`initial`

  The initial value of the carry value. Can be `null`.

### 戻り値

The return value of the final callback.

### 例

例1 Ds\Map::reduce() with initial value example

    <?php
    $map = new \Ds\Map(["a" => 1, "b" => 2, "c" => 3]);

    $callback = function($carry, $key, $value) {
        return $carry * $value;
    };

    var_dump($map->reduce($callback, 5));

    // Iterations:
    //
    // $carry = $initial = 5
    //
    // $carry = $carry * 1 =  5
    // $carry = $carry * 2 = 10
    // $carry = $carry * 3 = 30
    ?>

上の例の出力は、 たとえば以下のようになります。

    int(30)

例2 Ds\Map::reduce() without an initial value example

    <?php
    $map = new \Ds\Map(["a" => 1, "b" => 2, "c" => 3]);

    var_dump($map->reduce(function($carry, $key, $value) {
        return $carry + $value + 5;
    }));

    // Iterations:
    //
    // $carry = $initial = null
    //
    // $carry = $carry + 1 + 5 =  6
    // $carry = $carry + 2 + 5 = 13
    // $carry = $carry + 3 + 5 = 21
    ?>

上の例の出力は、 たとえば以下のようになります。

    int(21)

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