Reduces the set to a single value using a callback function

# Ds\Set::reduce

(PECL ds >= 1.0.0)

Ds\Set::reduce — Reduces the set to a single value using a callback function

### 说明

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

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

### 参数

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

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

  `value`

    The value of the current iteration.

`initial`

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

### 返回值

The return value of the final callback.

### 示例

示例 #1 Ds\Set::reduce() with initial value example

    <?php
    $set = new \Ds\Set([1, 2, 3]);

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

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

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

以上示例的输出类似于：

    int(30)

示例 #2 Ds\Set::reduce() without an initial value example

    <?php
    $set = new \Ds\Set([1, 2, 3]);

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

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

以上示例的输出类似于：

    int(21)

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