# reduceLeft

* [集合的化简 reduceLeft 操作](#集合的化简-reduceleft-操作)
  * [函数的定义](#函数的定义)
  * [Demo](#demo)

## 函数的定义

首先以 Seq 来查看一下函数的定义。

```scala
def reduceLeft[B >: A](op: (B, A) ⇒ B): B

    Applies a binary operator to all elements of this traversable or iterator, going left to right.

    Note: will not terminate for infinite-sized collections.

    Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

    B
    the result type of the binary operator.

    op
    the binary operator.

    returns
    the result of inserting op between consecutive elements of this traversable or iterator, going left to right:

        op( op( ... op(x_1, x_2) ..., x_{n-1}), x_n)

    where x1, ..., xn are the elements of this traversable or iterator.
```

1. reduceLeft 接收的函数类型为 `op: (B, A) ⇒ B`
2. 运行规则是，从左边开始执行，将得到的结果返回给第一个参数,然后继续和下一个元素运算，将得到的结果继续返回给第一个参数,继续执行。

## Demo

```scala
object ReduceLeftDemo {
  def main(args: Array[String]): Unit = {

    val list = List(1,2,3,5,6)

    val list1 = list.reduceLeft(sum)
    println(list1)
  }

  def sum(n1:Int,n2:Int):Int ={
    n1 + n2
  }

}
```

```scala
object ReduceLeftDemo1 {
  def main(args: Array[String]): Unit = {

    val list = List(1,2,3,5,6)

    val list1 = list.reduceLeft(_+_)
    println(list1)
  }

}
```

上面两个demo 的运行结果，其实也就是相当于下面的表达式

```scala
((((1 + 2) + 3) +5) +6)
```