Scala 中的 Foreach 与 Map [英] Foreach vs Map in Scala

问题描述

我尝试了两种方法来制作列表

i tried two ways for making a list out of something

var response = List[RS_TxnNested]
consumertxnlist.foreach(txData => {
  response = RS_TxnNested(blabla) +: response
})

其中 consumertxnlist 是 Seq[something] .

where consumertxnlist is a Seq[something] .

另一种方式是

var response = consumerTxnList._2.map(txData => RS_TxnNested(blabla))

有人可以帮助我澄清哪个更好以及为什么吗?

Can someone help me in clarifying which one is better and why?

推荐答案

正如您已经注意到的，Seq trait 中的 map 函数返回一个值.它的签名实际上是

As you've noticed already, the map function in the Seq trait returns a value. Its signature in fact is

def map[B](f: (A) ⇒ B): Seq[B]

它旨在将函数应用于扩展 Seq 特征的集合的每个元素并返回一个新集合.

It is designed to apply a function to every element of a collection extending the Seq trait and return a new collection.

另一方面，foreach 具有相同的特征，具有以下签名:

On the other hand, foreach, in the same trait, has the following signature:

def foreach(f: (A) ⇒ 单位): 单位

这意味着它旨在执行具有副作用的函数.

This means that it's designed to execute functions with side-effects.

在您的示例中，您将值分配给响应"的操作视为副作用.如果您的实际目标是返回一个值(就像您想要做的那样)，那么请使用 map.

In your example, you're treating the operation of assigning the value to "response" as a side effect. If you're actual goal is to return a value (like it seems you want to do), then use map.

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