问题描述
我试图了解使用foreach时如何执行操作.例如如何使用foreach
I am trying to understand how to perform an operation when using foreach. For e.g. how can I print element+1 of alist using foreach
scala>alist = List(1,3,5,7)
scala>alist.foreach(println(_+1)) #=> error: missing parameter type...
scala>alist.foreach(println(_).toInt + 1) #=> error: toInt is not a member of Unit
我知道下面有替代方法，但是我正在尝试是否可以使用foreach来完成.
I understand there are alternatives (below) but I am trying if it can be done using foreach.
scala>for(x<-alist) println(x+1) #=> 2,4,6,8
scala>alist.map(x => println(x + 1)) #=> 2,4,6,8
推荐答案
要回答更新的问题，您绝对可以使用foreach:
alist.foreach(x => println(x + 1))
您不能在println中使用占位符语法，因为编译器会将其推断为:
You can't use placeholder syntax inside println since the compler infers it as:
alist.foreach(x => println(x => x + 1))
根据它的扩展法则. println具有类型为Any的参数，因此无法使用plus方法将其绑定到具体类型.
According to it's expansion laws. println takes a parameter of type Any, so it can't bind it to a concrete type with a plus method.
如果您对占位符语法规则感兴趣，请参阅 Scala的隐藏功能
If you're interested in the rules of placeholder syntax, see Hidden features of Scala
由于列表是不可变的，并且foreach的返回类型为Unit，因此无法在Scala中使用foreach更改List[+A].您可以做的是使用map转换从现有列表中投影一个新列表:
You cannot mutate a List[+A] using foreach in Scala, since lists are immutable and foreach return type is Unit. What you can do is project a new list from an existing one using the map transformation:
scala> val list = List(1,2,3,4)
list: List[Int] = List(1, 2, 3, 4)
scala> val plusOneList = list.map(_ + 1)
plusOneList: List[Int] = List(2, 3, 4, 5)
如果查看foreach的签名，则会看到它具有一个函数:A => Unit，该函数采用类型A的元素并向后投射单元.此签名是副作用方法的标志，并且由于list是不可变的，因此对我们没有帮助.
If you look at the signature for foreach, you see that it takes a function: A => Unit, which takes an element of type A and projects back a Unit. This signature is a sign for a side effecting method and since list is immutable, that doesn't help us.
如果您使用了可变列表，例如ListBuffer，则可以对群体使用副作用:
If you used a mutable list, such as ListBuffer, then you could use side effects for population:
scala> val listBuffer = mutable.ListBuffer[Int]()
listBuffer: scala.collection.mutable.ListBuffer[Int] = ListBuffer()
scala> (0 to 4).foreach(x => listBuffer += x)
scala> listBuffer
res10: scala.collection.mutable.ListBuffer[Int] = ListBuffer(0, 1, 2, 3, 4)
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