Scala 单位类型
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Scala Unit type
提问by Don Ch
I use opencsv to parse csv files, and my code is
我使用opencsv来解析csv文件，我的代码是
while( (line = reader.readNext()) != null ) { .... }
I got a compiler warning saying:
我收到一个编译器警告说：
comparing values of types Unit and Null using `!=' will always yield true
[warn] while( (aLine = reader.readNext()) != null ) {
How should I do the while loop?
我应该怎么做while循环？
采纳答案by Vasil Remeniuk
In your case (line = reader.readNext()) is a functional literal that returns type Unit. You may rewrite the code as follows:
在您的情况下 (line = reader.readNext()) 是一个返回类型 Unit 的函数式文字。您可以按如下方式重写代码：
while( {line = reader.readNext(); line!= null} ) { .... }
回答by mkneissl
An assignment expression has type Unitin Scala. That is the reason for your compiler warning.
赋值表达式Unit在 Scala 中具有类型。这就是编译器警告的原因。
There is a nice idiom in Scala that avoids the while loop:
Scala 中有一个很好的习惯用法可以避免 while 循环：
val iterator = Iterator.continually(reader.readNext()).takeWhile(_ != null)
This gives you an iterator over whatever reader.readNextreturns.
这为您提供了任何reader.readNext返回的迭代器。
The continuallymethod returns an "infinite" iterator and takeWhiletakes the prefix of that, up to but not including the first null.
该continually方法返回一个“无限”迭代器并takeWhile采用它的前缀，直到但不包括第一个空值。
(Scala 2.8)
（斯卡拉 2.8）
回答by oxbow_lakes
You can use a Streamto get what you want:
你可以使用 aStream来获得你想要的：
Stream.continually(reader.readLine()).takeWhile( _ ne null) foreach { line =>
//do Stuff
}
This has the added advantage of other cool stuff as well:
这还有其他很酷的东西的额外优势：
Stream.continually(reader.readLine()).takeWhile( _ ne null) match {
case head #:: tail => //perhaps you need to do stuff with the first element?
case _ => //empty
}
EDIT- thanks to mkneissl for pointing out I should have included this warning:
编辑-感谢 mkneissl 指出我应该包含这个警告：
scala> Stream.continually(1).take(100000000).foreach(x=>())
scala> val s = Stream.continually(1).take(100000000)
s: scala.collection.immutable.Stream[Int] = Stream(1, ?)
scala> s.foreach(x=>()) java.lang.OutOfMemoryError: Java heap space
回答by Jesper
You are writing Scala code they way you would write it in Java. Try doing it in a more Scala-like way. To read a text file line by line and do something with each line, try this:
您正在编写 Scala 代码，就像用 Java 编写的那样。尝试以更像 Scala 的方式来做。要逐行读取文本文件并对每一行执行某些操作，请尝试以下操作：
import java.io.File
import scala.io.Source
Source.fromFile(new File("myfile.txt")).getLines.foreach { line =>
// Do something with the line, for example print it
println(line)
}
回答by Don Mackenzie
Assignment in Scala doesn't return a value, Unit is similar to void in C or C++.
Scala 中的赋值不返回值，Unit 类似于 C 或 C++ 中的 void。
try
尝试
var line = ""
while ({line = reader.readNext(); line != null}) { ... }
This works because the value of the last expression in a block is returned and in this case it is a Boolean which is required by the while
这是有效的，因为返回了块中最后一个表达式的值，在这种情况下，它是while所需的布尔值
