在 Scala 中将空字符串转换为 None

Question

我需要将两个可能为空的地址行连接成一个（两行之间有一个空格），但是None如果两个地址行都是None（此字段将进入一个Option[String]变量），我需要它返回一个。下面的命令让我得到我想要的连接：

Seq(myobj.address1, myobj.address2).flatten.mkString(" ")

但这给了我一个空字符串而不是一个None，以防 address1 和 address2 都是None。

Answer 1

这将单个字符串Option转换为 ，None如果它是null或空修剪的字符串，则将其转换为：

Option(x).collect { case x if x.trim.nonEmpty => x }

Answer 2

假设：

val list1 = List(Some("aaaa"), Some("bbbb"))
val list2 = List(None, None)

使用普通 Scala：

scala> Option(list1).map(_.flatten).filter(_.nonEmpty).map(_.mkString(" "))
res38: Option[String] = Some(aaaa bbbb)

scala> Option(list2).map(_.flatten).filter(_.nonEmpty).map(_.mkString(" "))
res39: Option[String] = None

或者使用 scalaz：

import scalaz._; import Scalaz._
scala> list1.flatten.toNel.map(_.toList.mkString(" "))
res35: Option[String] = Some(aaaa bbbb)

scala> list2.flatten.toNel.map(_.toList.mkString(" "))
res36: Option[String] = None

Answer 3

嗯，在 Scala 中，有一种Option[ T ]类型旨在消除由于空值而导致的各种运行时问题。

所以...这是您使用选项的方式，所以基本上 aOption[ T ]可以具有两种类型的值之一 -Some[ T ]或None

// A nice string
var niceStr = "I am a nice String"

// A nice String option
var noceStrOption: Option[ String ] = Some( niceStr )

// A None option
var noneStrOption: Option[ String ] = None

现在来谈谈你的问题部分：

// lets say both of your myobj.address1 and myobj.address2 were normal Strings... then you would not have needed to flatten them... this would have worked..
var yourString = Seq(myobj.address1, myobj.address2).mkString(" ")

// But since both of them were Option[ String ] you had to flatten the Sequence[ Option[ String ] ] to become a Sequence[ String ]
var yourString = Seq(myobj.address1, myobj.address2).flatten.mkString(" ")

//So... what really happens when you flatten a Sequence[ Option[ String ] ] ?

// Lets say we have Sequence[ Option [ String ] ], like this
var seqOfStringOptions = Seq( Some( "dsf" ), None, Some( "sdf" ) )

print( seqOfStringOptions )
// List( Some(dsf), None, Some(sdf))

//Now... lets flatten it out...
var flatSeqOfStrings = seqOfStringOptions.flatten

print( flatSeqOfStrings )
// List( dsf, sdf )

// So... basically all those Option[ String ] which were None are ignored and only Some[ String ] are converted to Strings.

// So... that means if both address1 and address2 were None... your flattened list would be empty.

// Now what happens when we create a String out of an empty list of Strings...

var emptyStringList: List[ String ] = List()
var stringFromEmptyList = emptyStringList.mkString( " " )
print( stringFromEmptyList ) 
// ""
// So... you get an empty String

// Which means we are sure that yourString will always be a String... though it can be empty (ie - "").

// Now that we are sure that yourString will alwyas be a String, we can use pattern matching to get out Option[ String ] .

// Getting an appropriate Option for yourString
var yourRequiredOption: Option[ String ] = yourString match {
    // In case yourString is "" give None.
    case "" => None
    // If case your string is not "" give Some[ yourString ] 
    case someStringVal => Some( someStringVal )
}