在 Scala 中附加到 Seq
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Appending to Seq in Scala
提问by ps0604
The following code is supposed to append to a Seq, but it prints an empty list, what's wrong here?
下面的代码应该附加到一个 Seq，但它打印了一个空列表，这里有什么问题？
object AppendToSeq extends App{
val x = Seq[Int]()
x :+ 1
x :+ 2
println(x)
}
回答by rogue-one
the value x created is an immutable Sequence and the method :+defined on the immutable sequence return a new Seq object.
创建的值 x 是一个不可变的序列，并且在不可变序列上:+定义的方法 返回一个新的 Seq 对象。
so your code should have x has a var (a mutable variable) and it should have its value re-assigned after each append (:+) operation as shown below.
所以你的代码应该有 x 有一个 var（一个可变变量），它应该在每次 append ( :+) 操作后重新分配它的值，如下所示。
scala> var x = Seq[Int]()
x: Seq[Int] = List()
scala> x = x :+ 1
x: Seq[Int] = List(1)
scala> x = x :+ 2
x: Seq[Int] = List(1, 2)
scala> x
res2: Seq[Int] = List(1, 2)
回答by jwvh
x :+ 1creates a new Seqby appending 1to the existing Seq, x, but the new Seqisn't saved anywhere, i.e. it isn't assigned to any variable, so it's just thrown away.
x :+ 1Seq通过附加1到现有的Seq,创建一个新的x，但新Seq的没有保存在任何地方，即它没有分配给任何变量，所以它只是被扔掉了。
If you want to modify an existing Seqyou can make the variable a varinstead of a val. Then when you create a new Seqyou can save it under the same name.
如果你想修改一个现有的，Seq你可以使变量 avar而不是 a val。然后当你创建一个新的时，Seq你可以用相同的名称保存它。
scala> var x = Seq[Int]()
x: Seq[Int] = List()
scala> x = x :+ 7
x: Seq[Int] = List(7)
