英文:
go variable scope and shadowing
问题
这是GOPL中的一个例子：“表达式x[i]和x + 'A' - 'a'分别指向外部块中的x的声明；我们稍后会解释这个。”但是解释从未出现。为什么x[i]引用外部作用域中的x？一旦在内部块中重新声明x，它应该遮蔽外部块中的x。为什么这样能行？
package main
import "fmt"
func main() {
x := "hello!"
for i := 0; i < len(x); i++ {
x := x[i]
if x != '!' {
x := x + 'A' - 'a'
fmt.Printf("%c", x)
}
}
}
链接：http://play.golang.org/p/NQxfkTeGzA
英文:
This is an example from GOPL - "The expressions x[i] and x + 'A' - 'a' each refer to a declaration of x from an outer block; we'll explain this in a moment."
The explanation never comes. Why is x[i] referring to the x in the outer scope? As soon as you redeclare x in an inner block it should shadow the x in the outer block. Why does this work?
package main
import "fmt"
func main() {
x := "hello!"
for i := 0; i < len(x); i++ {
x := x[i]
if x != '!' {
x := x + 'A' - 'a'
fmt.Printf("%c", x)
}
}
}
答案1
得分: 4
:= 运算符创建一个新变量，并将右侧的值赋给它。
在 for 循环的第一次迭代中，在步骤 x := x[i] 中，右侧看到的唯一的 x 是在步骤 x := "hello!" 中定义的 x。就右侧而言，x 尚未重新声明。
只有在内部块中重新声明 x 之后...
它还没有。只有在 x := x[i] 之后才重新声明。
在迭代结束时，新的 x 的作用域结束。它不会在新的迭代中被重用。
当发生新的迭代时，一切又重新开始。
英文:
:= operator creates a new variable and assigns the right hand side value to it.
At the first iteration of the for loop, in the step x := x[i], the only x the right hand side sees is the x defined in the step x := "hello!". As far as the right hand side sees x is not redeclared yet.
> As soon as you redeclare x in an inner block..
It is not yet. Its redeclared only after x := x[i].
And at the end of the iteration the new x's scope ends. It is not reused in a new iteration.
When a new iteration happens its the same thing all over again.
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