在Golang中，为什么我可以在for循环中多次重定义相同的变量，但是不能在循环之外？

Question

I have the following program.

package main

import (
    "fmt"
)

func main() {
    for i := 0; i < 2; i++ {
        x := 77
        fmt.Println(x)
    }
}

When executed I got:

77
77

As we can see, the x := 77 has been execute 2 times. However, If I modify it slightly like this:

package main

import (
    "fmt"
)

func main() {
    a := 77
    fmt.Println(a)
    a := 77
    fmt.Println(a)
}

I will get the error "no new variables on left side of :=". Why this? Can anyone help to explain it please? Thanks in advance.

Answer 1

There are a couple of things here. First let's address the second half of your question.

The default way to declare a variable is using the var keyword and then assign to it with the = operator.

var a int
a = 77

Go allows us a shortcut := that both declares a variable and assigns a value

a := 77

In your example when you use := a second time you're trying to redeclare a new variable named a in the same scope which is not allowed. The error no new variables on left side of := is trying to tell you this.

But now to your original question, why can you do this multiple times inside a for loop?

The reason is each time you enter a block of curly braces {} you're creating a new nested scope. When you declare the variable x at the top of the loop it is a new variable and it goes out of scope at the end of the loop. When the program comes back around to the top of the loop again it's another new scope.

For example look at this code

{
    x := 77
    fmt.Println(x)
}
fmt.Println(x) // Compile error

That second Println fails because x does not exist in that scope.