Same name

yourbasic.org/golang

Why doesn’t n change?

func main() {
    n := 0
    if true {
        n := 1
        n++
    }
    fmt.Println(n) // 0
}

Answer

The statement n := 1 declares a new variable which shadows the original n throughout the scope of the if statement.

To reuse n from the outer block, write n = 1 instead.

func main() {
    n := 0
    if true {
        n = 1
        n++
    }
    fmt.Println(n) // 2
}

Detecting shadowed variables

To help detect shadowed variables, you may use the experimental -shadow feature provided by the vet tool. It flags variables that may have been unintentionally shadowed. Passing the original version of the code to vet gives a warning message.

$ go vet -shadow main.go
main.go:4: declaration of "n" shadows declaration at main.go:2

Additionally, the Go compiler detects and disallows some cases of shadowing.

func Foo() (n int, err error) {
    if true {
        err := fmt.Errorf("Invalid")
        return
    }
    return
}
../main.go:4:3: err is shadowed during return

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