声明且未使用-如何在IF语句中更新变量

Question

I am trying to setup a bool variable with a default value and update it based on a condition, in Go Lang. The func foo compiles, But the function bar doesn't compile and gives an error "f declared and not used"

There is a related answer - which doesn't explain the next question

What is the correct pattern for this(bar function) in Go?

Here is the code:

package main

import (
"fmt"
"strconv"
)

func foo(m map[string]string) bool {
    f := false

if _, exists := m["READWRITE"]; exists {
    fmt.Println("Before Updating f : ", f)
    f, _ = strconv.ParseBool(m["READWRITE"])
    //if err != nil {
    //  panic(err)
    //}
}

fmt.Println("After Updating f : ", f)

return f
}

func bar(m map[string]string) bool {
    f := false

    if _, exists := m["READWRITE"]; exists {
         fmt.Println("Before Updating f : ", f)
         f, err := strconv.ParseBool(m["READWRITE"]) // error on this line "f declared and not used"
        if err != nil {
            panic(err)
        }
    }

    fmt.Println("After Updating f : ", f)

    return f
}

func main() {
    m := map[string]string{"READWRITE": "true"}

    fmt.Println(foo(m))
    fmt.Println(bar(m))
}

Answer 1

gives an error "f declared and not used"

f in your sample is a newly-declared variable. Its scope lies within the if block, and is different from the one originally declared at the start of your main().

Here is a sample commented code that may help clarify (also available as a runnable Go Playground snippet):

package main

import (
    "fmt"
)

func main() {
    food := "burger"                 // (1) variable 'food' is declared
    fmt.Println("food is", food)     // OUTPUT: "burger"

    if true {
        food := "fries"              // (2) a new 'food' variable is a declared here. it shadows the one in (1)
        fmt.Println("food is", food) // this accesses the 'food' var from (2). OUTPUT: "fries"
    }

    fmt.Println("food is", food)     // this accesses the variable from (1). OUTPUT: "burger"
}

Hope this helps clarify. Cheers,