声明并未使用的变量

I am trying to learn Go, but when trying out a simple for loop, I found it difficult to get it working. This code does not compile if I define the variable a in the main function, it gives an error 'a declared but not used'. I don't understand when a variable must be declared and when it must not be. Thanks.

package main

import "fmt"

func main() {   


    for a:=0;a<4;a++ {      
        fmt.Printf("value of a is %d
",a)
}

The reason, you have the 'not used error', is because the expression a:=0 declares a new variable with the same name in the scope of the loop. If you already have the variable 'a' declared before the loop, change it to for a=0; a<4; a++ (without the colon).

You have two options available

1. Declare the variable explicitly and then use

   var a int
   a = 0
   

2. Declare and assign in one statement without having to specify the type (it is inferred)

   a:=0
   

Note the difference in = and :=. If you use := twice, it counts as a redeclaration. In other words, = is for assignment only, whereas := is for declaration and assignment in a single step.