如何正确初始化一些变量？

Question

If I use a variable declaration into if, I get an error undefined: completeTime

    if completeTime, err := time.Parse(time.RFC3339Nano, "2016-06-06 18:11:24.617Z"); err != nil {
          return
        }

    fmt.Println(time.Since(completeTime).Seconds())

But if I declaration that, I get an error completeTime declared and not used

    var completeTime time.Time

    if completeTime, err := time.Parse(time.RFC3339Nano, "2016-06-06 18:11:24.617Z"); err != nil {
          return
        }

    fmt.Println(time.Since(completeTime).Seconds())

Why and how to do it right?

Answer 1

You have a scope problem in your code:

if completeTime, err := time.Parse(time.RFC3339Nano, "2016-06-06 18:11:24.617Z"); err != nil {
      return
}
fmt.Println(time.Since(completeTime).Seconds())

In this sample, completeTime is defined only for local scope inside the if statement, which means you cannot use it outside it.

var completeTime time.Time

if completeTime, err := time.Parse(time.RFC3339Nano, "2016-06-06 18:11:24.617Z"); err != nil {
      return
    }

fmt.Println(time.Since(completeTime).Seconds())

This one is a bit more tricky. You define a completeTime var with the correct scope at the beginning. Then, because of the := operator, you define another completeTime inside the if statement, which masks the first one.

Also, this second variable is not used, that's why you get the error. You can fix this problem by simply assigning completeTime and err outside the if:

completeTime, err := time.Parse(time.RFC3339Nano, "2016-06-06 18:11:24.617Z")

if err != nil {
    return
}

fmt.Println(time.Since(completeTime).Seconds())