Swift Programming Beginner：为什么在实现变量时我的循环中有错误？

Question

When I try and run my code in Xcode playground I get a warning:

Variable 'n' was never mutated; consider changing to 'let' constant.

First of all, I am changing the variable in the body of the loop so why is it telling me to change it to a let (constant) data type.

func multiples (n : Int) {
  var n = 1

  for _ in (3 ..< 1000) {

    var n = n + 1

    let multiple3 = 3 * n

    print(multiple3)
  }
}

Answer 1

I am changing the variable in the body of the loop

No, you're not. The one in the body of the loop is a different n .

To fix that, change

var n = n + 1

To

n = n + 1

Answer 2

3 little notes:

a) If you read carefully messages from Xcode, you will understand about vars' lifetime and usage. ( "Variable 'n' was never mutated; consider changing to 'let' constant" )

b) you have two var with same name in different scope

c) the you enter "for", n on the left will be computed using N in outer scope, so inner n will always be == 2

d) using debugger You will see as in picture.

Answer 3

Those are two different variables named n . One is unchanged and one is created for each new iteration of the for loop.
The reason you can have two variables with the same name is that they exist in different scopes and the one inside the for loop temporarily overrides the one outside the loop for the duration of the loop but only inside it.