variable declared and not used in a for loop

Question

This one has come up a couple times for Go here, but I think my experience is unique. Here are my codes.

type Stack []Weight

func newStack( size int, startSpread Spread ) Stack {
  stack := make(Stack, size)

  for _, curWeight := range stack {
    curWeight = Weight{ startSpread, rand.Float64( ), rand.Float64( ) }
  }

  return stack
}

Why is gc telling me I'm not using curWeight ?

Answer 1

Please note that the range construct ( for _, curWeight:= range stack ) does copy the elements, one after another. So, you are just copying a value, and then you do not use the copy for any further computations, printing or returning. You just drop the copy again.

So I guess your initial idea was to add the weight to the stack and return it. Let`s do that:

func newStack(size int, startSpread Spread) Stack {
    stack := make(Stack, size)

    for i := 0; i < size; i++ {
        stack[i] = Weight{startSpread, rand.Float64(), rand.Float64()}
    }

    return stack
}

Answer 2

You're assigning to curWeight twice, but you're not using the value in either place.

Go insists that if you assign a value to a variable, then you have to read that value back at some potential point in your program. If you're not going to read it, then assign to _ instead.