go - Access random rune element of string without using for ... range

Question

I recently asked this question and the answers increased my understanding, but they didn't solve the actual problem I had. So, I will try to ask a similar but different question as follows.

Suppose that I want to access random rune element of a string. One way is:

func RuneElement(str string, idx int) rune {
  var ret rune
  for i, c := range str {
    if i == idx {
      return c
    }
  }
  return ret // out of range -> proper handling is needed
}

What if I want to call such a function a lot of times? I guess what I am looking for is like an operator/function like str[i] (which returns a byte) that return the rune element at i-th position. Why this element can be accessed using for ... range but not through a funtcion like str.At(i) for example?

Answer 1

Waitting for answers