Golang Delete Slice Item in Range Problem

Problem

When range start, it is not iterate value one by one, it is iterate by index.

If you modified a slice while it is in range, it will induce some problem.

Example

Remove char a from chars.

• Incorrect Way

  chars := []string{"a", "a", "b"}
  
  for i, v := range chars {
      fmt.Printf("%+v, %d, %s\n", chars, i, v)
      if v == "a" {
          chars = append(chars[:i], chars[i+1:]...)
      }
  }
  fmt.Printf("%+v", chars)
  
  

  • Expected

  [a a b], 0, a
  [a b], 0, a
  [b], 0, b
  Result: [b]
  

  • Actual

  // Autual
  [a a b], 0, a
  [a b], 1, b
  [a b], 2, b
  Result: [a b]
  

  Playground

• Correct Way

  chars := []string{"a", "a", "b"}
  
  for i := 0; i < len(chars); i++ {
      if chars[i] == "a" {
          chars = append(chars[:i], chars[i+1:]...)
          i-- // form the remove item index to start iterate next item
      }
  }
  
  fmt.Printf("%+v", chars)
  

  Playground