String Concat In Loop (ARCH5070)

Key go.performance.string-concat-in-loop · Severity warning · Category performance

Strings are immutable in Go; concatenating with += inside a loop allocates and copies a new buffer on every iteration, yielding O(n^2) cost. Use a strings.Builder instead.

Incorrect

func join(items []string) string {
  var result string
  for _, item := range items {
    result += item + ","
  }
  return result
}

Correct

func join(items []string) string {
  var b strings.Builder
  for _, item := range items {
    b.WriteString(item)
    b.WriteByte(',')
  }
  return b.String()
}

Fix

Accumulate into a single strings.Builder instead of using += in the loop.