Repeated Work In Loop (ARCH5072)

Key go.performance.repeated-work-in-loop · Severity info · Category performance

A computation whose inputs do not depend on the loop variable re-runs on every iteration. Hoist loop-invariant expressions out of the loop.

Incorrect

func match(items []string, prefix string) []string {
  var out []string
  for _, item := range items {
    if strings.HasPrefix(item, strings.ToLower(prefix)) {
      out = append(out, item)
    }
  }
  return out
}

Correct

func match(items []string, prefix string) []string {
  lower := strings.ToLower(prefix)
  var out []string
  for _, item := range items {
    if strings.HasPrefix(item, lower) {
      out = append(out, item)
    }
  }
  return out
}

Fix

Move computations whose inputs don’t depend on the loop variable before the loop.