Large Collection Copy (ARCH5086)

Key go.performance.large-collection-copy · Severity info · Category performance

Appending to a zero-capacity slice when the final size is known causes repeated reallocation and copying. Preallocate capacity with make.

Incorrect

func double(in []int) []int {
  var out []int
  for _, n := range in {
    out = append(out, n*2)
  }
  return out
}

Correct

func double(in []int) []int {
  out := make([]int, 0, len(in))
  for _, n := range in {
    out = append(out, n*2)
  }
  return out
}

Fix

Create the target slice with preallocated capacity via make([]T, 0, len(in)).