Golang Slice

there are three way to declare a slice in golang.

define slice

// use slice literal
var slice []int
// use make without cap
var slice = make([]int, 10) // len == cap = 10
// use make with cap
var slice = make([]int, 0, 10) // len=0. cap = 10
// The append preformence compare
// slice literal
func AppendToNOMakeSlice(count int32) {
    var slice []int32
    var idx int32 = 0
    for ; idx < count; idx++ {
        slice = append(slice, idx)
    }
}

Bench mark

const numOfItems int32 = 1000000
func BenchmarkAppendToNOMakeSlice(b *testing.B) {
    for i := 0; i < b.N; i++ {
        AppendToNOMakeSlice(numOfItems)
    }
}
// bench result

/*goos: darwin
goarch: amd64
BenchmarkAppendToNOMakeSlice-8            100      12910359 ns/op
*/

make without cap

func AppendToMakeSlice(count int32) {
    var slice = make([]int32, count)
    var idx int32 = 0
    for ; idx < count; idx++ {
        slice[idx] = idx
    }
}
 func BenchmarkAppendToMakeSlice(b *testing.B) {
    for i := 0; i < b.N; i++ {
        AppendToMakeSlice(numOfItems)
    }
}

bench result

go test -v -bench=BenchmarkAppendToMakeSlice
goos: darwin
goarch: amd64
pkg: github.com/xxxx/go-common/bench
BenchmarkAppendToMakeSlice-8         474           2160812 ns/op

Make slice with cap

func AppendToMakeSliceWithCap(count int32) {
    var slice = make([]int32, 0, count)
    var idx int32 = 0
    for ; idx < count; idx++ {
        slice = append(slice, idx)
    }
}
/*
goos: darwin
goarch: amd64
BenchmarkAppendToCapSlice
BenchmarkAppendToCapSlice-8            388       2699622 ns/op
*/
func BenchmarkAppendToCapSlice(b *testing.B) {
    for i := 0; i < b.N; i++ {
        AppendToMakeSliceWithCap(numOfItems)
    }
}

Summary

• Literal slice define is not effective when append large amount of elements
• Slice declared with make is more efficient than literal declared.