Felix Hanley

Go string concatenation comparison

2018-05-01 go benchmark

There are a variety of methods of bulk string concatenation and some are faster than others:

UPDATE 20220704: Re-ran the benchmarks, they got slower!
UPDATE 20250819: Re-ran the benchmarks, fmt got slower

Concatenation using ‘+’

func BenchmarkPlus(b *testing.B) {
    var str string

    for n := 0; n < b.N; n++ {
        str += "x"
    }

    if len(str) != b.N {
        b.Errorf("got %d, want %d", len(str), b.N)
    }
}

Concatenation using fmt package

func BenchmarkFmt(b *testing.B) {
    var str string

    for n := 0; n < b.N; n++ {
        str = fmt.Sprintf("%s%s", str, "x")
    }

    if len(str) != b.N {
        b.Errorf("got %d, want %d", len(str), b.N)
    }
}

Concatenation using strings.Builder

func BenchmarkBuilder(b *testing.B) {
    var str strings.Builder

    for n := 0; n < b.N; n++ {
        str.WriteString("x")
    }

    if len(str.String()) != b.N {
        b.Errorf("got %d, want %d", len(str.String()), b.N)
    }
}

Comparison

$ go version
go version go1.24.5 linux/amd64
$ go test -bench=. -test.benchmem
goos: linux
goarch: amd64
pkg: benchmarks
cpu: 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz
BenchmarkFmt-8          1000000    152187 ns/op 1006183 B/op          3 allocs/op
BenchmarkPlus-8         1000000     49599 ns/op  503996 B/op          1 allocs/op
BenchmarkBuilder-8      557699485           1.856 ns/op       5 B/op          0 allocs/op
PASS
ok      benchmarks  204.096s
$ go version
go version go1.18.3 darwin/amd64
$ go test -bench=. -test.benchmem
goos: darwin
goarch: amd64
cpu: Intel(R) Core(TM) i7-8850H CPU @ 2.60GHz
BenchmarkFmt-12          1000000            128244 ns/op         1006374 B/op          4 allocs/op
BenchmarkPlus-12         1000000             62706 ns/op          503994 B/op          1 allocs/op
BenchmarkBuilder-12     520606197                2.449 ns/op           5 B/op          0 allocs/op
PASS
ok      _/tmp/concat 194.673s

Interesting!

$ go1.11.13 test -bench=. -test.benchmem
goos: darwin
goarch: amd64
BenchmarkFmt-12          1000000             78188 ns/op          523073 B/op          2 allocs/op
BenchmarkPlus-12         1000000             43801 ns/op          503992 B/op          1 allocs/op
BenchmarkBuilder-12     300000000                3.35 ns/op            5 B/op          0 allocs/op
PASS
ok      _/tmp/concat     124.349s