How to Efficiently Concatenate Strings in Go

Question

In Go, a string is a primitive type, which means it is read-only, and every manipulation of it will create a new string.

So if I want to concatenate strings many times without knowing the length of the resulting string, what's the best way to do it?

The naive way would be:

var s string
for i := 0; i < 1000; i++ {
    s += getShortStringFromSomewhere()
}
return s

but that does not seem very efficient.

Accepted Answer

New Way:

From Go 1.10 there is a strings.Builder type, please take a look at this answer for more detail.

Old Way:

Use the bytes package. It has a Buffer type which implements io.Writer.

package main

import (
    "bytes"
    "fmt"
)

func main() {
    var buffer bytes.Buffer

    for i := 0; i < 1000; i++ {
        buffer.WriteString("a")
    }

    fmt.Println(buffer.String())
}

This does it in O(n) time.

Answer 2

In Go 1.10+ there is strings.Builder, here.

A Builder is used to efficiently build a string using Write methods. It minimizes memory copying. The zero value is ready to use.

Must Read

Example

It's almost the same with bytes.Buffer.

package main

import (
    "strings"
    "fmt"
)

func main() {
    // ZERO-VALUE:
    //
    // It's ready to use from the get-go.
    // You don't need to initialize it.
    var sb strings.Builder

    for i := 0; i < 1000; i++ {
        sb.WriteString("a")
    }

    fmt.Println(sb.String())
}

Click to see this on the playground.

How to Efficiently Concatenate Strings in Go

19 Answers

New Way:

Old Way:

Must Read

Example

Marcus Vance