Concatenating with strings.Builder Quickly in Golang

The Go standard library makes concatenating strings easy. Concatenation is just a fancy word for adding strings together to make a larger string. For example, if we concatenate "hello", " " and "world" we’d get "hello world".

The built-in [fmt.Sprintf](https://golang.org/pkg/fmt/#Sprintf) function takes a format and a variadic list of interfaces as input.

func Sprintf(format string, a ...interface{}) string

The formatting option lets us template out how the final string will look, then we can add inputs that will be interpolated into the string.

s := fmt.Sprintf("%v has been subscribed since %v.\n", user.Name, user.CreatedAt)

%v is a simple token that will be replaced by the default format of whatever the given arguments are. In our case, it was a string and a time.Time. Check out the documentation for all the formatting options.

Efficient string concatenation

Go 1.10+ released the awesome [strings.Builder](https://golang.org/pkg/strings/#Builder) type, which lets us more efficiently build strings. Because it minimizes memory copying, strings.Builder is a high-performance option when it comes to doing bulk string concatenation.

Quickly writing a user list - example

First, let’s create an empty builder.

var builder strings.Builder

Next, let’s add a title string to our list.

b.WriteString("user list\n")

Now we’ll iterate from 0-9, and for each number write a line containing “user #{i}”. Because strings.Builder implements the io.Writer interface, we can use the standard fmt.Fprintf function.

for i := 0; i < 10; i++ {
    fmt.Fprintf(&b, "user #%d\n", i)
}

To actually print the full string we can use the String() method.

fmt.Println(b.String())

Full code:

package main

import (
    "fmt"
    "strings"
)

func main() {
    var b strings.Builder
    b.WriteString("user list\n")
    for i := 0; i < 10; i++ {
        fmt.Fprintf(&b, "user #%d\n", i)
    }
    fmt.Println(b.String())
}

Preallocation for more speed

If you really want to speed up your string building, and you already know the size of your final string, you can use the builder’s [Grow()](https://golang.org/pkg/strings/#Builder.Grow) method to preallocate the size of the buffer. This saves your code from needing to grow the memory dynamically.

// Grow grows b's capacity, if necessary, to guarantee space for another n bytes.
// After Grow(n), at least n bytes can be written to b without another allocation.
// If n is negative, Grow panics.
func (b *Builder) Grow(n int)

So to preallocate for our example we could do the following.

package main

import (
    "fmt"
    "strings"
)

func main() {
    var b strings.Builder
    b.Grow(90) // we will be writing 90 bytes
    b.WriteString("user list\n")
    for i := 0; i < 10; i++ {
        fmt.Fprintf(&b, "user #%d\n", i)
    }
    fmt.Println(b.String())
}