Efficient string concatenation [full guide]

yourbasic.org/golang

Clean and simple string building (fmt)
High-performance string concatenation (stringbuilder)
Before Go 1.10 (bytebuffer)

Clean and simple string building

For simple cases where performance is a non-issue, fmt.Sprintf is your friend. It’s clean, simple and fairly efficient.

s := fmt.Sprintf("Size: %d MB.", 85) // s == "Size: 85 MB."

The fmt cheat sheet lists the most common formatting verbs and flags.

High-performance string concatenationGo 1.10

A strings.Builder is used to efficiently append strings using write methods.

It offers a subset of the bytes.Buffer methods that allows it to safely avoid extra copying when converting a builder to a string.
You can use the fmt package for formatting since the builder implements the io.Writer interface.
The Grow method can be used to preallocate memory when the maximum size of the string is known.

var b strings.Builder
b.Grow(32)
for i, p := range []int{2, 3, 5, 7, 11, 13} {
	fmt.Fprintf(&b, "%d:%d, ", i+1, p)
}
s := b.String()   // no copying
s = s[:b.Len()-2] // no copying (removes trailing ", ")
fmt.Println(s)

1:2, 2:3, 3:5, 4:7, 5:11, 6:13

Before Go 1.10

Use fmt.Fprintf to print into a bytes.Buffer.

var buf bytes.Buffer
for i, p := range []int{2, 3, 5, 7, 11, 13} {
	fmt.Fprintf(&buf, "%d:%d, ", i+1, p)
}
buf.Truncate(buf.Len() - 2) // Remove trailing ", "
s := buf.String()           // Copy into a new string
fmt.Println(s)

1:2, 2:3, 3:5, 4:7, 5:11, 6:13

This solution is pretty efficient but may generate some excess garbage. For higher performance, you can try to use the append functions in package strconv.

buf := []byte("Size: ")
buf = strconv.AppendInt(buf, 85, 10)
buf = append(buf, " MB."...)
s := string(buf)

If the expected maximum length of the string is known, you may want to preallocate the slice.

buf := make([]byte, 0, 16)
buf = append(buf, "Size: "...)
buf = strconv.AppendInt(buf, 85, 10)
buf = append(buf, " MB."...)
s := string(buf)

Efficient string concatenation [full guide]

Clean and simple string building

High-performance string concatenationGo 1.10

Before Go 1.10

Further reading