Concatenate strings efficiently with a string builder

yourbasic.org/golang

Clean and simple

For simple cases where performance is a non-issue, fmt.Sprintf is your friend.

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

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

Efficient string concatenationGo 1.10

The strings.Builder type is used to efficiently append strings using write methods.

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)

Further reading

String functions cheat sheet

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