Efficient string concatenation in a loop

This example discusses common problems when building a string in a loop and ways to fix them.

func fn() {
    result := ""
    for i := 0; i < 100000; i++ {
        result += fmt.Sprintf("%d", i)
    }
}

Problems:

• The operation result += ... creates a new string each time, copying the content, which leads to O(n²) time and high memory usage.
• Inside the loop, fmt.Sprintf is used, which is also costly as it formats the string.

Recommendations for improvement:

1. Use strings.Builder (or bytes.Buffer) – it can accumulate bytes without repeated copying.
2. Convert the number to a string without Sprintf, for example using strconv.Itoa.

func fn() {
    var b strings.Builder
    b.Grow(600000) // approximate size estimate to avoid reallocations
    for i := 0; i < 100000; i++ {
        b.WriteString(strconv.Itoa(i))
    }
    result := b.String()
    _ = result // use further
}

This approach reduces execution time and memory consumption to O(n).