What might be wrong with the following small program?

func main() {
	scanner := bufio.NewScanner(strings.NewReader(`one
two
three
four
`))
	var (
		text string
		n    int
	)
	for scanner.Scan() {
		n++
		text += fmt.Sprintf("%d. %s\n", n, scanner.Text())
	}
	fmt.Print(text)

	// Output:
	// 1. One
	// 2. Two
	// 3. Three
	// 4. Four
}

The program numbers the lines in a buffer and uses the text/scanner to read the input line-by-line. What might be wrong with it?

Answer:

First, it is not necessary to collect the input in the string before putting it out to standard output. This example is slightly contrived.

Second, the string text is not modified with the += operator, it is created anew for every line. This is a significant difference between strings and []byte slices — strings in Go are non-modifiable. If you need to modify a string, use a []byte slice.

Here’s a provided small program, written in a better way:

func main() {
	scanner := bufio.NewScanner(strings.NewReader(`one
two
three
four
`))
	var (
		text []byte
		n    int
	)
	for scanner.Scan() {
		n++
		text = append(text, fmt.Sprintf("%d. %s\n", n, scanner.Text())...)
	}
	os.Stdout.Write(text)
	// 1. One
	// 2. Two
	// 3. Three
	// 4. Four
}

That is the point of the existence of both bytes and strings packages.