package main

import (
	"fmt"
	"strings"
)

func main() {
	var b strings.Builder
	// a Builder appends without reallocating each time
	for i := 0; i < 3; i++ {
		fmt.Fprintf(&b, "line %d\n", i)
	}
	b.WriteString("done")
	fmt.Println(b.String())
}

grow a string efficiently