String Programs

Learn string programs through an executable example, then connect the small demonstration to production code.

Go strings contain bytes and are typically UTF-8 encoded. Many beginner string exercises must distinguish byte indexes, Unicode code points, and user-perceived characters.

The example reverses runes, not raw bytes. It preserves multi-byte code points, though grapheme clusters can still contain more than one rune.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

func reverseRunes(s string) string {
	chars := []rune(s)
	for left, right := 0, len(chars)-1; left < right; left, right = left+1, right-1 {
		chars[left], chars[right] = chars[right], chars[left]
	}
	return string(chars)
}
func main() { fmt.Println(reverseRunes("Go世界")) }

Output

界世oG

The string becomes a rune slice containing G, o, 世 and 界. Reversing rune order yields 界世oG without corrupting UTF-8 bytes.

• Reversing runes is not always equivalent to reversing grapheme clusters.
• len(string) counts bytes, not Unicode characters.

Count runes and bytes for the same input, then try an accent made from combining characters.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced String Programs in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.