Go Program to Convert String to Lowercase

In Go, you can convert a string to lowercase using the strings.ToLower function from the strings package like this: lower := strings.ToLower(yourString).

Examples

InputHELLO

Outputhello

InputGoLang

Outputgolang

Input123ABC!@#

Output123abc!@#

How to Think About It

To convert a string to lowercase in Go, think of it as changing all uppercase letters to their lowercase versions while leaving other characters unchanged. The strings package provides a ready-made function ToLower that handles this for you, so you just pass your string to it and get the lowercase result.

Algorithm

1

Import the strings package.

2

Get the input string you want to convert.

3

Call the strings.ToLower function with the input string.

4

Store or use the returned lowercase string.

Code

go

package main

import (
	"fmt"
	"strings"
)

func main() {
	input := "Hello, GoLang!"
	lower := strings.ToLower(input)
	fmt.Println(lower)
}

Output

hello, golang!

Dry Run

Let's trace the string "Hello, GoLang!" through the code

1

Input string

input = "Hello, GoLang!"

2

Convert to lowercase

lower = strings.ToLower(input) // lower = "hello, golang!"

3

Print result

Output: hello, golang!

Step	Input	Output
1	Hello, GoLang!
2	Hello, GoLang!	hello, golang!
3		hello, golang!

Why This Works

Step 1: Import strings package

The strings package contains useful functions for string manipulation, including ToLower.

Step 2: Use strings.ToLower

Calling strings.ToLower on a string returns a new string with all uppercase letters converted to lowercase.

Step 3: Print the result

The converted lowercase string is printed to the console using fmt.Println.

Alternative Approaches

Manual conversion using rune iteration

go

package main

import (
	"fmt"
)

func toLowerManual(s string) string {
	result := []rune{}
	for _, r := range s {
		if r >= 'A' && r <= 'Z' {
			r += 'a' - 'A'
		}
		result = append(result, r)
	}
	return string(result)
}

func main() {
	input := "Hello, GoLang!"
	fmt.Println(toLowerManual(input))
}

This method manually converts uppercase letters to lowercase by checking rune values. It is less efficient and more complex than using strings.ToLower.

Complexity: O(n) time, O(n) space

Time Complexity

The function processes each character once, so the time grows linearly with the string length.

Space Complexity

A new string is created for the lowercase result, so space usage is proportional to the input size.

Which Approach is Fastest?

Using strings.ToLower is optimized and faster than manual rune conversion, and it handles Unicode correctly.

Approach	Time	Space	Best For
strings.ToLower	O(n)	O(n)	Simple, reliable lowercase conversion
Manual rune iteration	O(n)	O(n)	Learning purpose, custom logic