Project Reusable stringutil package

Context

Build a reusable package stringutil that provides common string utilities: Reverse, ToUpper, ToLower, Contains, etc. Then use it in a main program.

This project demonstrates:

• Package creation
• Visibility (exported vs unexported)
• Importing your own package
• Writing tests

Step‑by‑step code

Directory structure

stringutil/
├── go.mod
├── stringutil.go
├── stringutil_test.go
└── cmd/
    └── demo/
        └── main.go

go.mod

module github.com/yourusername/stringutil

go 1.24

stringutil.go

package stringutil

import "strings"

// Reverse returns its argument string reversed rune-wise.
func Reverse(s string) string {
    r := []rune(s)
    for i, j := 0, len(r)-1; i < j; i, j = i+1, j-1 {
        r[i], r[j] = r[j], r[i]
    }
    return string(r)
}

// ToUpper converts the string to uppercase.
func ToUpper(s string) string {
    return strings.ToUpper(s)
}

// ToLower converts the string to lowercase.
func ToLower(s string) string {
    return strings.ToLower(s)
}

// Contains checks if substring is present.
func Contains(s, substr string) bool {
    return strings.Contains(s, substr)
}

// IsPalindrome checks if a string reads the same forward and backward.
func IsPalindrome(s string) bool {
    rev := Reverse(s)
    return s == rev
}

stringutil_test.go

package stringutil

import "testing"

func TestReverse(t *testing.T) {
    cases := []struct {
        in, want string
    }{
        {"hello", "olleh"},
        {"world", "dlrow"},
        {"", ""},
        {"世 界", "界 世"},
    }
    for _, c := range cases {
        got := Reverse(c.in)
        if got != c.want {
            t.Errorf("Reverse(%q) == %q, want %q", c.in, got, c.want)
        }
    }
}

func TestIsPalindrome(t *testing.T) {
    if !IsPalindrome("racecar") {
        t.Error("racecar should be palindrome")
    }
    if IsPalindrome("hello") {
        t.Error("hello should not be palindrome")
    }
}

cmd/demo/main.go

package main

import (
    "fmt"
    "github.com/yourusername/stringutil"
)

func main() {
    s := "Hello, Go!"
    fmt.Println("Original:", s)
    fmt.Println("Reversed:", stringutil.Reverse(s))
    fmt.Println("Uppercase:", stringutil.ToUpper(s))
    fmt.Println("Lowercase:", stringutil.ToLower(s))
    fmt.Println("Contains 'Go':", stringutil.Contains(s, "Go"))
    fmt.Println("Is palindrome? (racecar):", stringutil.IsPalindrome("racecar"))
}

Example output

Original: Hello, Go!
Reversed: !oG ,olleH
Uppercase: HELLO, GO!
Lowercase: hello, go!
Contains 'Go': true
Is palindrome? (racecar): true

Useful links

• Writing Go packages
• Testing in Go