Creating custom packages in Go - Try It Yourself

Creating custom packages

📖 Scenario: You are building a small Go program that uses a custom package to organize your code better. This is like having a toolbox with different compartments for different tools.

🎯 Goal: Create a custom package named greetings that has a function to return a welcome message. Then use this package in your main program to print the message.

📋 What You'll Learn

Create a package named greetings in a separate file

Inside greetings, write a function WelcomeMessage that returns a string

In the main program, import the greetings package

Call greetings.WelcomeMessage() and print its result

💡 Why This Matters

🌍 Real World

Custom packages help organize code into reusable parts, making programs easier to manage and understand.

💼 Career

Knowing how to create and use packages is essential for writing clean, maintainable Go programs in professional software development.

1

Create the greetings package

Create a file named greetings.go with a package called greetings. Inside it, write a function named WelcomeMessage that returns the string "Hello from the greetings package!".

Go

package greetings

// Write the WelcomeMessage function below
// Your code here

Hint

Remember to start with package greetings and define a function that returns the exact string.

2

Set up the main program

Create a file named main.go. Write the main package and import the fmt package and your custom greetings package.

Go

package main

import (
    "fmt"
    "greetings"
)

// Your code here

Hint

Use import with parentheses to include both fmt and greetings.

3

Call the WelcomeMessage function

Inside the main function, call greetings.WelcomeMessage() and store the result in a variable named message.

Go

package greetings

func WelcomeMessage() string {
    return "Hello from the greetings package!"
}

package main

import (
    "fmt"
    "greetings"
)

func main() {
    // Call greetings.WelcomeMessage() and save to message
    // Your code here
}

Hint

Use := to declare and assign the variable message.

4

Print the welcome message

Use fmt.Println to print the variable message inside the main function.

Go

package greetings

func WelcomeMessage() string {
    return "Hello from the greetings package!"
}

package main

import (
    "fmt"
    "greetings"
)

func main() {
    message := greetings.WelcomeMessage()
    // Print the message
    // Your code here
}

Hint

Use fmt.Println(message) to show the message on the screen.

Practice

1. What is the main purpose of creating a custom package in Go?

easy

A. To organize code into reusable parts

B. To make the program run faster

C. To avoid using functions

D. To write code without imports

Solution

1. Step 1: Understand the role of packages

   Packages help group related code together for reuse and clarity.

2. Step 2: Identify the main benefit

   Custom packages allow you to reuse code in different parts of your program or in other programs.

3. Final Answer:

   To organize code into reusable parts -> Option A

4. Quick Check:

   Custom packages = reusable code [OK]

Hint: Packages group code for reuse and clarity [OK]

Common Mistakes:

• Thinking packages speed up code execution
• Believing packages remove the need for functions
• Assuming packages eliminate imports

2. Which of the following is the correct way to export a function from a custom package in Go?

easy

A. func MyFunction() {}

B. func my_function() {}

C. func _MyFunction() {}

D. func myFunction() {}

Solution

1. Step 1: Recall Go export rules

   In Go, only names starting with an uppercase letter are exported (visible outside the package).

2. Step 2: Check function names

   Only 'MyFunction' starts with uppercase, so it is exported.

3. Final Answer:

   func MyFunction() {} -> Option A

4. Quick Check:

   Uppercase start = exported function [OK]

Hint: Exported names start with uppercase letters [OK]

Common Mistakes:

• Using lowercase to export functions
• Using underscores to export
• Assuming snake_case exports functions

3. Given a package named mathutil with an exported function Square, what is the output of this code?

package main

import (
  "fmt"
  "mathutil"
)

func main() {
  fmt.Println(mathutil.Square(4))
}

medium

A. Square(4)

B. 16

C. Error: undefined mathutil

D. 0

Solution

1. Step 1: Understand the function call

   The code calls mathutil.Square(4), which should return 4 squared (4*4).

2. Step 2: Calculate the expected output

   4 multiplied by 4 equals 16, so the output is 16.

3. Final Answer:

   16 -> Option B

4. Quick Check:

   Square(4) = 16 [OK]

Hint: Square means multiply number by itself [OK]

Common Mistakes:

• Assuming function name prints as string
• Thinking import fails without path
• Expecting zero as default output

4. What is wrong with this custom package usage?

package main

import (
  "fmt"
  "mypkg"
)

func main() {
  fmt.Println(mypkg.helper())
}

Assuming helper is a function in mypkg.

medium

A. Package name must be Main to import

B. fmt.Println cannot print function results

C. Import path must be a URL

D. Function name should start with uppercase to be accessible

Solution

1. Step 1: Check function visibility rules

   In Go, only functions starting with uppercase letters are exported and accessible outside their package.

2. Step 2: Identify the function name issue

   The function is named 'helper' (lowercase), so it is not accessible from main package.

3. Final Answer:

   Function name should start with uppercase to be accessible -> Option D

4. Quick Check:

   Lowercase function = not exported [OK]

Hint: Exported functions start uppercase [OK]

Common Mistakes:

• Thinking package name must be Main
• Believing import path must be URL
• Assuming fmt.Println can't print function output

5. You want to create a custom package stringutil with a function Reverse that reverses a string. Which of the following is the correct way to define and use it in your main.go?

hard

A. In stringutil package: func Reverse(s string) string { /* code */ }In main.go: import "stringutil" and call Reverse("hello")

B. In stringutil package: func reverse(s string) string { /* code */ }In main.go: import "stringutil" and call stringutil.reverse("hello")

C. In stringutil package: func Reverse(s string) string { /* code */ }In main.go: import "stringutil" and call stringutil.Reverse("hello")

D. In stringutil package: func Reverse(s string) string { /* code */ }In main.go: no import needed, call Reverse("hello")

Solution

1. Step 1: Define exported function in package

   The function must start with uppercase 'Reverse' to be accessible outside the package.

2. Step 2: Import package and call function with package name

   In main.go, import "stringutil" and call the function as stringutil.Reverse("hello").

3. Final Answer:

   Define func Reverse and call stringutil.Reverse("hello") after import -> Option C

4. Quick Check:

   Uppercase function + import + package prefix = correct usage [OK]

Hint: Exported func uppercase + import + package.func call [OK]

Common Mistakes:

• Using lowercase function name and expecting export
• Calling function without package prefix after import
• Not importing the custom package