Creating custom packages in Go - Practice Exercises

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple custom package usage

What is the output of this Go program that uses a custom package named greet?

Go

package main

import (
	"fmt"
	"example.com/mypkg/greet"
)

func main() {
	message := greet.Hello("Alice")
	fmt.Println(message)
}

// Package greet in example.com/mypkg/greet/greet.go
package greet

func Hello(name string) string {
	return "Hello, " + name + "!"
}

A Hello Alice

B Error: undefined greet.Hello

C hello, Alice!

D Hello, Alice!

💡 Hint

Check how the Hello function formats the string and if it includes punctuation.

The Hello function returns the string "Hello, " concatenated with the name and an exclamation mark. So the output is exactly "Hello, Alice!".

🧠 Conceptual

intermediate

Understanding package visibility rules

In Go, which of the following statements about identifiers in custom packages is true?

A Identifiers starting with a lowercase letter are accessible outside the package.

B Only identifiers starting with an uppercase letter are exported and accessible outside the package.

C All identifiers in a package are accessible outside the package regardless of case.

D Identifiers starting with an underscore are exported automatically.

💡 Hint

Think about how Go controls what is visible outside a package.

In Go, only identifiers (functions, variables, types) that start with an uppercase letter are exported and can be accessed from other packages.

🔧 Debug

advanced

Fix the import path error in custom package usage

Given this main.go file, what is the cause of the error when trying to build the program? main.go: package main import ( "fmt" "mypkg/greet" ) func main() { fmt.Println(greet.Hello("Bob")) } The error is: cannot find package "mypkg/greet"

A The greet package is not in the same directory as main.go.

B The greet package does not have a Hello function exported.

C The import path "mypkg/greet" is incorrect; it should be a full module path or relative path.

D The main package is missing a main function.

💡 Hint

Check how Go modules and import paths work for custom packages.

Go requires import paths to be full module paths or relative paths. "mypkg/greet" is not a valid import path unless it matches the module path. The import path should match the module name or be relative.

📝 Syntax

advanced

Identify the syntax error in custom package declaration

Which option shows the correct syntax for declaring a custom package named mathutil in Go?

A

package mathutil

func Add(a int, b int) int {
	return a + b
}

B

package MathUtil

func Add(a int, b int) int {
	return a + b
}

C

package mathutil {

func Add(a int, b int) int {
	return a + b
}
}

D

package mathutil

func Add(a int, b int) int {
return a + b
}

💡 Hint

Remember how package declarations are written in Go.

The package declaration must be a single line starting with the keyword 'package' followed by the package name without braces. Option A follows this syntax correctly.

🚀 Application

expert

Number of exported functions in a custom package

Consider a Go package named tools with the following functions: func ExportedOne() {} func exportedTwo() {} func ExportedThree() {} func exportedFour() {} How many functions are accessible (exported) when importing the tools package in another Go file?

A 2

B 0

C 4

D 3

💡 Hint

Recall Go's rule for exported identifiers based on capitalization.

Only functions starting with an uppercase letter are exported. Here, ExportedOne and ExportedThree are exported, so 2 functions are accessible.

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