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.