Package scope rules in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of accessing package-level variables

What is the output of this Go program?

Go

package main

import "fmt"

var packageVar = 10

func main() {
    fmt.Println(packageVar)
}

A Compilation error: packageVar undefined

B 0

C 10

D Runtime panic

💡 Hint

Package-level variables are accessible inside functions within the same package.

The variable packageVar is declared at package level and initialized to 10. It is accessible inside main(), so printing it outputs 10.

❓ Predict Output

intermediate

Accessing unexported variable from another package

Given two packages, main and utils, where utils has an unexported variable, what happens when main tries to access it?

Go

package utils

var secret = 42

package main

import (
    "fmt"
    "utils"
)

func main() {
    fmt.Println(utils.secret)
}

A 0

B 42

C Runtime panic

D Compilation error: secret is not exported by package utils

💡 Hint

In Go, identifiers starting with lowercase letters are unexported and not accessible outside their package.

The variable secret starts with a lowercase letter, so it is unexported and cannot be accessed from main package. This causes a compilation error.

🔧 Debug

advanced

Why does this code fail to compile?

Consider this Go code snippet. Why does it fail to compile?

Go

package main

import "fmt"

func main() {
    fmt.Println(helperVar)
}

var helperVar = 5

A Compilation error: undefined helperVar

B It compiles and prints 5

C Compilation error: helperVar used before declaration

D Runtime panic: nil pointer dereference

💡 Hint

Package-level variables are initialized before main runs, regardless of their position in the file.

In Go, package-level variables are initialized before main() runs, so helperVar is accessible even if declared after main(). The code compiles and prints 5.

❓ Predict Output

advanced

Shadowing package-level variable inside function

What is the output of this Go program?

Go

package main

import "fmt"

var count = 100

func main() {
    count := 50
    fmt.Println(count)
}

A 50

B 100

C Compilation error: count redeclared

D 0

💡 Hint

A variable declared inside a function with := shadows the package-level variable.

Inside main(), count := 50 declares a new local variable count that shadows the package-level count. So printing count outputs 50.

❓ Predict Output

expert

Package variable initialization order with init functions

What is the output of this Go program?

Go

package main

import "fmt"

var x = initializeX()

func initializeX() int {
    fmt.Println("initializeX called")
    return 42
}

func init() {
    fmt.Println("init function called")
}

func main() {
    fmt.Println("main function called")
    fmt.Println(x)
}

A

initializeX called
init function called
main function called
42

B

init function called
initializeX called
main function called
42

C

main function called
initializeX called
init function called
42

D Compilation error

💡 Hint

Package variables are initialized before init functions run.

First, package-level variables are initialized, so initializeX() runs and prints its message. Then init() runs. Finally main() runs. So the output order is initializeX called, init function called, main function called, 42.

Practice

1. In Go, what does it mean if a variable name starts with a lowercase letter inside a package?

easy

A. It is private to the package and cannot be accessed from other packages.

B. It is accessible from any package.

C. It is a constant value.

D. It is a global variable accessible everywhere.

Solution

1. Step 1: Understand Go naming conventions

   In Go, identifiers starting with a lowercase letter are unexported, meaning they are private to the package.

2. Step 2: Apply package scope rule

   Such variables cannot be accessed from outside their package, ensuring encapsulation.

3. Final Answer:

   It is private to the package and cannot be accessed from other packages. -> Option A

4. Quick Check:

   Lowercase start = private [OK]

Hint: Lowercase start means private inside package [OK]

Common Mistakes:

• Thinking lowercase variables are accessible outside package
• Confusing lowercase with constants
• Assuming lowercase means global

2. Which of the following is the correct way to declare an exported function in a Go package?

easy

A. func MyFunction() {}

B. func myFunction() {}

C. func _MyFunction() {}

D. func my_function() {}

Solution

1. Step 1: Recall Go export rules

   Functions starting with an uppercase letter are exported and accessible outside the package.

2. Step 2: Check each option

   func MyFunction() {} starts with uppercase 'M', so it is exported. Others start with lowercase or invalid characters.

3. Final Answer:

   func MyFunction() {} -> Option A

4. Quick Check:

   Uppercase start = exported function [OK]

Hint: Function name uppercase means exported [OK]

Common Mistakes:

• Using lowercase to export functions
• Using underscores which are allowed but don't affect export
• Confusing export with naming style

3. Given the following Go code in package 'mathops':

package mathops

var result = 10

func Result() int {
    return result
}

What will happen if another package tries to access mathops.result?

medium

A. It will access the value 10 successfully.

B. It will cause a compile-time error due to unexported variable.

C. It will cause a runtime panic.

D. It will access a zero value.

Solution

1. Step 1: Identify variable scope

   The variable 'result' starts with lowercase, so it is unexported and private to the package.

2. Step 2: Understand access from other packages

   Accessing 'mathops.result' from outside the package is not allowed and causes a compile-time error.

3. Final Answer:

   It will cause a compile-time error due to unexported variable. -> Option B

4. Quick Check:

   Lowercase variable = inaccessible outside package [OK]

Hint: Lowercase vars cause compile error if accessed outside [OK]

Common Mistakes:

• Assuming private variables can be accessed
• Thinking runtime panic occurs instead of compile error
• Confusing function export with variable export

4. Consider this Go code snippet inside a package:

package example

func calculate() int {
    return 42
}

func Calculate() int {
    return calculate()
}

What is the main reason for defining both calculate and Calculate functions?

medium

A. To confuse the compiler with similar names.

B. To create two independent exported functions.

C. To cause a compile-time error due to duplicate names.

D. To have a private helper function and a public wrapper function.

Solution

1. Step 1: Analyze function names and scope

   'calculate' starts lowercase, so it is private to the package; 'Calculate' starts uppercase, so it is exported.

2. Step 2: Understand design pattern

   This pattern allows internal logic in 'calculate' while exposing a public function 'Calculate' that calls it.

3. Final Answer:

   To have a private helper function and a public wrapper function. -> Option D

4. Quick Check:

   Lowercase private + uppercase public wrapper [OK]

Hint: Use lowercase for private helpers, uppercase for public API [OK]

Common Mistakes:

• Thinking both functions are exported
• Expecting compile error from similar names
• Assuming no reason for two functions

5. You want to create a Go package that exposes only a function GetData but keeps the data source variable private. Which code snippet correctly implements this using package scope rules?

hard

A. package data

var datasource = "secret"

func getData() string {
return datasource
}

B. package data

var DataSource = "secret"

func GetData() string {
return DataSource
}

C. package data

var datasource = "secret"

func GetData() string {
return datasource
}

D. package data

var DataSource = "secret"

func getData() string {
return DataSource
}

Solution

1. Step 1: Identify private data variable

   The data source variable should start lowercase to be private to the package, so 'datasource' is correct.

2. Step 2: Identify exported function

   The function 'GetData' starts uppercase, so it is exported and accessible outside the package.

3. Step 3: Verify function returns private variable

   The function returns the private variable, exposing data safely.

4. Final Answer:

   package data var datasource = "secret" func GetData() string { return datasource } -> Option C

5. Quick Check:

   Lowercase var + uppercase func = private data, public access [OK]

Hint: Private vars lowercase, public funcs uppercase [OK]

Common Mistakes:

• Making data variable exported by uppercase
• Making function unexported by lowercase
• Returning unexported function instead of exported