Map use cases in Go - Mini Project: Build & Apply

Map use cases

📖 Scenario: You are managing a small library system. You want to keep track of books and their availability status using a map in Go.

🎯 Goal: Build a Go program that uses a map to store book titles as keys and their availability (true for available, false for checked out) as values. Then, filter and display only the available books.

📋 What You'll Learn

Create a map called library with exact book titles as keys and boolean values for availability

Create a variable called availableBooks to store available books

Use a for loop with variables title and available to iterate over library

Add available book titles to availableBooks

Print the availableBooks slice

💡 Why This Matters

🌍 Real World

Maps are useful to store and quickly access data by keys, like book titles in a library system.

💼 Career

Understanding maps and slices is essential for backend development, data processing, and building efficient Go applications.

1

Create the library map

Create a map called library with these exact entries: "The Hobbit": true, "1984": false, "Pride and Prejudice": true, "The Catcher in the Rye": false

Go

// Create the map called library with the given book titles and availability
// Your code here

Hint

Use map[string]bool to create the map with book titles as keys and availability as boolean values.

2

Create the availableBooks slice

Create a variable called availableBooks as an empty slice of strings to store titles of available books

Go

package main

import "fmt"

func main() {
    library := map[string]bool{
        "The Hobbit": true,
        "1984": false,
        "Pride and Prejudice": true,
        "The Catcher in the Rye": false,
    }
    // Create an empty slice called availableBooks
    // Your code here
}

Hint

Use []string{} to create an empty slice of strings.

3

Filter available books

Use a for loop with variables title and available to iterate over library. Inside the loop, if available is true, append title to availableBooks

Go

package main

import "fmt"

func main() {
    library := map[string]bool{
        "The Hobbit": true,
        "1984": false,
        "Pride and Prejudice": true,
        "The Catcher in the Rye": false,
    }
    availableBooks := []string{}
    // Loop over library and add available books to availableBooks
    // Your code here
}

Hint

Use for title, available := range library to loop over the map. Use append to add to the slice.

4

Print available books

Write fmt.Println(availableBooks) to display the list of available books

Go

package main

import "fmt"

func main() {
    library := map[string]bool{
        "The Hobbit": true,
        "1984": false,
        "Pride and Prejudice": true,
        "The Catcher in the Rye": false,
    }
    availableBooks := []string{}
    for title, available := range library {
        if available {
            availableBooks = append(availableBooks, title)
        }
    }
    // Print the availableBooks slice
    // Your code here
}

Hint

Use fmt.Println(availableBooks) to print the slice.

Practice

1. What is the primary use of a map in Go?

easy

A. To store data with keys for fast lookup

B. To perform mathematical calculations

C. To create user interfaces

D. To handle file input/output

Solution

1. Step 1: Understand the purpose of a map

   A map in Go is a collection of key-value pairs, designed for fast data retrieval using keys.

2. Step 2: Compare with other options

   Options B, C, and D describe unrelated tasks like calculations, UI, or file handling, which are not the main use of maps.

3. Final Answer:

   To store data with keys for fast lookup -> Option A

4. Quick Check:

   Maps = key-value storage [OK]

Hint: Maps store key-value pairs for quick access [OK]

Common Mistakes:

• Confusing maps with slices or arrays
• Thinking maps are for calculations
• Assuming maps handle UI or files

2. Which of the following is the correct way to create an empty map in Go?

easy

A. m := make([]string, 0)

B. var m map[string]int = make(map[string]int)

C. m := map[string]int

D. m := []int{}

Solution

1. Step 1: Identify map creation syntax

   In Go, maps are created using make(map[keyType]valueType). var m map[string]int = make(map[string]int) uses this correctly.

2. Step 2: Check other options

   m := []int{} creates a slice, not a map. m := map[string]int is invalid syntax for map creation. m := make([]string, 0) creates a slice, not a map.

3. Final Answer:

   var m map[string]int = make(map[string]int) -> Option B

4. Quick Check:

   Use make() to create maps [OK]

Hint: Use make() to create maps before use [OK]

Common Mistakes:

• Using slice syntax instead of map syntax
• Not using make() to initialize maps
• Confusing map and slice creation

3. What will be the output of this Go code?

m := make(map[string]int)
m["apple"] = 5
m["banana"] = 3
fmt.Println(m["apple"] + m["banana"])

medium

A. 8

B. 53

C. 0

D. Compilation error

Solution

1. Step 1: Understand map assignments

   The map m stores integers with string keys. "apple" maps to 5, "banana" maps to 3.

2. Step 2: Calculate the sum

   Adding m["apple"] + m["banana"] equals 5 + 3 = 8.

3. Final Answer:

   8 -> Option A

4. Quick Check:

   5 + 3 = 8 [OK]

Hint: Sum values by accessing keys directly [OK]

Common Mistakes:

• Concatenating values as strings
• Expecting zero for existing keys
• Thinking code causes error

4. What is wrong with this Go code snippet?

var m map[string]int
m["key"] = 10
fmt.Println(m["key"])

medium

A. Cannot assign int values to map

B. Key "key" is invalid syntax

C. Map is not initialized with make() before use

D. fmt.Println cannot print map values

Solution

1. Step 1: Check map initialization

   The map m is declared but not initialized with make(). This causes a runtime panic on assignment.

2. Step 2: Understand consequences

   Without initialization, m["key"] = 10 causes a panic: assignment to nil map.

3. Final Answer:

   Map is not initialized with make() before use -> Option C

4. Quick Check:

   Initialize maps with make() [OK]

Hint: Always initialize maps with make() before adding keys [OK]

Common Mistakes:

• Forgetting to initialize map before use
• Assuming declaration is enough
• Misunderstanding map assignment rules

5. Given a map m := map[string]int{"a": 1, "b": 0, "c": 3}, which code snippet correctly checks if key "b" exists and its value is non-zero?

hard

A. if _, ok := m["b"]; ok { fmt.Println("Exists and non-zero") }

B. if m["b"] == 0 { fmt.Println("Exists and non-zero") }

C. if m["b"] >= 0 { fmt.Println("Exists and non-zero") }

D. if val, ok := m["b"]; ok && val != 0 { fmt.Println("Exists and non-zero") }

Solution

1. Step 1: Understand map key existence check

   Using val, ok := m["b"] lets us check if key "b" exists (ok) and get its value (val).

2. Step 2: Check value is non-zero

   The condition ok && val != 0 ensures key exists and value is not zero.

3. Final Answer:

   if val, ok := m["b"]; ok && val != 0 { fmt.Println("Exists and non-zero") } -> Option D

4. Quick Check:

   Check key existence and value together [OK]

Hint: Use val, ok := map[key] to check existence and value [OK]

Common Mistakes:

• Checking value without verifying key existence
• Assuming zero means key missing
• Using only ok without value check