Map creation in Go - Mini Project: Build & Apply

Map creation

📖 Scenario: You are creating a simple program to store and look up the prices of fruits in a store.

🎯 Goal: Build a Go program that creates a map with fruit names as keys and their prices as values.

📋 What You'll Learn

Create a map called fruitPrices with exact entries

Add a variable called newFruit with the value "Mango"

Add a new entry to the map using newFruit as the key and 120 as the value

Print the entire fruitPrices map

💡 Why This Matters

🌍 Real World

Maps are used to store related data where you want to quickly find a value by a key, like looking up prices by product names.

💼 Career

Understanding maps is important for many programming jobs because they help organize and access data efficiently.

1

Create the initial map

Create a map called fruitPrices with these exact entries: "Apple": 100, "Banana": 30, "Cherry": 75

Go

# Your code here

Hint

Use map[string]int to create a map with string keys and int values.

2

Add a new fruit variable

Add a variable called newFruit and set it to the string "Mango"

Go

package main

import "fmt"

func main() {
    fruitPrices := map[string]int{"Apple": 100, "Banana": 30, "Cherry": 75}
    // Your code here
}

Hint

Use newFruit := "Mango" to create the variable.

3

Add the new fruit to the map

Add a new entry to the fruitPrices map using newFruit as the key and 120 as the value

Go

package main

import "fmt"

func main() {
    fruitPrices := map[string]int{"Apple": 100, "Banana": 30, "Cherry": 75}
    newFruit := "Mango"
    // Your code here
}

Hint

Use fruitPrices[newFruit] = 120 to add the new entry.

4

Print the map

Print the entire fruitPrices map using fmt.Println

Go

package main

import "fmt"

func main() {
    fruitPrices := map[string]int{"Apple": 100, "Banana": 30, "Cherry": 75}
    newFruit := "Mango"
    fruitPrices[newFruit] = 120
    // Your code here
}

Hint

Use fmt.Println(fruitPrices) to display the map.

Practice

1. What is the correct way to create an empty map with string keys and int values in Go?

easy

A. m := make(map[string]int)

B. m := map[string]int()

C. m := new(map[string]int)

D. m := []string{}

Solution

1. Step 1: Understand map creation syntax

   In Go, the built-in function make is used to create maps with specified key and value types.

2. Step 2: Identify the correct syntax for an empty map

   make(map[string]int) creates an empty map with string keys and int values.

3. Final Answer:

   m := make(map[string]int) -> Option A

4. Quick Check:

   Use make(map[keyType]valueType) to create maps [OK]

Hint: Use make() with map[keyType]valueType to create maps [OK]

Common Mistakes:

• Using new() instead of make() for maps
• Trying to create map with slice syntax
• Confusing map literal with empty slice

2. Which of the following is a syntax error when creating a map in Go?

easy

A. m := make(map[int]string)

B. m := make(map[float64]int)

C. m := make(map[[]int]string)

D. m := map[int]string{}

Solution

1. Step 1: Check key type validity

   Map keys must be comparable types. Slices ([]int) are not comparable in Go.

2. Step 2: Identify invalid map key type

   Using []int as a key type causes a syntax error because slices cannot be map keys.

3. Final Answer:

   m := make(map[[]int]string) -> Option C

4. Quick Check:

   Map keys must be comparable types [OK]

Hint: Map keys cannot be slices or maps, only comparable types [OK]

Common Mistakes:

• Using slices or maps as map keys
• Confusing map literal with slice literal
• Ignoring key type restrictions

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

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

medium

A. 53

B. 8

C. 0

D. Compilation error

Solution

1. Step 1: Understand map assignments

   The map m stores key-value pairs: "apple"=5 and "banana"=3.

2. Step 2: Calculate the sum of values

   Adding m["apple"] (5) and m["banana"] (3) results in 8.

3. Final Answer:

   8 -> Option B

4. Quick Check:

   5 + 3 = 8 [OK]

Hint: Sum map values by accessing keys directly [OK]

Common Mistakes:

• Concatenating values as strings instead of adding
• Expecting zero for existing keys
• Forgetting to initialize map before use

4. Identify the error in the following Go code snippet:

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

medium

A. No error, prints 10

B. Syntax error in map declaration

C. Key type must be int, not string

D. Map declared but not initialized, causes runtime panic

Solution

1. Step 1: Check map initialization

   The map m is declared but not initialized with make, so it is nil.

2. Step 2: Understand runtime behavior

   Assigning to a nil map causes a runtime panic (error) because the map is not ready to store values.

3. Final Answer:

   Map declared but not initialized, causes runtime panic -> Option D

4. Quick Check:

   Declare + make before use to avoid panic [OK]

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

Common Mistakes:

• Forgetting to initialize map before assignment
• Assuming declaration alone creates usable map
• Confusing map declaration with slice declaration

5. Given a slice of strings keys := []string{"a", "b", "c"} and a slice of ints values := []int{1, 2, 3}, which code snippet correctly creates a map from keys to values?

hard

A. m := make(map[string]int)
for i := range keys {
m[keys[i]] = values[i]
}

B. m := map[string]int{}
for i, v := range values {
m[v] = keys[i]
}

C. m := make(map[int]string)
for i := 0; i < len(keys); i++ {
m[i] = keys[i]
}

D. m := make(map[string]int)
for i := 0; i < len(values); i++ {
m[values[i]] = keys[i]
}

Solution

1. Step 1: Match keys and values correctly

   Keys are strings, values are ints. The map must have type map[string]int.

2. Step 2: Verify loop and assignment

   m := make(map[string]int) for i := range keys { m[keys[i]] = values[i] } loops over keys and assigns m[keys[i]] = values[i], correctly pairing keys to values.

3. Final Answer:

   m := make(map[string]int) for i := range keys { m[keys[i]] = values[i] } -> Option A

4. Quick Check:

   Loop keys, assign values to map[key] [OK]

Hint: Loop keys, assign values to map[key] for correct pairing [OK]

Common Mistakes:

• Swapping keys and values in map assignment
• Using wrong map key or value types
• Not initializing map before assignment