Map creation in Go

Introduction

A map in Go helps you store pairs of keys and values. It is like a real-life dictionary where you look up a word (key) to find its meaning (value).

When you want to quickly find a value using a unique key, like looking up a phone number by a person's name.

When you need to count how many times each word appears in a text.

When you want to store settings or options with names and values.

When you want to group data by categories, like storing students by their class.

Syntax

Go

var mapName map[keyType]valueType
mapName = make(map[keyType]valueType)

// Or combined:
mapName := make(map[keyType]valueType)

You must use make to create a map before adding items.

Keys must be a type that can be compared, like strings or integers.

Examples

This creates a map named ages where keys are strings and values are integers.

Go

var ages map[string]int
ages = make(map[string]int)

This creates and initializes a map named scores with string keys and float64 values.

Go

scores := make(map[string]float64)

This creates a map with initial key-value pairs for colors and their hex codes.

Go

colors := map[string]string{"red": "#ff0000", "green": "#00ff00"}

Sample Program

This program creates a map to store ages by name, adds two entries, and prints the map and one value.

Go

package main

import "fmt"

func main() {
    // Create a map to store ages
    ages := make(map[string]int)

    // Add some data
    ages["Alice"] = 30
    ages["Bob"] = 25

    // Print the map
    fmt.Println("Ages:", ages)

    // Access a value
    fmt.Println("Alice's age:", ages["Alice"])
}

Important Notes

If you try to read a key that does not exist, Go returns the zero value for the value type (like 0 for int).

You can check if a key exists using the comma-ok idiom: value, ok := mapName[key].

Summary

Maps store key-value pairs for fast lookup.

Create maps with make before use.

Keys must be comparable types like strings or ints.

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