Map use cases in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a map in Go?

A map in Go is a built-in data type that stores key-value pairs. It allows you to quickly look up a value by its key, similar to a dictionary in real life where you find a word's meaning by looking it up.

beginner

How do you declare and initialize a map in Go?

You declare a map using the syntax: var m map[string]int. To initialize it, use m = make(map[string]int) or declare and initialize together with m := map[string]int{"key": 1}.

beginner

What happens if you try to access a key that does not exist in a Go map?

If you access a key that does not exist, Go returns the zero value for the map's value type. For example, if the map stores integers, it returns 0. You can also check if the key exists using the two-value assignment: value, ok := m[key].

beginner

Name a common use case for maps in Go.

Maps are commonly used to count occurrences, like counting how many times each word appears in a text. They are also used for fast lookups, caching data, and grouping related information by keys.

beginner

How do you delete a key-value pair from a map in Go?

You use the built-in delete function: delete(m, key). This removes the key and its value from the map.

How do you check if a key exists in a Go map?

A value, ok := m[key]

B exists := m.contains(key)

C if m[key] != nil

D m.hasKey(key)

In Go, you use the two-value assignment: value, ok := m[key]. The variable ok is true if the key exists.

What is the zero value returned when accessing a missing key in a map of type map[string]int?

A 0

B "" (empty string)

C nil

D false

For int values, the zero value is 0, so accessing a missing key returns 0.

Which function initializes a map in Go?

A init()

B new()

C make()

D create()

The make() function initializes maps, slices, and channels in Go.

How do you remove a key-value pair from a map?

A m.remove(key)

B delete(m, key)

C m.pop(key)

D m.clear(key)

The built-in delete(m, key) function removes a key and its value from the map.

Which of these is NOT a typical use case for maps?

A Caching data

B Fast data lookup by key

C Counting word frequencies

D Storing ordered lists

Maps do not maintain order, so they are not used for storing ordered lists.

Explain how you would use a map in Go to count how many times each word appears in a sentence.

Think about how a map can store words as keys and counts as values.

Describe how to safely check if a key exists in a Go map and handle the case when it does not.

Remember the special syntax Go provides for map lookups.

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