Map use cases in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Map key existence check output

What is the output of this Go program that checks if a key exists in a map?

Go

package main
import "fmt"
func main() {
    m := map[string]int{"apple": 5, "banana": 3}
    val, ok := m["orange"]
    fmt.Println(val, ok)
}

A 3 true

B 0 true

C 0 false

D Compilation error

💡 Hint

Check what happens when you access a map with a key that does not exist.

In Go, when you access a map with a key that does not exist, the zero value of the value type is returned (0 for int) and the boolean 'ok' is false.

❓ Predict Output

intermediate

Map iteration order output

What is the output of this Go program that iterates over a map?

Go

package main
import "fmt"
func main() {
    m := map[int]string{1: "one", 2: "two", 3: "three"}
    for k, v := range m {
        fmt.Printf("%d:%s ", k, v)
    }
}

A 1:one 2:two 3:three

B The output order is random and may vary each run

C 3:three 2:two 1:one

D Compilation error

💡 Hint

Remember how Go handles map iteration order.

Go does not guarantee the order of map iteration. The output order can change each time the program runs.

🔧 Debug

advanced

Identify the runtime error in map access

What runtime error will this Go program produce when run?

Go

package main
func main() {
    var m map[string]int
    m["key"] = 10
}

A panic: index out of range

B Compilation error: map not initialized

C No error, program runs fine

D panic: assignment to entry in nil map

💡 Hint

Think about what happens if you assign to a nil map.

Maps must be initialized before assigning values. Assigning to a nil map causes a runtime panic.

🧠 Conceptual

advanced

Map key type restrictions

Which of the following types can be used as keys in Go maps?

A Structs with only comparable fields

B Arrays

C Slices

D Maps

💡 Hint

Map keys must be comparable types in Go.

Go map keys must be types that can be compared with '=='. Slices and maps are not comparable, arrays and structs are if their fields are comparable.

🚀 Application

expert

Count word frequency using a map

What is the final value of the map after running this Go program that counts word frequency?

Go

package main
import (
    "fmt"
    "strings"
)
func main() {
    text := "go go gophers go"
    words := strings.Fields(text)
    freq := make(map[string]int)
    for _, w := range words {
        freq[w]++
    }
    fmt.Println(freq)
}

A map[go:3 gophers:1]

B map[go:4 gophers:1]

C map[go:3 gophers:0]

D map[go:2 gophers:1]

💡 Hint

Count how many times 'go' appears in the text.

The word 'go' appears 3 times and 'gophers' once. The map counts each word's frequency correctly.

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