Adding and updating values in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Go code when adding and updating map values?

Consider the following Go program that adds and updates values in a map. What will it print?

Go

package main
import "fmt"
func main() {
    scores := map[string]int{"Alice": 10, "Bob": 15}
    scores["Alice"] += 5
    scores["Charlie"] = 20
    fmt.Println(scores)
}

A map[Alice:15 Bob:15 Charlie:20]

B map[Alice:10 Bob:15 Charlie:20]

C map[Alice:15 Bob:15]

D map[Alice:5 Bob:15 Charlie:20]

💡 Hint

Remember that updating a map value with += adds to the existing value.

The map starts with Alice:10 and Bob:15. Alice's score is increased by 5, making it 15. Charlie is added with 20. So the final map has Alice:15, Bob:15, and Charlie:20.

❓ Predict Output

intermediate

What happens when you update a map value for a non-existent key?

Look at this Go code snippet. What will be the output?

Go

package main
import "fmt"
func main() {
    ages := map[string]int{"John": 30}
    ages["Mary"] += 5
    fmt.Println(ages)
}

A map[John:30 Mary:5]

B map[John:30 Mary:0]

C map[John:30]

D Compilation error

💡 Hint

When you add to a zero value in a map, Go initializes it to zero first.

Mary is not in the map initially, so ages["Mary"] returns 0. Adding 5 sets Mary to 5. The map now has John:30 and Mary:5.

🔧 Debug

advanced

Why does this Go code cause a runtime panic when updating a map?

Examine the code below. It causes a runtime panic. What is the reason?

Go

package main
func main() {
    var data map[string]int
    data["key"] = 1
}

A The map must be declared with 'make' keyword to allow updates.

B The key 'key' is invalid for map assignment.

C The map 'data' is nil and not initialized before assignment.

D The map keys must be integers, not strings.

💡 Hint

Maps must be initialized before you can add or update values.

The variable 'data' is declared but not initialized, so it is nil. Assigning to a nil map causes a runtime panic.

❓ Predict Output

advanced

What is the output after updating nested map values?

Given this Go code with nested maps, what will be printed?

Go

package main
import "fmt"
func main() {
    users := map[string]map[string]int{
        "user1": {"score": 10},
        "user2": {"score": 20},
    }
    users["user1"]["score"] += 5
    users["user3"] = map[string]int{"score": 15}
    fmt.Println(users)
}

A Compilation error due to nested map assignment

B map[user1:map[score:15] user2:map[score:20] user3:map[score:15]]

C map[user1:map[score:15] user2:map[score:20]]

D map[user1:map[score:10] user2:map[score:20] user3:map[score:15]]

💡 Hint

You can update nested map values by accessing keys step by step.

user1's score is increased from 10 to 15. user3 is added with score 15. user2 remains unchanged.

🧠 Conceptual

expert

How many items are in the map after these operations?

Consider this Go code snippet. How many key-value pairs does the map contain at the end?

Go

package main
import "fmt"
func main() {
    m := make(map[int]string)
    for i := 0; i < 5; i++ {
        m[i] = fmt.Sprintf("val%d", i)
    }
    m[2] = "updated"
    delete(m, 4)
    fmt.Println(len(m))
}

A 6

B 5

C 3

D 4

💡 Hint

Remember that deleting a key reduces the map size by one.

Initially 5 keys (0 to 4) are added. Key 2 is updated but not added anew. Key 4 is deleted, so 4 keys remain.

Practice

1. What happens when you assign a value to a new key in a Go map using mapName[key] = value?

easy

A. A new key-value pair is added to the map.

B. The map is deleted.

C. The map becomes empty.

D. The program crashes.

Solution

1. Step 1: Understand map assignment

   Assigning a value to a new key in a map adds that key with the value.

2. Step 2: Differentiate from update

   If the key exists, the value updates; if not, a new pair is created.

3. Final Answer:

   A new key-value pair is added to the map. -> Option A

4. Quick Check:

   Adding new key = new pair added [OK]

Hint: Assigning to new key adds pair, not error or delete [OK]

Common Mistakes:

• Thinking map clears on new key assignment
• Confusing adding with deleting
• Assuming program crashes on new key

2. Which of the following is the correct syntax to update the value for key "age" to 30 in a Go map named person?

easy

A. person["age"] := 30

B. person["age"] = 30

C. person.age = 30

D. person.update("age", 30)

Solution

1. Step 1: Recall map update syntax

   In Go, map values are updated by assigning with = operator: map[key] = value.

2. Step 2: Check each option

   person["age"] = 30 uses correct syntax. person["age"] := 30 uses := which is for variable declaration, not map update. person.age = 30 uses dot notation which is invalid for maps. person.update("age", 30) uses a method that does not exist.

3. Final Answer:

   person["age"] = 30 -> Option B

4. Quick Check:

   Use = to update map value [OK]

Hint: Use =, not := or dot, to update map values [OK]

Common Mistakes:

• Using := instead of = for map update
• Using dot notation for map keys
• Trying to call update method on map

3. What is the output of this Go code?

m := map[string]int{"a": 1, "b": 2}
m["a"] = 5
fmt.Println(m["a"])

medium

A. 1

B. 2

C. 5

D. 0

Solution

1. Step 1: Analyze initial map values

   The map m starts with keys "a":1 and "b":2.

2. Step 2: Update value for key "a"

   m["a"] = 5 changes the value for "a" from 1 to 5.

3. Step 3: Print updated value

   fmt.Println(m["a"]) prints the updated value 5.

4. Final Answer:

   5 -> Option C

5. Quick Check:

   Updated value prints 5 [OK]

Hint: Updated map key prints new value, not old [OK]

Common Mistakes:

• Expecting old value instead of updated
• Confusing key "b" value with "a"
• Thinking print shows zero value

4. Identify the error in this Go code snippet that tries to add a new key-value pair to a map:

var m map[string]int
m["new"] = 10

medium

A. Key "new" is a reserved word and cannot be used.

B. Syntax error: missing semicolon after map declaration.

C. Value 10 must be a string, not int.

D. Map m is nil and not initialized before adding a key.

Solution

1. Step 1: Understand map initialization

   Declaring var m map[string]int creates a nil map. It must be initialized with make() before adding keys.

2. Step 2: Identify error cause

   Assigning m["new"] = 10 on a nil map causes runtime panic because map is not initialized.

3. Final Answer:

   Map m is nil and not initialized before adding a key. -> Option D

4. Quick Check:

   Maps must be initialized before adding keys [OK]

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

Common Mistakes:

• Assuming var declares initialized map
• Thinking key "new" is invalid
• Confusing value types for map

5. Given a map inventory := map[string]int{"apple": 5, "banana": 3}, write the correct Go code to add 2 more apples to the inventory without overwriting the existing count.

hard

A. inventory["apple"] += 2

B. inventory["apple"] = inventory["apple"] - 2

C. inventory["apple"] = 2

D. inventory.add("apple", 2)

Solution

1. Step 1: Understand current value

   Current count for "apple" is 5 in the map.

2. Step 2: Add 2 without overwriting

   Use += operator to add 2 to existing value: inventory["apple"] += 2.

3. Step 3: Check other options

   inventory["apple"] = 2 overwrites with 2, not add. inventory["apple"] = inventory["apple"] - 2 subtracts 2. inventory.add("apple", 2) uses a non-existent method.

4. Final Answer:

   inventory["apple"] += 2 -> Option A

5. Quick Check:

   Use += to add to map value [OK]

Hint: Use += to add to existing map value, not = [OK]

Common Mistakes:

• Overwriting value instead of adding
• Subtracting instead of adding
• Using non-existent map methods