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.