Accessing map values in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of accessing a missing key in a map?

Consider the following Go code. What will be printed when accessing a key that does not exist in the map?

Go

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

A nil

B 0

C panic: runtime error: invalid memory address or nil pointer dereference

D Compilation error

💡 Hint

In Go, accessing a map with a key that does not exist returns the zero value of the map's value type.

When you access a map with a key that is not present, Go returns the zero value for the value type. Since the map stores ints, the zero value is 0.

❓ Predict Output

intermediate

How to check if a key exists in a map?

What will be 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{"x": 10}
    val, ok := m["y"]
    fmt.Println(val, ok)
}

A 0 false

B 0 true

C Compilation error

D 10 false

💡 Hint

The second value returned when accessing a map key tells if the key exists.

Since "y" is not a key in the map, val is zero value 0 and ok is false.

🔧 Debug

advanced

Why does this code panic when accessing a map?

This Go code panics at runtime. What is the cause?

Go

package main
import "fmt"
func main() {
    var m map[string]int
    fmt.Println(m["key"])
}

A The key "key" does not exist, so it returns zero value 0 without panic.

B The code has a syntax error due to missing map initialization.

C The map m is nil and not initialized, so accessing it causes a panic.

D The program compiles but prints garbage value.

💡 Hint

Maps must be initialized before use to avoid runtime panic.

A nil map does not cause a runtime panic when accessed for reading. Accessing a nil map returns the zero value for the value type. However, writing to a nil map causes a panic.

❓ Predict Output

advanced

What is the output when modifying a map value?

What will this Go program print?

Go

package main
import "fmt"
func main() {
    m := map[string]int{"a": 1}
    m["a"] += 5
    fmt.Println(m["a"])
}

A Compilation error: cannot assign to map element

B 1

C 6

D Runtime panic

💡 Hint

You can update map values by accessing and assigning to the key.

The value for key "a" is incremented by 5, so 1 + 5 = 6 is printed.

❓ Predict Output

expert

What is the output of this map access with struct keys?

Given this Go code, what will be printed?

Go

package main
import "fmt"

type Point struct { X, Y int }

func main() {
    m := map[Point]string{
        {X: 1, Y: 2}: "A",
        {X: 3, Y: 4}: "B",
    }
    p := Point{X: 1, Y: 2}
    fmt.Println(m[p])
}

A B

B Empty string

C Compilation error: invalid map key type

D A

💡 Hint

Structs can be used as map keys if all their fields are comparable.

The map has a key Point{1,2} with value "A", so printing m[p] outputs "A".