Iterating over maps in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of iterating over a map with range

What is the output of this Go program?

Go

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

A The output order is random but all key:value pairs are printed separated by spaces

B c:3 b:2 a:1

C a:1 b:2 c:3

D Compilation error due to map iteration

💡 Hint

Maps in Go do not guarantee order when iterating.

In Go, iterating over a map with range produces keys in a random order each time. All key:value pairs are printed but order is not fixed.

❓ Predict Output

intermediate

Counting items in a map during iteration

What is the value of count after running this Go code?

Go

package main
import "fmt"
func main() {
    m := map[int]string{1: "one", 2: "two", 3: "three"}
    count := 0
    for range m {
        count++
    }
    fmt.Println(count)
}

A Runtime panic

B 0

C 3

D Compilation error due to missing variables in range

💡 Hint

Range over a map iterates over all keys.

The loop runs once for each key in the map, so count increments 3 times.

❓ Predict Output

advanced

Modifying map values during iteration

What is the output of this Go program?

Go

package main
import "fmt"
func main() {
    m := map[string]int{"x": 1, "y": 2}
    for k := range m {
        m[k] = m[k] * 10
    }
    fmt.Println(m)
}

A map[x:1 y:2]

B map[x:10 y:20]

C Compilation error: cannot assign to map element during iteration

D Runtime panic: concurrent map iteration and map write

💡 Hint

You can modify map values during iteration safely in Go.

Go allows modifying map values during iteration without error. The values are updated as expected.

❓ Predict Output

advanced

Iterating over a nil map

What happens when this Go code runs?

Go

package main
import "fmt"
func main() {
    var m map[string]int
    for k, v := range m {
        fmt.Println(k, v)
    }
    fmt.Println("Done")
}

A Prints nothing then prints "Done"

B Runtime panic: nil map iteration

C Compilation error: map not initialized

D Infinite loop

💡 Hint

Iterating over a nil map is safe and acts like empty map.

In Go, iterating over a nil map does nothing and does not panic. The program prints "Done".

🧠 Conceptual

expert

Why is map iteration order random in Go?

Which reason best explains why Go's map iteration order is randomized?

A To guarantee keys are sorted alphabetically during iteration

B Because Go does not allow iteration over maps

C Because Go maps are implemented as linked lists which have no order

D To prevent programs from relying on order and to improve security by avoiding predictable iteration

💡 Hint

Think about why predictable order might be a problem.

Go randomizes map iteration order to discourage code relying on order and to reduce certain security risks.