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.

Practice

1.

What does the range keyword do when used with a map in Go?

easy

A. It creates a copy of the map.

B. It sorts the map by keys before looping.

C. It only loops over the keys, ignoring values.

D. It allows you to loop over all key-value pairs in the map.

Solution

1. Step 1: Understand the purpose of range with maps

   The range keyword in Go is used to iterate over elements in data structures like arrays, slices, and maps.

2. Step 2: Identify what happens with maps

   When used with a map, range returns both the key and the value for each item in the map.

3. Final Answer:

   It allows you to loop over all key-value pairs in the map. -> Option D

4. Quick Check:

   range with map = loop keys and values [OK]

Hint: Remember: range gives key and value for maps [OK]

Common Mistakes:

• Thinking range sorts the map
• Assuming range only returns keys
• Believing range copies the map

2.

Which of the following is the correct syntax to iterate over a map m in Go?

?

easy

A. for key, value := range m { /* code */ }

B. for (key, value) in m { /* code */ }

C. foreach key, value in m { /* code */ }

D. for key, value from m { /* code */ }

Solution

1. Step 1: Recall Go's for-range syntax

   In Go, the correct way to loop over a map is using for key, value := range mapName.

2. Step 2: Compare options

   Only for key, value := range m { /* code */ } uses the correct Go syntax with := and range keyword.

3. Final Answer:

   for key, value := range m { /* code */ } -> Option A

4. Quick Check:

   Go map iteration syntax = for key, value := range map [OK]

Hint: Use := with range to get key and value [OK]

Common Mistakes:

• Using Python or JavaScript style loops
• Missing := in the loop
• Using wrong keywords like foreach or from

3.

What will be the output of the following Go code?

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

medium

A. Either 'a 1\nb 2' or 'b 2\na 1' (order not fixed)

B. b 2\na 1

C. a 1\nb 2

D. Compilation error

Solution

1. Step 1: Understand map iteration order in Go

   Maps in Go do not guarantee order when iterating. The keys can appear in any order.

2. Step 2: Analyze the output possibilities

   The code prints each key and value pair. The output could be 'a 1' then 'b 2' or 'b 2' then 'a 1'.

3. Final Answer:

   Either 'a 1\nb 2' or 'b 2\na 1' (order not fixed) -> Option A

4. Quick Check:

   Map iteration order = unpredictable [OK]

Hint: Map order is random; output order varies [OK]

Common Mistakes:

• Assuming map iteration order is fixed
• Expecting sorted output
• Thinking code causes error

4.

Identify the error in this Go code snippet that tries to iterate over a map m:

for key, value = range m {
    fmt.Println(key, value)
}

medium

A. Using 'range' incorrectly; should be 'for each'.

B. Using '=' instead of ':=' in the for loop.

C. Missing parentheses around key and value.

D. Map 'm' is not declared.

Solution

1. Step 1: Check the for loop variable declaration

   In Go, variables declared in a for-range loop must use ':=' to declare new variables.

2. Step 2: Identify the incorrect operator

   The code uses '=' which is assignment, not declaration, causing a syntax error.

3. Final Answer:

   Using '=' instead of ':=' in the for loop. -> Option B

4. Quick Check:

   Use ':=' to declare loop variables [OK]

Hint: Use ':=' not '=' in for-range loops [OK]

Common Mistakes:

• Using '=' instead of ':='
• Adding parentheses around variables
• Confusing 'range' with other keywords

5.

Given a map m := map[string]int{"x": 0, "y": 2, "z": 0}, how can you create a new map containing only keys with non-zero values by iterating over m?

hard

A. Use range but skip keys with zero values by deleting them from original map.

B. Use for key := range m and copy all keys regardless of value.

C. Create a new map and add pairs where value != 0 inside a for-range loop.

D. Sort the map and then filter keys with zero values.

Solution

1. Step 1: Understand the goal

   We want a new map with only keys whose values are not zero.

2. Step 2: Use for-range to check each key-value pair

   Iterate over m with for k, v := range m, and add to new map only if v != 0.

3. Step 3: Avoid modifying original map

   Do not delete keys from original map during iteration; create a new map instead.

4. Final Answer:

   Create a new map and add pairs where value != 0 inside a for-range loop. -> Option C

5. Quick Check:

   Filter map by value using new map and for-range [OK]

Hint: Build new map, add only non-zero values in loop [OK]

Common Mistakes:

• Deleting keys from map while iterating
• Copying all keys without filtering
• Trying to sort a map (not supported)