Iterating over maps in Go - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to iterate over the map and print keys.

Go

for [1] := range myMap {
    fmt.Println([1])
}

Drag options to blanks, or click blank then click option'

Aindex

Bkey

Citem

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'value' instead of 'key' for the first variable.

Using variable names that don't represent keys.

In Go, when iterating over a map, the first variable is the key.

2fill in blank

medium

Complete the code to iterate over the map and print keys and values.

Go

for [1], value := range myMap {
    fmt.Println([1], value)
}

Drag options to blanks, or click blank then click option'

Akey

Bval

Citem

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'val' or 'value' as the first variable instead of the key.

Using the same variable name for both key and value.

The first variable in the range loop is the key, which we name 'key' here.

3fill in blank

hard

Fix the error in the code to correctly iterate over the map and print keys and values.

Go

for key, [1] := range myMap {
    fmt.Println(key, [1])
}

Drag options to blanks, or click blank then click option'

Akey

Bitem

Cvalue

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'key' for both variables.

Using variable names that don't represent values.

The second variable in the range loop is the value, so use 'value'.

4fill in blank

hard

Fill both blanks to create a map comprehension that stores lengths of keys longer than 3 characters.

Go

lengths := map[string]int{}
for [1], [2] := range myMap {
    if len([1]) > 3 {
        lengths[[1]] = len([1])
    }
}

Drag options to blanks, or click blank then click option'

Akey

Bvalue

Citem

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same variable for both blanks.

Using 'item' or 'index' which are not typical for maps.

Use 'key' and 'value' to iterate over the map. The key is used to check length and store in the new map.

5fill in blank

hard

Fill all three blanks to create a map that stores uppercase keys with their values if the value is greater than 10.

Go

filtered := map[string]int{}
for [1], [2] := range myMap {
    if [2] > 10 {
        filtered[strings.ToUpper([1])] = [2]
    }
}

Drag options to blanks, or click blank then click option'

Akey

Bvalue

Cval

Ditem

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently.

Using the key variable where the value is needed.

Use 'key' and 'value' to iterate. Check if value > 10, then store with uppercase key and value.

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)