Iterating over maps in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a map in Go?

A map in Go is a collection of key-value pairs where each key is unique. It is like a dictionary or a phone book where you look up a value using a key.

beginner

How do you start iterating over a map in Go?

You use a for loop with the range keyword. It lets you access each key and value in the map one by one.

beginner

What does the range keyword return when used on a map?

It returns two values: the key and the value for each item in the map during the loop.

intermediate

Can you control the order of iteration over a map in Go?

No, the order of iteration over a map is random and can change each time you run the program. Maps do not keep items in order.

intermediate

How do you ignore the value or the key when iterating over a map?

Use the blank identifier _ to ignore either the key or the value. For example, for k := range m ignores the value, and for _, v := range m ignores the key.

What does the range keyword return when used in a for loop over a map?

A Key and value

B Only keys

C Only values

D Index and value

When iterating over a map, range returns both the key and the value for each element.

How can you ignore the key when iterating over a map?

A Use <code>_</code> for the key: <code>for _, v := range m</code>

B Use <code>_</code> for the value: <code>for k, _ := range m</code>

C Use <code>ignoreKey()</code> function

D You cannot ignore the key

Using the blank identifier _ for the key lets you ignore it and only use the value.

Is the order of iteration over a Go map guaranteed to be the same every time?

A Yes, always the same order

B No, the order is random

C Only if the map is sorted

D Depends on the Go version

Maps in Go do not keep order, so iteration order is random and can change each run.

Which of these is the correct syntax to iterate over a map named m?

A foreach (k, v in m) { /* code */ }

B for i := 0; i < len(m); i++ { /* code */ }

C for k, v := range m { /* code */ }

D while (k, v := range m) { /* code */ }

The for k, v := range m syntax is the correct way to loop over a map in Go.

What happens if you use for k := range m?

A You get both keys and values

B You get only the values from the map

C It causes a compile error

D You get only the keys from the map

This syntax returns only the keys, ignoring the values.

Explain how to iterate over a map in Go and how to access keys and values inside the loop.

Think about how you use a for loop with range to get both key and value.

Describe why the order of iteration over a map in Go is not predictable and what that means for your code.

Consider how a phone book might be shuffled each time you look at it.

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)