Iterating over maps in Go

Introduction

We use iteration to look at each item in a map one by one. This helps us work with all the data stored in the map.

When you want to print all keys and values in a map.

When you need to find a specific value by checking each item.

When you want to change or use every value in the map.

When you want to count or summarize data stored in a map.

Syntax

Go

for key, value := range mapName {
    // use key and value here
}

The range keyword helps go through each item in the map.

You get both the key and value for each item during the loop.

Examples

This prints each key and value in the map.

Go

for k, v := range myMap {
    fmt.Println(k, v)
}

This prints only the keys, ignoring the values.

Go

for k := range myMap {
    fmt.Println(k)
}

This prints only the values, ignoring the keys.

Go

for _, v := range myMap {
    fmt.Println(v)
}

Sample Program

This program creates a map of fruit codes to fruit names. It then loops over the map and prints each key and its value.

Go

package main

import "fmt"

func main() {
    fruits := map[string]string{
        "a": "Apple",
        "b": "Banana",
        "c": "Cherry",
    }

    for key, value := range fruits {
        fmt.Printf("Key: %s, Value: %s\n", key, value)
    }
}

Important Notes

The order of items when iterating over a map is random each time.

You can ignore the key or value by using an underscore _ if you don't need it.

Summary

Use for key, value := range map to look at all items in a map.

You get both keys and values during the loop.

The order of items is not fixed, so don't rely on 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)