Iterating over maps in Go - Mini Project: Build & Apply

Iterating over maps

📖 Scenario: You are managing a small store's inventory. You have a list of products with their stock counts. You want to check which products are available and print their names with the stock count.

🎯 Goal: Build a Go program that creates a map of products and their stock counts, then iterates over the map to print each product and its stock.

📋 What You'll Learn

Create a map called inventory with string keys and int values

Add a variable called minStock to set the minimum stock to consider a product available

Use a for loop with range to iterate over inventory

Print the product name and stock count only if the stock is greater than or equal to minStock

💡 Why This Matters

🌍 Real World

Managing inventory is a common task in stores and warehouses. Knowing how to store and check stock counts helps keep track of products.

💼 Career

Many programming jobs require working with maps or dictionaries to store and process data efficiently. Iterating over maps is a fundamental skill.

1

Create the inventory map

Create a map called inventory with these exact entries: "Apples": 10, "Bananas": 5, "Oranges": 0, "Grapes": 7

Go

# Create the inventory map with the given products and stock counts
# Your code here

Hint

Use map[string]int to create the map and include the exact key-value pairs.

2

Add minimum stock variable

Add an integer variable called minStock and set it to 1 to represent the minimum stock count to consider a product available.

Go

package main

func main() {
    inventory := map[string]int{"Apples": 10, "Bananas": 5, "Oranges": 0, "Grapes": 7}
    
    // Add minStock variable here
    // Your code here
}

Hint

Declare minStock as an integer and assign it the value 1.

3

Iterate over the inventory map

Use a for loop with range to iterate over inventory with variables product and stock. Inside the loop, check if stock is greater than or equal to minStock.

Go

package main

func main() {
    inventory := map[string]int{"Apples": 10, "Bananas": 5, "Oranges": 0, "Grapes": 7}
    minStock := 1

    // Iterate over inventory and check stock
    // Your code here
}

Hint

Use for product, stock := range inventory and an if statement to check stock.

4

Print available products

Inside the if block, use fmt.Println to print the product name and stock count in this exact format: "Apples: 10". Remember to import the fmt package.

Go

package main

import "fmt"

func main() {
    inventory := map[string]int{"Apples": 10, "Bananas": 5, "Oranges": 0, "Grapes": 7}
    minStock := 1

    for product, stock := range inventory {
        if stock >= minStock {
            // Print product and stock here
            // Your code here
        }
    }
}

Hint

Use fmt.Println(product + ": " + fmt.Sprint(stock)) to print the product and stock.

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)