Adding and updating values in Go - Mini Project: Build & Apply

Adding and updating values

📖 Scenario: You are managing a small store's inventory. You want to keep track of how many items you have for each product.

🎯 Goal: Create a Go program that stores product quantities in a map, adds a new product, updates the quantity of an existing product, and then shows the final inventory.

📋 What You'll Learn

Create a map called inventory with initial products and quantities

Add a new product with a quantity to the inventory map

Update the quantity of an existing product in the inventory map

Print the final inventory map

💡 Why This Matters

🌍 Real World

Stores and businesses often track their inventory using maps or dictionaries to know how many items they have.

💼 Career

Understanding how to add and update values in maps is essential for managing data in many programming jobs, especially in backend development and data processing.

1

Create the initial inventory map

Create a map called inventory with these exact entries: "apple": 10, "banana": 5, and "orange": 8.

Go

// Create the inventory map with initial products
// Your code here

Hint

Use map[string]int to create a map with string keys and int values.

2

Add a new product to the inventory

Add a new product "grape" with quantity 12 to the inventory map.

Go

package main

import "fmt"

func main() {
    inventory := map[string]int{"apple": 10, "banana": 5, "orange": 8}
    // Add grape with quantity 12
    // Your code here
}

Hint

Use inventory["grape"] = 12 to add a new key and value to the map.

3

Update the quantity of an existing product

Update the quantity of "banana" in the inventory map to 7.

Go

package main

import "fmt"

func main() {
    inventory := map[string]int{"apple": 10, "banana": 5, "orange": 8}
    inventory["grape"] = 12
    // Update banana quantity to 7
    // Your code here
}

Hint

Assign the new quantity to inventory["banana"].

4

Print the final inventory

Print the inventory map to show all products and their quantities.

Go

package main

import "fmt"

func main() {
    inventory := map[string]int{"apple": 10, "banana": 5, "orange": 8}
    inventory["grape"] = 12
    inventory["banana"] = 7
    // Print the inventory map
    // Your code here
}

Hint

Use fmt.Println(inventory) to print the map.

Practice

1. What happens when you assign a value to a new key in a Go map using mapName[key] = value?

easy

A. A new key-value pair is added to the map.

B. The map is deleted.

C. The map becomes empty.

D. The program crashes.

Solution

1. Step 1: Understand map assignment

   Assigning a value to a new key in a map adds that key with the value.

2. Step 2: Differentiate from update

   If the key exists, the value updates; if not, a new pair is created.

3. Final Answer:

   A new key-value pair is added to the map. -> Option A

4. Quick Check:

   Adding new key = new pair added [OK]

Hint: Assigning to new key adds pair, not error or delete [OK]

Common Mistakes:

• Thinking map clears on new key assignment
• Confusing adding with deleting
• Assuming program crashes on new key

2. Which of the following is the correct syntax to update the value for key "age" to 30 in a Go map named person?

easy

A. person["age"] := 30

B. person["age"] = 30

C. person.age = 30

D. person.update("age", 30)

Solution

1. Step 1: Recall map update syntax

   In Go, map values are updated by assigning with = operator: map[key] = value.

2. Step 2: Check each option

   person["age"] = 30 uses correct syntax. person["age"] := 30 uses := which is for variable declaration, not map update. person.age = 30 uses dot notation which is invalid for maps. person.update("age", 30) uses a method that does not exist.

3. Final Answer:

   person["age"] = 30 -> Option B

4. Quick Check:

   Use = to update map value [OK]

Hint: Use =, not := or dot, to update map values [OK]

Common Mistakes:

• Using := instead of = for map update
• Using dot notation for map keys
• Trying to call update method on map

3. What is the output of this Go code?

m := map[string]int{"a": 1, "b": 2}
m["a"] = 5
fmt.Println(m["a"])

medium

A. 1

B. 2

C. 5

D. 0

Solution

1. Step 1: Analyze initial map values

   The map m starts with keys "a":1 and "b":2.

2. Step 2: Update value for key "a"

   m["a"] = 5 changes the value for "a" from 1 to 5.

3. Step 3: Print updated value

   fmt.Println(m["a"]) prints the updated value 5.

4. Final Answer:

   5 -> Option C

5. Quick Check:

   Updated value prints 5 [OK]

Hint: Updated map key prints new value, not old [OK]

Common Mistakes:

• Expecting old value instead of updated
• Confusing key "b" value with "a"
• Thinking print shows zero value

4. Identify the error in this Go code snippet that tries to add a new key-value pair to a map:

var m map[string]int
m["new"] = 10

medium

A. Key "new" is a reserved word and cannot be used.

B. Syntax error: missing semicolon after map declaration.

C. Value 10 must be a string, not int.

D. Map m is nil and not initialized before adding a key.

Solution

1. Step 1: Understand map initialization

   Declaring var m map[string]int creates a nil map. It must be initialized with make() before adding keys.

2. Step 2: Identify error cause

   Assigning m["new"] = 10 on a nil map causes runtime panic because map is not initialized.

3. Final Answer:

   Map m is nil and not initialized before adding a key. -> Option D

4. Quick Check:

   Maps must be initialized before adding keys [OK]

Hint: Always initialize maps with make() before adding keys [OK]

Common Mistakes:

• Assuming var declares initialized map
• Thinking key "new" is invalid
• Confusing value types for map

5. Given a map inventory := map[string]int{"apple": 5, "banana": 3}, write the correct Go code to add 2 more apples to the inventory without overwriting the existing count.

hard

A. inventory["apple"] += 2

B. inventory["apple"] = inventory["apple"] - 2

C. inventory["apple"] = 2

D. inventory.add("apple", 2)

Solution

1. Step 1: Understand current value

   Current count for "apple" is 5 in the map.

2. Step 2: Add 2 without overwriting

   Use += operator to add 2 to existing value: inventory["apple"] += 2.

3. Step 3: Check other options

   inventory["apple"] = 2 overwrites with 2, not add. inventory["apple"] = inventory["apple"] - 2 subtracts 2. inventory.add("apple", 2) uses a non-existent method.

4. Final Answer:

   inventory["apple"] += 2 -> Option A

5. Quick Check:

   Use += to add to map value [OK]

Hint: Use += to add to existing map value, not = [OK]

Common Mistakes:

• Overwriting value instead of adding
• Subtracting instead of adding
• Using non-existent map methods