Accessing map values in Go - Mini Project: Build & Apply

Accessing map values

📖 Scenario: You are managing a small store's inventory. You want to check the quantity of certain products quickly.

🎯 Goal: Build a Go program that creates a map of products and their quantities, then accesses and prints the quantity of a specific product.

📋 What You'll Learn

Create a map with product names as keys and quantities as values

Access the quantity of a specific product using its key

Print the quantity to the console

💡 Why This Matters

🌍 Real World

Maps are useful for storing and looking up data quickly, like product inventories or user information.

💼 Career

Understanding maps and how to access their values is essential for many programming tasks in software development.

1

Create the product inventory map

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

Go

// Your code here

Hint

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

2

Create a variable for the product to check

Create a variable called product and set it to the string "banana".

Go

package main

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

Hint

Use product := "banana" to create the variable.

3

Access the quantity of the product

Create a variable called quantity and set it to the value from inventory for the key stored in product.

Go

package main

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

Hint

Use quantity := inventory[product] to get the value from the map.

4

Print the quantity

Use fmt.Println to print the value of quantity.

Go

package main

import "fmt"

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

Hint

Use fmt.Println(quantity) to print the quantity.

Practice

1. What does the expression value, ok := myMap["key"] do in Go?

easy

A. It retrieves the value for "key" and checks if the key exists in the map.

B. It deletes the key "key" from the map.

C. It adds a new key "key" with value "ok" to the map.

D. It returns the number of keys in the map.

Solution

1. Step 1: Understand map access syntax

   In Go, value, ok := myMap["key"] retrieves the value for the key and a boolean indicating if the key exists.

2. Step 2: Explain the boolean ok

   The ok is true if the key is present, false otherwise.

3. Final Answer:

   It retrieves the value for "key" and checks if the key exists in the map. -> Option A

4. Quick Check:

   Map access with existence check = value, ok [OK]

Hint: Remember: value, ok checks key presence in map [OK]

Common Mistakes:

• Thinking it deletes the key
• Assuming it adds a key
• Confusing it with map length

2. Which of the following is the correct way to check if a key exists in a Go map named m for key k?

easy

A. value, ok := m[k]; if ok { ... }

B. value := m[k]; if value != nil { ... }

C. if m.contains(k) { ... }

D. if m[k] != 0 { ... }

Solution

1. Step 1: Recall Go map existence check syntax

   In Go, to check if a key exists, use value, ok := m[k] and then check ok.

2. Step 2: Analyze options

   value := m[k]; if value != nil { ... } wrongly assumes value is nil for missing keys, which is not always true. if m.contains(k) { ... } uses a method that does not exist. if m[k] != 0 { ... } checks value but zero value can be valid.

3. Final Answer:

   value, ok := m[k]; if ok { ... } -> Option A

4. Quick Check:

   Use value, ok to check key presence [OK]

Hint: Use value, ok := map[key] to check key existence [OK]

Common Mistakes:

• Checking value against nil or zero instead of ok
• Using non-existent map methods
• Assuming zero value means key missing

3. What will be the output of this Go code?

m := map[string]int{"a": 10, "b": 20}
value, ok := m["c"]
fmt.Println(value, ok)

medium

A. Compilation error

B. 0 true

C. 20 true

D. 0 false

Solution

1. Step 1: Understand map lookup for missing key

   Key "c" is not in map m, so value gets zero value for int, which is 0, and ok is false.

2. Step 2: Print output values

   Printing value and ok prints "0 false".

3. Final Answer:

   0 false -> Option D

4. Quick Check:

   Missing key returns zero value and false [OK]

Hint: Missing map keys return zero value and false [OK]

Common Mistakes:

• Assuming missing key returns true
• Expecting runtime error for missing key
• Confusing zero value with actual stored value

4. Identify the error in this Go code snippet:

m := map[string]int{"x": 5}
value := m["y"]
if value {
    fmt.Println("Key exists")
}

medium

A. Missing declaration of map m.

B. Map key "y" does not exist, causing runtime panic.

C. Cannot use int value as boolean in if condition.

D. Syntax error in map initialization.

Solution

1. Step 1: Analyze the if condition

   The variable value is an int, but Go requires a boolean expression in if conditions.

2. Step 2: Understand Go type rules

   Using an int directly as a boolean causes a compile-time error.

3. Final Answer:

   Cannot use int value as boolean in if condition. -> Option C

4. Quick Check:

   if conditions need boolean, not int [OK]

Hint: if needs boolean, not int value [OK]

Common Mistakes:

• Assuming missing key causes panic
• Ignoring type mismatch in if condition
• Thinking map declaration is missing

5. Given a map m := map[string]int{"a": 1, "b": 0, "c": 3}, how can you print only keys that exist and have a non-zero value?

hard

A. for k := range m { if m[k] { fmt.Println(k) } }

B. for k, v := range m { if v != 0 { fmt.Println(k) } }

C. for k, v := range m { if m[k] != nil { fmt.Println(k) } }

D. for k, v := range m { if v { fmt.Println(k) } }

Solution

1. Step 1: Iterate over map with key and value

   Use for k, v := range m to get keys and values.

2. Step 2: Check for non-zero values

   Check if v != 0 to filter keys with non-zero values.

3. Step 3: Print keys that meet condition

   Print the key k inside the if block.

4. Final Answer:

   for k, v := range m { if v != 0 { fmt.Println(k) } } -> Option B

5. Quick Check:

   Filter keys by value != 0 using range loop [OK]

Hint: Use for k,v := range map and check v != 0 [OK]

Common Mistakes:

• Using value as boolean directly
• Checking map key against nil
• Ignoring zero values in condition