Adding and updating values in Go - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a new key-value pair to the map.

Go

package main

import "fmt"

func main() {
	ages := map[string]int{"Alice": 30}
	ages["Bob"] = [1]
	fmt.Println(ages)
}

Drag options to blanks, or click blank then click option'

Aage

B"25"

C25

Dnil

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string value instead of an integer.

Using an undefined variable.

The value for the map key must be an integer. Here, 25 is the correct integer to add.

2fill in blank

medium

Complete the code to update the value for the key "Alice" in the map.

Go

package main

import "fmt"

func main() {
	ages := map[string]int{"Alice": 30, "Bob": 25}
	ages["Alice"] = [1]
	fmt.Println(ages)
}

Drag options to blanks, or click blank then click option'

Aage

B35

C"35"

Dnil

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string instead of an integer.

Trying to update a key that does not exist.

To update the value for "Alice", assign a new integer value like 35.

3fill in blank

hard

Fix the error in the code by completing the blank to add a new key-value pair.

Go

package main

import "fmt"

func main() {
	ages := map[string]int{"Alice": 30}
	ages[[1]] = 40
	fmt.Println(ages)
}

Drag options to blanks, or click blank then click option'

A"Bob"

BBob

C40

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Using an unquoted string as a key.

Using a value instead of a key.

Map keys of type string must be enclosed in quotes when used as literals. "Bob" is correct.

4fill in blank

hard

Fill both blanks to add a new key-value pair and update an existing one.

Go

package main

import "fmt"

func main() {
	ages := map[string]int{"Alice": 30, "Bob": 25}
	ages[[1]] = 40
	ages[[2]] = 35
	fmt.Println(ages)
}

Drag options to blanks, or click blank then click option'

A"Charlie"

B"Alice"

C"Bob"

D"David"

Attempts:

3 left

💡 Hint

Common Mistakes

Using keys without quotes.

Trying to update a key that does not exist.

"Charlie" is a new key to add, and "Bob" is an existing key to update.

5fill in blank

hard

Fill all three blanks to add a new key-value pair, update an existing one, and print a value from the updated map.

Go

package main

import "fmt"

func main() {
	ages := map[string]int{"Alice": 30, "Bob": 25}
	ages[[1]] = 40
	ages[[2]] = 35
	fmt.Println(ages[[3]])
}

Drag options to blanks, or click blank then click option'

A"Charlie"

B"Bob"

C"Alice"

D"David"

Attempts:

3 left

💡 Hint

Common Mistakes

Using keys without quotes.

Printing a key that was not added or updated.

"Charlie" is added with 40, "Alice" is updated to 35, and the value for "Bob" is printed.

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