Dictionary methods and default values in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift code using dictionary default values?

Consider the following Swift code. What will be printed when it runs?

Swift

var scores = ["Alice": 10, "Bob": 5]

scores["Charlie", default: 0] += 3
print(scores)

A ["Alice": 10, "Bob": 5, "Charlie": 3]

B ["Alice": 10, "Bob": 5]

C ["Alice": 10, "Bob": 5, "Charlie": 0]

D Runtime error

💡 Hint

Look at how the default value is used to add or update the dictionary entry.

Using the default value 0 for key "Charlie", the code adds 3 to it, creating a new entry with value 3.

❓ Predict Output

intermediate

What does this Swift code print when using the removeValue method?

Given this Swift code, what will be printed?

Swift

var fruits = ["apple": 3, "banana": 2, "orange": 5]

let removed = fruits.removeValue(forKey: "banana")
print(removed ?? "none")
print(fruits)

A

nil
["apple": 3, "banana": 2, "orange": 5]

B

none
["apple": 3, "orange": 5]

C

2
["apple": 3, "banana": 2, "orange": 5]

D

2
["apple": 3, "orange": 5]

💡 Hint

removeValue returns the removed value or nil if key not found.

The key "banana" exists with value 2, so removeValue returns 2 and removes the key from the dictionary.

🔧 Debug

advanced

Why does this Swift code cause a runtime error?

Examine this Swift code snippet. Why does it crash at runtime?

Swift

var dict = ["x": 1, "y": 2]

let value = dict["z"]!
print(value)

A Because the dictionary is immutable and cannot be accessed.

B Because key "z" does not exist, force unwrap causes a crash.

C Because the dictionary keys must be integers, not strings.

D Because the print statement is missing parentheses.

💡 Hint

Check what happens when you force unwrap a nil optional.

Accessing a non-existent key returns nil. Force unwrapping nil causes a runtime crash.

🧠 Conceptual

advanced

How does the default value subscript work in Swift dictionaries?

Which statement best describes how the default value subscript works in Swift dictionaries?

A It always inserts the default value for the key, even if the key exists.

B It returns nil if the key is missing and does not modify the dictionary.

C It returns the value for the key if present; otherwise, it inserts the default value and returns it.

D It throws an error if the key is not found.

💡 Hint

Think about whether the dictionary changes when using the default value subscript.

The default value subscript inserts the default value if the key is missing, then returns it, allowing modification.

❓ Predict Output

expert

What is the final content of the dictionary after this Swift code runs?

Analyze the code below and determine the final dictionary content printed.

Swift

var inventory = ["pen": 3, "notebook": 5]

inventory["pen", default: 0] += 2
inventory["eraser", default: 0] += 1
inventory["notebook", default: 0] -= 3
print(inventory)

A ["pen": 5, "notebook": 2, "eraser": 1]

B ["pen": 3, "notebook": 5, "eraser": 1]

C ["pen": 5, "notebook": -3, "eraser": 1]

D ["pen": 5, "notebook": 2]

💡 Hint

Remember the default value subscript inserts missing keys and allows arithmetic updates.

The code adds 2 to "pen" (3+2=5), adds 1 to new key "eraser", and subtracts 3 from "notebook" (5-3=2).