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).

Practice

1. What does the updateValue(_:forKey:) method do in a Swift dictionary?

easy

A. It updates the value for a given key and returns the old value if it existed.

B. It removes the key and its value from the dictionary.

C. It returns the value for a key or a default if the key is missing.

D. It creates a new dictionary with updated keys.

Solution

1. Step 1: Understand updateValue method purpose

   The updateValue(_:forKey:) method changes the value for a specified key in the dictionary.

2. Step 2: Check return behavior

   This method returns the old value if the key existed, otherwise nil.

3. Final Answer:

   It updates the value for a given key and returns the old value if it existed. -> Option A

4. Quick Check:

   updateValue returns old value or nil [OK]

Hint: Remember updateValue returns old value if key exists [OK]

Common Mistakes:

• Confusing updateValue with removeValue
• Thinking it returns the new value
• Assuming it creates a new dictionary

2. Which of the following is the correct syntax to get a value from a dictionary with a default in Swift?

easy

A. let value = dict[key, default: 0]

B. let value = dict.get(key, default: 0)

C. let value = dict[key] ?? 0

D. let value = dict.value(forKey: key, default: 0)

Solution

1. Step 1: Recall Swift dictionary default value syntax

   Swift dictionaries allow accessing values with a default using dict[key, default: defaultValue].

2. Step 2: Compare options

   let value = dict[key, default: 0] matches the correct syntax. let value = dict[key] ?? 0 uses nil-coalescing but is not the dictionary default syntax.

3. Final Answer:

   let value = dict[key, default: 0] -> Option A

4. Quick Check:

   Use dict[key, default:] for default values [OK]

Hint: Use square brackets with default: for safe dictionary access [OK]

Common Mistakes:

• Using get() method which doesn't exist
• Confusing nil-coalescing with dictionary default syntax
• Using value(forKey:) which is for NSObject

3. What is the output of this Swift code?

var scores = ["Alice": 10, "Bob": 8]
let oldValue = scores.updateValue(12, forKey: "Alice")
let missingValue = scores.updateValue(5, forKey: "Charlie")
print(oldValue, missingValue, scores)

medium

A. nil Optional(5) ["Alice": 12, "Bob": 8, "Charlie": 5]

B. 10 5 ["Alice": 12, "Bob": 8, "Charlie": 5]

C. Optional(10) nil ["Alice": 12, "Bob": 8, "Charlie": 5]

D. Optional(12) Optional(5) ["Alice": 10, "Bob": 8, "Charlie": 5]

Solution

1. Step 1: Understand updateValue return values

   Updating "Alice" from 10 to 12 returns old value Optional(10). Adding "Charlie" returns nil because key was missing.

2. Step 2: Check dictionary after updates

   Dictionary now has "Alice":12, "Bob":8, and "Charlie":5.

3. Final Answer:

   Optional(10) nil ["Alice": 12, "Bob": 8, "Charlie": 5] -> Option C

4. Quick Check:

   updateValue returns old value or nil [OK]

Hint: updateValue returns old value wrapped in Optional or nil if new key [OK]

Common Mistakes:

• Expecting updateValue to return new value
• Forgetting Optional wrapping
• Not updating dictionary correctly

4. Identify the error in this Swift code snippet:

var dict = ["a": 1, "b": 2]
let removed = dict.removeValue(forKey: "c")
print(removed!)

medium

A. Dictionary keys must be integers, not strings.

B. Force unwrapping removed causes runtime crash if key missing.

C. removeValue(forKey:) does not exist for dictionaries.

D. print statement syntax is incorrect.

Solution

1. Step 1: Understand removeValue return type

   removeValue(forKey:) returns an Optional value. If key is missing, it returns nil.

2. Step 2: Analyze force unwrapping

   Force unwrapping removed! when key "c" is missing causes a runtime crash.

3. Final Answer:

   Force unwrapping removed causes runtime crash if key missing. -> Option B

4. Quick Check:

   Force unwrap nil causes crash [OK]

Hint: Avoid force unwrapping optional from removeValue without check [OK]

Common Mistakes:

• Assuming removeValue always returns a value
• Ignoring optional unwrapping
• Thinking removeValue removes all keys

5. You have a dictionary var inventory = ["apple": 3, "banana": 5]. Write code to safely add 2 to the count of "orange", even if it doesn't exist yet, using dictionary default values. What is the correct way?

hard

A. inventory["orange"] = inventory["orange"]! + 2

B. inventory["orange"] = inventory["orange"] ?? 0 + 2

C. inventory.updateValue(inventory["orange"]! + 2, forKey: "orange")

D. inventory["orange"] = inventory["orange", default: 0] + 2

Solution

1. Step 1: Understand safe addition with default

   Using inventory["orange", default: 0] returns 0 if "orange" key is missing.

2. Step 2: Increment value safely

   Adding 2 to this default value and assigning back to inventory["orange"] updates or creates the key safely.

3. Final Answer:

   inventory["orange"] = inventory["orange", default: 0] + 2 -> Option D

4. Quick Check:

   dict[key] = dict[key, default: 0] + value for safe increments [OK]

Hint: Use dict[key] = dict[key, default: 0] + amount to add safely [OK]

Common Mistakes:

• Force unwrapping missing keys causing crash
• Using += on read-only default subscript
• Using updateValue without checking for nil