Extending built-in types in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a method that returns the square of an Int.

Swift

extension Int {
    func squared() -> Int {
        return self [1] self
    }
}

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

A-

B+

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using + instead of * will add the number to itself, not square it.

Using - or / will cause incorrect results or errors.

The * operator multiplies the integer by itself to get the square.

2fill in blank

medium

Complete the code to add a computed property that returns the number of characters in a String.

Swift

extension String {
    var length: Int {
        return [1]
    }
}

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

Acount

Bself.length

Clength

Dself.count

Attempts:

3 left

💡 Hint

Common Mistakes

Using just count without self causes an error.

Using length is not a valid property in Swift strings.

self.count returns the number of characters in the string instance.

3fill in blank

hard

Fix the error in the extension that adds a method to check if an Int is even.

Swift

extension Int {
    func isEven() -> Bool {
        return self [1] 2 == 0
    }
}

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

A%

B/

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using division / instead of modulo causes a type mismatch.

Using multiplication or addition operators will not check evenness.

The modulo operator % returns the remainder. If remainder is 0 when divided by 2, the number is even.

4fill in blank

hard

Fill both blanks to create an extension that returns a new String with the first letter capitalized and the rest lowercase.

Swift

extension String {
    func capitalizedFirst() -> String {
        return self.prefix(1).[1]() + self.dropFirst().[2]()
    }
}

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

Auppercased

Blowercased

Ccapitalized

DtrimmingCharacters

Attempts:

3 left

💡 Hint

Common Mistakes

Using capitalized() on the first letter only does not work.

Using trimmingCharacters() is unrelated to case changes.

uppercased() makes the first letter uppercase, and lowercased() makes the rest lowercase.

5fill in blank

hard

Fill all three blanks to create an extension that returns a dictionary with characters as keys and their counts as values, filtering only characters that appear more than once.

Swift

extension String {
    func repeatedCharacters() -> [Character: Int] {
        let counts = self.reduce(into: [:]) { counts, char in
            counts[char, default: [1]] += 1
        }
        return counts.filter { $0.value [2] [3] }
    }
}

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

A0

B>

C1

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing counts with 1 causes incorrect counts.

Using equality instead of greater than filters only characters appearing exactly once.

Initialize counts with 0, then filter characters with count greater than 1.

Practice

1. What is the main purpose of extending built-in types in Swift?

easy

A. To change the stored properties of existing types

B. To delete existing methods from built-in types

C. To add new methods or computed properties to existing types

D. To create subclasses of built-in types

Solution

1. Step 1: Understand what extensions do

   Extensions allow adding new methods or computed properties to existing types without subclassing.

2. Step 2: Identify what extensions cannot do

   Extensions cannot add stored properties or delete existing methods.

3. Final Answer:

   To add new methods or computed properties to existing types -> Option C

4. Quick Check:

   Extensions add methods/properties = B [OK]

Hint: Remember: extensions add features, not stored data [OK]

Common Mistakes:

• Thinking extensions can add stored properties
• Confusing extensions with subclassing
• Believing extensions can remove methods

2. Which of the following is the correct syntax to extend the built-in Int type with a method isEven() that returns a Bool?

easy

A. extension Int { func isEven() -> Bool { return self % 2 == 0 } }

B. extend Int { func isEven() -> Bool { return self % 2 == 0 } }

C. extension Int { func isEven { return self % 2 == 0 } }

D. extension Int { isEven() -> Bool { return self % 2 == 0 } }

Solution

1. Step 1: Identify the correct keyword for extension

   Swift uses the keyword extension to add methods to existing types.

2. Step 2: Check method syntax

   The method must have parentheses and a return type: func isEven() -> Bool. The body uses self % 2 == 0.

3. Final Answer:

   extension Int { func isEven() -> Bool { return self % 2 == 0 } } -> Option A

4. Quick Check:

   Correct extension syntax = D [OK]

Hint: Use 'extension' keyword and full method signature [OK]

Common Mistakes:

• Using 'extend' instead of 'extension'
• Omitting parentheses in method declaration
• Missing 'func' keyword before method name

3. Consider this Swift code:

extension String {
    func reversedString() -> String {
        return String(self.reversed())
    }
}

let word = "hello"
print(word.reversedString())

What is the output when this code runs?

medium

A. Error: reversed() not found

B. hello

C. nil

D. olleh

Solution

1. Step 1: Understand the extension method

   The method reversedString() returns a new String made by reversing the characters of the original string.

2. Step 2: Apply method to 'hello'

   Reversing "hello" results in "olleh".

3. Final Answer:

   olleh -> Option D

4. Quick Check:

   String reversed = olleh [OK]

Hint: Remember reversed() returns characters reversed [OK]

Common Mistakes:

• Expecting original string output
• Thinking reversed() returns an error
• Confusing reversed() with reverse()

4. What is wrong with this Swift extension code?

extension Double {
    var squared: Double {
        return self * self
    }

    var storedValue: Double = 0.0
}

medium

A. Extensions cannot add stored properties like 'storedValue'

B. Computed properties must use 'func' keyword

C. The 'squared' property must be a method, not a property

D. The extension must be declared as 'class extension'

Solution

1. Step 1: Review extension property rules

   Extensions can add computed properties but cannot add stored properties.

2. Step 2: Identify the stored property

   The line var storedValue: Double = 0.0 tries to add a stored property, which is not allowed.

3. Final Answer:

   Extensions cannot add stored properties like 'storedValue' -> Option A

4. Quick Check:

   No stored properties in extensions = A [OK]

Hint: Remember: only computed properties allowed in extensions [OK]

Common Mistakes:

• Trying to add stored properties in extensions
• Confusing computed properties with stored properties
• Thinking extensions require 'class' keyword

5. You want to extend the built-in Array type in Swift to add a method secondElement() that returns the second element if it exists, or nil otherwise. Which of the following extensions correctly implements this?

hard

A. extension Array {
func secondElement() -> Element {
return self[1]
}
}

B. extension Array {
func secondElement() -> Element? {
return self.count > 1 ? self[1] : nil
}
}

C. extension Array {
var secondElement: Element? {
if self.count >= 2 { return self[2] } else { return nil }
}
}

D. extension Array {
func secondElement() -> Element? {
if self.count > 0 { return self[1] } else { return nil }
}
}

Solution

1. Step 1: Check method signature and return type

   The method should return an optional Element (Element?) because the second element might not exist.

2. Step 2: Verify index and condition

   Arrays are zero-indexed, so the second element is at index 1. The condition should check if count > 1 to safely access index 1.

3. Step 3: Compare options

   The correct one uses ternary operator: return self.count > 1 ? self[1] : nil. The non-optional return crashes on short arrays. The computed property uses index 2 (third element). The if-else checks count > 0, allowing index 1 access when count == 1, causing runtime crash.

4. Final Answer:

   extension Array { func secondElement() -> Element? { return self.count > 1 ? self[1] : nil } } -> Option B

5. Quick Check:

   Safe optional return with index 1 = A [OK]

Hint: Check array count > 1 and return optional Element [OK]

Common Mistakes:

• Accessing wrong index (like 2 instead of 1)
• Returning non-optional Element causing crash
• Using computed property but with wrong index