Shorthand argument names ($0, $1) in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of map using shorthand argument names

What is the output of this Swift code using shorthand argument names?

Swift

let numbers = [1, 2, 3]
let doubled = numbers.map { $0 * 2 }
print(doubled)

A [0, 2, 4]

B [2, 4, 6]

C [1, 2, 3]

D Compilation error

💡 Hint

Remember $0 refers to the first argument in the closure.

The map function applies the closure to each element. $0 is each element, so multiplying by 2 doubles each number.

❓ Predict Output

intermediate

Using $0 and $1 in sorted(by:)

What does this Swift code print?

Swift

let words = ["apple", "banana", "cherry"]
let sortedWords = words.sorted { $0.count < $1.count }
print(sortedWords)

A ["apple", "cherry", "banana"]

B ["apple", "banana", "cherry"]

C ["banana", "apple", "cherry"]

D Runtime error

💡 Hint

Check the length of each word and how the closure compares them.

The closure compares the length of two words using $0 and $1. It sorts words by length ascending. Since "banana" and "cherry" have equal lengths, Swift's stable sorting preserves their relative order from the original array.

🔧 Debug

advanced

Identify the error with shorthand argument names

Why does this Swift code cause a compilation error?

Swift

let pairs = [(1, 2), (3, 4)]
let sums = pairs.map { $0 + $1 }
print(sums)

A Cannot use $1 because map closure takes only one argument

B Cannot add tuples directly

C Missing return keyword in closure

D Pairs array is empty

💡 Hint

map closure receives one argument which is a tuple, not two separate arguments.

The map closure receives each tuple as a single argument ($0). There is no $1 because the closure has only one parameter.

❓ Predict Output

advanced

Output of reduce with shorthand arguments

What is the output of this Swift code?

Swift

let numbers = [1, 2, 3, 4]
let result = numbers.reduce(0) { $0 + $1 }
print(result)

A 1

B 24

C 10

D Compilation error

💡 Hint

reduce starts with initial value 0 and adds each element using $0 and $1.

reduce accumulates the sum starting from 0. $0 is the running total, $1 is the current element.

🧠 Conceptual

expert

Understanding shorthand argument names in nested closures

Consider this Swift code snippet. What is the output printed?

Swift

let array = [[1, 2], [3, 4]]
let flattened = array.flatMap { $0.map { $0 * 2 } }
print(flattened)

A [1, 2, 3, 4]

B [[2, 4], [6, 8]]

C Compilation error due to ambiguous $0

D [2, 4, 6, 8]

💡 Hint

The inner closure uses $0 for elements inside each subarray, the outer closure uses $0 for each subarray.

The outer flatMap takes each subarray ($0) and maps its elements multiplying by 2. flatMap then flattens the result into a single array.

Practice

1. What does the shorthand argument $0 represent in a Swift closure?

easy

A. The last argument passed to the closure

B. The first argument passed to the closure

C. The closure itself

D. The return value of the closure

Solution

1. Step 1: Understand shorthand argument names

   In Swift closures, $0 refers to the first argument passed to the closure.

2. Step 2: Identify $0 meaning

   Since $0 is always the first argument, it represents the first input value the closure receives.

3. Final Answer:

   The first argument passed to the closure -> Option B

4. Quick Check:

   $0 = first argument [OK]

Hint: Remember $0 is always the first input in closures [OK]

Common Mistakes:

• Confusing $0 with the return value
• Thinking $0 is the closure itself
• Mixing $0 with the last argument

2. Which of the following is the correct way to use shorthand argument names to sort an array of integers in ascending order?

easy

A. let sorted = numbers.sorted { $1 > $0 }

B. let sorted = numbers.sorted { $1 < $0 }

C. let sorted = numbers.sorted { $0 > $1 }

D. let sorted = numbers.sorted { $0 < $1 }

Solution

1. Step 1: Understand sorting with closures

   To sort ascending, the closure should return true if the first element is less than the second.

2. Step 2: Match shorthand arguments to sorting logic

   Using $0 < $1 means the first element is less than the second, which sorts ascending.

3. Final Answer:

   let sorted = numbers.sorted { $0 < $1 } -> Option D

4. Quick Check:

   Ascending sort uses $0 < $1 [OK]

Hint: Use $0 < $1 for ascending sort closures [OK]

Common Mistakes:

• Reversing $0 and $1 in comparison
• Using > instead of < for ascending
• Confusing sorting order logic

3. What is the output of the following Swift code?

let names = ["Anna", "Bob", "Charlie"]
let reversed = names.map { String($0.reversed()) }
print(reversed)

medium

A. ["annA", "boB", "eilrahc"]

B. ["Anna", "Bob", "Charlie"]

C. ["annA", "boB", "eilrahC"]

D. ["a n n A", "b o B", "c h a r l i e"]

Solution

1. Step 1: Understand the map and reversed usage

   The map applies the closure to each name. $0 is each string. $0.reversed() reverses characters.

2. Step 2: Convert reversed characters back to String

   Wrapping String($0.reversed()) creates a reversed string for each name.

3. Final Answer:

   ["annA", "boB", "eilrahC"] -> Option C

4. Quick Check:

   map + reversed string = reversed names [OK]

Hint: Remember map applies closure to each element using $0 [OK]

Common Mistakes:

• Forgetting to convert reversed characters back to String
• Expecting original array without changes
• Confusing reversed string with spaced characters

4. Identify the error in this Swift code using shorthand argument names:

let numbers = [3, 1, 2]
let sorted = numbers.sorted { $2 < $1 }
print(sorted)

medium

A. Using $2 which does not exist in this closure

B. Missing return keyword in closure

C. Incorrect array declaration syntax

D. Using sorted instead of sort method

Solution

1. Step 1: Check shorthand argument usage

   Shorthand arguments start at $0 for the first, $1 for the second. There is no $2 in a two-argument closure.

2. Step 2: Identify invalid argument index

   Using $2 causes a compile error because only $0 and $1 exist here.

3. Final Answer:

   Using $2 which does not exist in this closure -> Option A

4. Quick Check:

   Only $0 and $1 allowed in two-argument closures [OK]

Hint: Use only $0 and $1 for two-parameter closures [OK]

Common Mistakes:

• Assuming $2 exists in two-argument closures
• Forgetting shorthand argument limits
• Confusing sorted and sort method differences

5. Given the array let pairs = [(3, 5), (1, 2), (4, 4)], which closure using shorthand argument names correctly sorts the array by the sum of each tuple's elements in descending order?

hard

A. pairs.sorted { $0.0 + $0.1 > $1.0 + $1.1 }

B. pairs.sorted { $1.0 + $1.1 > $0.0 + $0.1 }

C. pairs.sorted { $0 + $1 > $1 + $0 }

D. pairs.sorted { $0.0 > $1.1 }

Solution

1. Step 1: Understand tuple access and sorting logic

   Each element is a tuple. To sort by sum descending, compare sums of $0 and $1 tuples.

2. Step 2: Check each option's correctness

   pairs.sorted { $0.0 + $0.1 > $1.0 + $1.1 } sums $0.0 + $0.1 and compares to $1.0 + $1.1 with > for descending order, which is correct.

3. Final Answer:

   pairs.sorted { $0.0 + $0.1 > $1.0 + $1.1 } -> Option A

4. Quick Check:

   Sum tuples and use > for descending sort [OK]

Hint: Sum tuple elements with $0.0 + $0.1 for sorting [OK]

Common Mistakes:

• Mixing $0 and $1 order in comparison
• Trying to add tuples directly without accessing elements
• Using incorrect tuple element indices