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.