Shorthand argument names ($0, $1) in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print the squares of numbers using shorthand argument names.

Swift

let numbers = [1, 2, 3, 4]
let squares = numbers.map { $[1] * $[1] }
print(squares)

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

A2

B1

C0

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using $1 when there is only one argument.

Using variable names instead of shorthand arguments.

The shorthand argument $0 refers to the first argument passed to the closure, which is each element in the array.

2fill in blank

medium

Complete the code to filter numbers greater than 3 using shorthand argument names.

Swift

let numbers = [1, 2, 3, 4, 5]
let filtered = numbers.filter { $[1] > 3 }
print(filtered)

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

A0

B1

Cindex

Dself

Attempts:

3 left

💡 Hint

Common Mistakes

Using $1 when only one argument is passed.

Using variable names instead of shorthand arguments.

The closure passed to filter takes each element as the first argument, accessible via $0.

3fill in blank

hard

Fix the error in the code by using the correct shorthand argument name to sort pairs by their second value.

Swift

let pairs = [("a", 3), ("b", 1), ("c", 2)]
let sorted = pairs.sorted { $[1].1 < $[2].1 }
print(sorted)

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

A$0, $1

B0, 1

C1, 2

D$1, $0

Attempts:

3 left

💡 Hint

Common Mistakes

Using numeric literals instead of shorthand arguments.

Swapping $0 and $1 incorrectly.

The closure for sorted takes two arguments, accessible as $0 and $1. We compare their second elements with .1.

4fill in blank

hard

Fill both blanks to create a closure that adds two numbers using shorthand argument names.

Swift

let add = { $[1] + $[2] }
print(add(5, 7))

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

A$0

B$1

C0

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using numeric literals instead of shorthand arguments.

Using the same shorthand argument for both blanks.

The closure takes two arguments, accessible as $0 and $1, which we add together.

5fill in blank

hard

Fill all three blanks to create a dictionary from an array of tuples using shorthand argument names.

Swift

let items = [("apple", 3), ("banana", 5), ("cherry", 2)]
let dict = Dictionary(uniqueKeysWithValues: items.map { ( $[1].0, $[2].1 ) })
print(dict)

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

A$0

B$1

Attempts:

3 left

💡 Hint

Common Mistakes

Using $1 when only one argument is passed.

Mixing up tuple element indices.

The map closure takes one argument (a tuple), accessible as $0. We use $0.0 for the key and $0.1 for the value.

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