Sorted and custom comparators in Swift - Mini Project: Build & Apply

Sorting a List of Fruits with Custom Order in Swift

📖 Scenario: You have a list of fruits, but you want to sort them in a special order that you decide, not just alphabetically.

🎯 Goal: Build a Swift program that sorts a list of fruits using a custom order you define.

📋 What You'll Learn

Create an array called fruits with the exact values: "apple", "banana", "cherry", "date", "elderberry"

Create a dictionary called priority that assigns each fruit a number representing its custom order

Use the sorted(by:) method with a custom comparator closure to sort fruits by the priority values

Print the sorted array exactly as shown

💡 Why This Matters

🌍 Real World

Sorting items by custom rules is common in apps like shopping lists, games, or task managers where order matters beyond simple alphabetical sorting.

💼 Career

Understanding how to use custom comparators and sorting is important for software developers working on user interfaces, data processing, and any feature that requires ordered data.

1

Create the fruits array

Create an array called fruits with these exact string values in this order: "apple", "banana", "cherry", "date", "elderberry"

Swift

# Create the fruits array with the exact values
# Your code here

Hint

Use square brackets [] to create an array and separate the fruit names with commas.

2

Create the priority dictionary

Create a dictionary called priority that assigns these exact values: "banana": 1, "apple": 2, "date": 3, "cherry": 4, "elderberry": 5

Swift

let fruits = ["apple", "banana", "cherry", "date", "elderberry"]
# Create the priority dictionary with the exact key-value pairs
# Your code here

Hint

Use square brackets [] with key-value pairs separated by colons and commas.

3

Sort fruits using the custom priority

Create a new constant called sortedFruits that sorts fruits using sorted(by:) with a closure comparing the priority values of two fruits

Swift

let fruits = ["apple", "banana", "cherry", "date", "elderberry"]
let priority = ["banana": 1, "apple": 2, "date": 3, "cherry": 4, "elderberry": 5]
# Sort fruits using sorted(by:) with a closure comparing priority values
# Your code here

Hint

Use fruits.sorted { priority[$0]! < priority[$1]! } to sort by priority values.

4

Print the sorted fruits

Print the sortedFruits array exactly using print(sortedFruits)

Swift

let fruits = ["apple", "banana", "cherry", "date", "elderberry"]
let priority = ["banana": 1, "apple": 2, "date": 3, "cherry": 4, "elderberry": 5]
let sortedFruits = fruits.sorted { priority[$0]! < priority[$1]! }
# Print the sortedFruits array
# Your code here

Hint

Use print(sortedFruits) to show the sorted list.

Practice

1. What does the sorted(by:) method do in Swift?

easy

A. It sorts an array using a custom rule you provide.

B. It removes duplicate elements from an array.

C. It reverses the order of elements in an array.

D. It filters elements based on a condition.

Solution

1. Step 1: Understand the purpose of sorted(by:)

   This method sorts elements of an array based on a closure that defines the order.

2. Step 2: Compare with other array methods

   Unlike filtering or reversing, sorted(by:) changes order based on a custom rule.

3. Final Answer:

   It sorts an array using a custom rule you provide. -> Option A

4. Quick Check:

   sorted(by:) = custom sorting [OK]

Hint: Remember: sorted(by:) arranges items your way [OK]

Common Mistakes:

• Confusing sorting with filtering
• Thinking it removes duplicates
• Assuming it reverses the array

2. Which of the following is the correct syntax to sort an array of integers in descending order using sorted(by:)?

easy

A. let sortedArray = numbers.sorted(by: { $1 > $0 })

B. let sortedArray = numbers.sorted(by: { $0 < $1 })

C. let sortedArray = numbers.sorted()

D. let sortedArray = numbers.sorted(by: { $0 > $1 })

Solution

1. Step 1: Understand sorting order

   To sort descending, the comparator must return true when the first element is greater than the second.

2. Step 2: Check each option

   let sortedArray = numbers.sorted(by: { $0 > $1 }) uses { $0 > $1 } which sorts descending. let sortedArray = numbers.sorted(by: { $0 < $1 }) sorts ascending. let sortedArray = numbers.sorted() sorts ascending by default. let sortedArray = numbers.sorted(by: { $1 > $0 }) reverses the comparison incorrectly.

3. Final Answer:

   let sortedArray = numbers.sorted(by: { $0 > $1 }) -> Option D

4. Quick Check:

   Descending sort uses $0 > $1 [OK]

Hint: Use $0 > $1 for descending sort [OK]

Common Mistakes:

• Using $0 < $1 for descending
• Forgetting to provide closure for custom order
• Confusing parameter order in closure

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

let names = ["Anna", "Bob", "Charlie"]
let sortedNames = names.sorted(by: { $0.count < $1.count })
print(sortedNames)

medium

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

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

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

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

Solution

1. Step 1: Understand the sorting rule

   The closure sorts by string length ascending: shorter names come first.

2. Step 2: Apply sorting to the array

   "Bob" has 3 letters, "Anna" 4, "Charlie" 7. So order is ["Bob", "Anna", "Charlie"].

3. Final Answer:

   ["Bob", "Anna", "Charlie"] -> Option B

4. Quick Check:

   Sort by length ascending = ["Bob", "Anna", "Charlie"] [OK]

Hint: Sort by $0.count < $1.count for shortest first [OK]

Common Mistakes:

• Sorting alphabetically instead of by length
• Confusing < with > in closure
• Expecting original order

4. Identify the error in this Swift code that tries to sort an array of tuples by the second value:

let items = [("a", 3), ("b", 1), ("c", 2)]
let sortedItems = items.sorted(by: { $0.1 > $1.1 })
print(sortedItems)

medium

A. There is no error; the code sorts descending by second value.

B. The tuple elements cannot be accessed by index like $0.1.

C. The closure parameters should be $1.1 > $0.1 for ascending order.

D. The sorting closure must return an Int, not Bool.

Solution

1. Step 1: Check tuple access syntax

   Accessing tuple elements by $0.1 and $1.1 is correct in Swift closures.

2. Step 2: Verify sorting closure logic

   The closure returns true if first element's second value is greater, sorting descending by second value.

3. Final Answer:

   There is no error; the code sorts descending by second value. -> Option A

4. Quick Check:

   Tuple access and Bool return are correct [OK]

Hint: Tuple elements accessed as $0.1; closure returns Bool [OK]

Common Mistakes:

• Thinking tuple elements can't be accessed by index
• Expecting closure to return Int
• Confusing ascending and descending order

5. You have an array of dictionaries representing people: let people = [["name": "Alice", "age": 30], ["name": "Bob", "age": 25], ["name": "Charlie", "age": 30]]. How do you sort this array first by age ascending, then by name ascending if ages are equal?

hard

A. people.sorted { $0["age"] as! Int < $1["age"] as! Int && $0["name"] as! String < $1["name"] as! String }

B. people.sorted(by: { $0["age"] as! Int > $1["age"] as! Int || $0["name"] as! String > $1["name"] as! String })

C. people.sorted { ($0["age"] as! Int, $0["name"] as! String) < ($1["age"] as! Int, $1["name"] as! String) }

D. people.sorted(by: { $0["name"] as! String < $1["name"] as! String })

Solution

1. Step 1: Understand sorting by multiple keys

   Sorting by age ascending, then name ascending means comparing tuples of (age, name).

2. Step 2: Check each option's logic

   people.sorted { ($0["age"] as! Int, $0["name"] as! String) < ($1["age"] as! Int, $1["name"] as! String) } compares tuples directly, which sorts by age first, then name if ages equal. people.sorted { $0["age"] as! Int < $1["age"] as! Int && $0["name"] as! String < $1["name"] as! String } uses && which fails for correct ordering. people.sorted(by: { $0["age"] as! Int > $1["age"] as! Int || $0["name"] as! String > $1["name"] as! String }) sorts descending and mixes conditions incorrectly. people.sorted(by: { $0["name"] as! String < $1["name"] as! String }) sorts only by name.

3. Final Answer:

   people.sorted { ($0["age"] as! Int, $0["name"] as! String) < ($1["age"] as! Int, $1["name"] as! String) } -> Option C

4. Quick Check:

   Tuple comparison sorts by multiple keys [OK]

Hint: Compare tuples (age, name) for multi-level sorting [OK]

Common Mistakes:

• Using && instead of tuple comparison
• Sorting only by one key
• Mixing ascending and descending incorrectly