Predicates and sorting in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

Filtering a list with NSPredicate

Given an array of names, which option correctly filters names starting with the letter 'J' using NSPredicate?

iOS Swift

let names = ["John", "Alice", "Jack", "Bob"]
let predicate = NSPredicate(format: "SELF BEGINSWITH[c] %@", "J")
let filtered = (names as NSArray).filtered(using: predicate) as! [String]
print(filtered)

A ["John", "Jack"]

B ["Alice", "Bob"]

C ["John", "Alice", "Jack", "Bob"]

D []

💡 Hint

Look for the predicate that matches names starting with 'J' ignoring case.

The predicate "SELF BEGINSWITH[c] %@" with argument "J" filters strings starting with 'J' case-insensitively. So only "John" and "Jack" remain.

📝 Syntax

intermediate

Correct sorting syntax with NSSortDescriptor

Which option correctly sorts an array of numbers in descending order using NSSortDescriptor?

iOS Swift

let numbers = [3, 1, 4, 2]
let sortDescriptor = NSSortDescriptor(key: nil, ascending: false)
let sorted = (numbers as NSArray).sortedArray(using: [sortDescriptor]) as! [Int]
print(sorted)

A [3, 1, 4, 2]

B [4, 3, 2, 1]

C SyntaxError

D [1, 2, 3, 4]

💡 Hint

Check the ascending parameter to get descending order.

Setting ascending to false sorts the array from highest to lowest.

🔧 Debug

advanced

Why does this NSPredicate cause a crash?

What error does this code cause and why? let predicate = NSPredicate(format: "age > %@", 18) let filtered = people.filter { predicate.evaluate(with: $0) }

iOS Swift

let predicate = NSPredicate(format: "age > %@", NSNumber(value: 18))
let filtered = people.filter { predicate.evaluate(with: $0) }

A NSInvalidArgumentException because %@ expects an object, not a primitive Int

B No error, filters correctly

C TypeError because 'age' is undefined

D Runtime crash due to missing key 'age'

💡 Hint

Check the placeholder %@ and the type of argument passed.

The %@ placeholder expects an object like NSNumber, but 18 is a primitive Int. This mismatch causes NSInvalidArgumentException.

🧠 Conceptual

advanced

Understanding sorting stability with NSSortDescriptor

If you sort an array of people first by last name ascending, then by first name ascending using two NSSortDescriptors, what is the expected behavior?

A The array is sorted randomly

B The array is sorted only by first name, ignoring last name

C The array is sorted only by last name, ignoring first name

D The array is sorted by last name, and within same last names, sorted by first name

💡 Hint

Think about how multiple sort descriptors combine sorting criteria.

Multiple NSSortDescriptors apply sorting in order: first by last name, then by first name for ties.

❓ lifecycle

expert

When is NSPredicate evaluated in a Core Data fetch request?

At what point is the NSPredicate applied when fetching data from Core Data?

A NSPredicate is ignored in Core Data fetch requests

B After fetching all objects into memory, filtering happens in app code

C When the fetch request is executed, filtering happens at the persistent store level

D Filtering happens only when saving data

💡 Hint

Consider performance and data transfer when fetching from Core Data.

NSPredicate filters data at the persistent store level during fetch, reducing memory usage and improving performance.