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.

Practice

1. What does an NSPredicate do in Swift when working with arrays?

easy

A. Removes duplicates from the array

B. Sorts the array in ascending or descending order

C. Adds new items to the array

D. Filters the array to include only items matching a condition

Solution

1. Step 1: Understand the role of NSPredicate

   An NSPredicate is used to specify conditions to filter collections like arrays.

2. Step 2: Differentiate filtering from sorting

   Filtering means selecting items that meet a condition, while sorting changes order.

3. Final Answer:

   Filters the array to include only items matching a condition -> Option D

4. Quick Check:

   Predicate = Filter [OK]

Hint: Predicate means filter, not sort or add [OK]

Common Mistakes:

• Confusing predicate with sorting
• Thinking predicate adds or removes items
• Assuming predicate changes order

2. Which of the following is the correct syntax to create an NSPredicate filtering for names equal to "John"?

easy

A. NSPredicate(format: "name == 'John'")

B. NSPredicate(format: "name = 'John'")

C. NSPredicate(format: "name equals 'John'")

D. NSPredicate(format: "name === 'John'")

Solution

1. Step 1: Recall NSPredicate format syntax

   NSPredicate uses format strings with == for equality comparison.

2. Step 2: Identify correct equality operator

   == is the correct operator; = or === are invalid in NSPredicate format.

3. Final Answer:

   NSPredicate(format: "name == 'John'") -> Option A

4. Quick Check:

   Use == for equality in NSPredicate [OK]

Hint: Use double equals (==) for equality in predicates [OK]

Common Mistakes:

• Using single equals (=) instead of double (==)
• Using === which is not valid in NSPredicate
• Writing 'equals' as a word instead of operator

3. Given this Swift code snippet, what will be the output?

let names = ["Anna", "John", "Bob", "Alice"]
let predicate = NSPredicate(format: "SELF BEGINSWITH[c] 'A'")
let filtered = (names as NSArray).filtered(using: predicate) as! [String]
let sorted = filtered.sorted(by: { $0 > $1 })
print(sorted)

medium

A. ["Alice", "Anna"]

B. ["Anna", "Alice"]

C. ["Anna"]

D. ["Alice"]

Solution

1. Step 1: Filter names starting with 'A' ignoring case

   The predicate selects names beginning with 'A' or 'a': "Anna" and "Alice".

2. Step 2: Sort filtered names descending

   Sorting with $0 > $1 means descending order alphabetically: "Alice" comes before "Anna".

3. Final Answer:

   ["Alice", "Anna"] -> Option A

4. Quick Check:

   Filter by 'A', then descending sort = ["Alice", "Anna"] [OK]

Hint: Filter first, then sort; descending means bigger first [OK]

Common Mistakes:

• Sorting ascending instead of descending
• Ignoring case sensitivity in predicate
• Confusing filter and sort order

4. Identify the error in this Swift code using NSPredicate and sorting:

let numbers = [5, 3, 8, 1]
let predicate = NSPredicate(format: "SELF > 4")
let filtered = numbers.filter { predicate.evaluate(with: $0) }
let sorted = filtered.sorted(by: { $0 < $1 })
print(sorted)

medium

A. NSPredicate format string is incorrect

B. No error; code works as expected

C. Filtering with predicate.evaluate(with:) is correct

D. Cannot use NSPredicate with Int values

Solution

1. Step 1: Check NSPredicate format

   The format "SELF > 4" is valid for filtering numbers greater than 4.

2. Step 2: Verify filtering and sorting logic

   Using predicate.evaluate(with:) inside filter is valid; sorting ascending is correct.

3. Final Answer:

   No error; code works as expected -> Option B

4. Quick Check:

   Predicate with Int and filter works fine [OK]

Hint: Evaluate predicate inside filter is valid for Int arrays [OK]

Common Mistakes:

• Thinking NSPredicate can't filter Ints
• Assuming format string is wrong
• Expecting error in sorting syntax

5. You have an array of dictionaries representing people:

let people = [
  ["name": "John", "age": 30],
  ["name": "Alice", "age": 25],
  ["name": "Bob", "age": 30],
  ["name": "Carol", "age": 25]
]

How do you filter for people aged 25 and sort them by name ascending using NSPredicate and Swift sorting?

hard

A. Use NSPredicate(format: "age = 25") and sort with sorted(by: { $0["name"]! > $1["name"]! })

B. Use NSPredicate(format: "age >= 25") and sort with sorted(by: { $0["age"]! < $1["age"]! })

C. Use NSPredicate(format: "age == 25") and sort with sorted(by: { $0["name"]! < $1["name"]! })

D. Use NSPredicate(format: "age == '25'") and sort with sorted(by: { $0["name"]! < $1["name"]! })

Solution

1. Step 1: Write correct NSPredicate for age 25

   Use "age == 25" to filter exactly age 25; single equals is invalid.

2. Step 2: Sort filtered array by name ascending

   Sort by comparing names with < operator for ascending order.

3. Final Answer:

   Use NSPredicate(format: "age == 25") and sort with sorted(by: { $0["name"]! < $1["name"]! }) -> Option C

4. Quick Check:

   Filter age == 25, sort by name ascending [OK]

Hint: Use == for equality and < for ascending sort [OK]

Common Mistakes:

• Using single = instead of ==
• Comparing age as string '25' instead of number
• Sorting by age instead of name