Predicates and sorting in iOS Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an NSPredicate in Swift?

An NSPredicate is a way to filter collections or Core Data fetch requests by defining conditions that items must meet. Think of it like a filter that only lets through items matching certain rules.

beginner

How do you create a predicate to find names starting with "A"?

You create it like this: NSPredicate(format: "name BEGINSWITH[c] %@", "A"). This means it looks for names starting with 'A', ignoring case.

beginner

What is NSSortDescriptor used for?

NSSortDescriptor tells Swift how to sort a list of items. For example, you can sort by a property like 'age' in ascending or descending order.

intermediate

How do you sort an array of people by age descending using NSSortDescriptor?

You create a sort descriptor like this: NSSortDescriptor(key: "age", ascending: false) and use it to sort the array.

intermediate

Why use predicates and sort descriptors together?

Using predicates filters your data to only what you want, and sort descriptors organize that filtered data in the order you want. Together, they help you get exactly the right data, nicely ordered.

What does NSPredicate do in Swift?

A Filters data based on conditions

B Sorts data in ascending order

C Creates user interface elements

D Manages memory allocation

NSPredicate is used to filter data by specifying conditions.

Which of these is the correct way to create a predicate to find items where 'age' is greater than 18?

A NSPredicate(format: "age > 18")

B NSPredicate(format: "age >= 18")

C NSPredicate(format: "age < 18")

D NSPredicate(format: "age == 18")

The predicate format "age > 18" filters for ages greater than 18.

What does NSSortDescriptor(key: "name", ascending: true) do?

A Filters items with key 'name'

B Filters items with name 'true'

C Sorts items by name in descending order

D Sorts items by name in ascending order

It sorts items by the 'name' property in ascending order.

How can you combine multiple sort descriptors?

A Chain them with '+' operator

B Put them in an array and apply together

C Use only one sort descriptor at a time

D Use NSPredicate instead

Multiple NSSortDescriptors can be combined in an array to sort by multiple keys.

Which is true about predicates and sorting?

A Both sort data

B Predicates order data; sorting filters data

C Predicates filter data; sorting orders data

D Both filter data

Predicates filter data by conditions; sorting arranges data order.

Explain how you would filter a list of contacts to only those whose names start with 'J' and then sort them by last name ascending.

Think about how to write the predicate format and then create a sort descriptor.

Describe the difference between NSPredicate and NSSortDescriptor and how they work together in data handling.

Imagine filtering a list first, then arranging it.

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