Predicates and sorting in iOS Swift

Introduction

Predicates help you find specific items in a list. Sorting arranges items in order, like alphabetically or by number.

You want to show only contacts whose names start with 'A'.

You need to list tasks sorted by their due date.

Filtering a list of products to show only those under $20.

Sorting messages by newest first.

Finding all photos taken in a certain year.

Syntax

iOS Swift

let predicate = NSPredicate(format: "propertyName CONTAINS[c] %@", "value")
let sortedArray = array.sorted { $0.property < $1.property }

NSPredicate uses a format string to describe the filter condition.

Sorting uses the sorted method with a closure to compare items.

Examples

This finds items where the name starts with 'J', ignoring case.

iOS Swift

let predicate = NSPredicate(format: "name BEGINSWITH[c] %@", "J")

This finds items where the age is 18 or older.

iOS Swift

let predicate = NSPredicate(format: "age >= %d", 18)

This sorts people by age from youngest to oldest.

iOS Swift

let sortedArray = people.sorted { $0.age < $1.age }

This sorts people by name alphabetically, ignoring case.

iOS Swift

let sortedArray = people.sorted { $0.name.localizedCaseInsensitiveCompare($1.name) == .orderedAscending }

Sample App

This program filters a list of people to only those whose names start with 'a' or 'A'. Then it sorts them by age from youngest to oldest. Finally, it prints their names and ages.

iOS Swift

import UIKit

class Person: NSObject {
  let name: String
  let age: Int
  init(name: String, age: Int) {
    self.name = name
    self.age = age
    super.init()
  }
}

let people = [
  Person(name: "Alice", age: 30),
  Person(name: "Bob", age: 20),
  Person(name: "Charlie", age: 25),
  Person(name: "anna", age: 22)
]

// Filter people whose name starts with 'A' or 'a'
let predicate = NSPredicate(format: "name BEGINSWITH[c] %@", "A")
let filteredPeople = people.filter { predicate.evaluate(with: $0) }

// Sort filtered people by age ascending
let sortedPeople = filteredPeople.sorted { $0.age < $1.age }

for person in sortedPeople {
  print("\(person.name) - \(person.age)")
}

Important Notes

NSPredicate is powerful for filtering arrays or Core Data fetch requests.

Sorting closures return true if the first item should come before the second.

Use [c] in predicates to ignore case when matching strings.

Summary

Predicates filter lists by conditions you set.

Sorting arranges items in order using a comparison.

Combine filtering and sorting to show exactly what you want in the right order.

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