Predicates and sorting in iOS Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a predicate that filters names starting with "A".

iOS Swift

let predicate = NSPredicate(format: "name BEGINSWITH [1]")

Drag options to blanks, or click blank then click option'

A'A'

B"A"

C"a"

DA

Attempts:

3 left

💡 Hint

Common Mistakes

Using single quotes instead of double quotes.

Not quoting the letter at all.

Using lowercase 'a' which won't match uppercase names.

The predicate format requires the string "A" to be in double quotes to match names starting with uppercase A.

2fill in blank

medium

Complete the code to sort an array of strings in ascending order.

iOS Swift

let sortedNames = names.sorted(by: { $0 [1] $1 })

Drag options to blanks, or click blank then click option'

A<

B>

C==

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' which sorts in descending order.

Using '==' which does not sort.

Using '>=' which is not a valid sorting comparator.

Using '<' sorts the array in ascending order alphabetically.

3fill in blank

hard

Fix the error in the predicate format to filter ages greater than 18.

iOS Swift

let predicate = NSPredicate(format: "age [1] 18")

Drag options to blanks, or click blank then click option'

A==

B=

C<

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' instead of '==' or '>'.

Using '<' which filters ages less than 18.

Using '==' which filters only age exactly 18.

The '>' operator correctly filters for ages greater than 18.

4fill in blank

hard

Fill both blanks to create a predicate filtering names containing "John" and sort them descending.

iOS Swift

let predicate = NSPredicate(format: "name CONTAINS[c] [1]")
let sortedNames = names.sorted(by: { $0 [2] $1 })

Drag options to blanks, or click blank then click option'

A"John"

B<

C>

D"john"

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase 'john' which may not match case-insensitively without [c].

Using '<' which sorts ascending instead of descending.

Not quoting the string in the predicate.

The predicate uses "John" with quotes and the sorting uses '>' for descending order.

5fill in blank

hard

Fill all three blanks to create a dictionary of names and ages filtered by age > 20 and sorted by name ascending.

iOS Swift

let filtered = people.filter { $0.age [1] 20 }
let sorted = filtered.sorted(by: { $0.name [2] $1.name })
let nameAgeDict = Dictionary(uniqueKeysWithValues: sorted.map { ([3], $0.age) })

Drag options to blanks, or click blank then click option'

A>

B<

C$0.name

D$1.name

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' in filter which filters ages less than 20.

Using '>' in sort which sorts descending.

Using '$1.name' as key in map which is invalid.

Filter uses '>' for age > 20, sort uses '<' for ascending by name, and map uses '$0.name' as key.

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