Collection slicing and indices in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get a slice of the array from index 1 to 3.

Swift

let numbers = [10, 20, 30, 40, 50]
let slice = numbers[[1]]

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

A1...3

B0...2

C3...5

D2...4

Attempts:

3 left

💡 Hint

Common Mistakes

Using a range that starts at 0 instead of 1.

Using an open range operator which excludes the last index.

The slice 1...3 gets elements at indices 1, 2, and 3.

2fill in blank

medium

Complete the code to get the first three elements of the array using prefix.

Swift

let fruits = ["apple", "banana", "cherry", "date"]
let firstThree = fruits.[1](3)

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

Asuffix

BdropLast

CdropFirst

Dprefix

Attempts:

3 left

💡 Hint

Common Mistakes

Using suffix instead of prefix.

Using dropFirst which removes elements instead of selecting.

The prefix(3) method returns the first three elements of the collection.

3fill in blank

hard

Fix the error in the code to correctly get a slice from index 2 to the end.

Swift

let letters = ["a", "b", "c", "d", "e"]
let slice = letters[[1]...]

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

A2

B3

Cletters.startIndex

Dletters.endIndex

Attempts:

3 left

💡 Hint

Common Mistakes

Using endIndex which is one past the last valid index.

Using startIndex which gets the whole array.

Using 2... gets the slice from index 2 to the end.

4fill in blank

hard

Fill both blanks to create a slice of the array from index 1 up to but not including index 4.

Swift

let colors = ["red", "green", "blue", "yellow", "purple"]
let slice = colors[[1]..<[2]]

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

A1

B3

C4

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using a closed range which includes index 4.

Using wrong start or end indices.

The half-open range 1..<4 includes indices 1, 2, and 3.

5fill in blank

hard

Fill all three blanks to create a dictionary with keys as uppercase letters and values as their index, only for letters with index greater than 1.

Swift

let letters = ["a", "b", "c", "d", "e"]
let dict = [[1]: [2] for (index, letter) in letters.enumerated() if index [3] 1]

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

Aletter.uppercased()

Bindex

C>

Dletter

Attempts:

3 left

💡 Hint

Common Mistakes

Using the letter itself instead of uppercase for keys.

Using wrong comparison operator in the condition.

This comprehension creates a dictionary where keys are uppercase letters and values are their indices, only for indices greater than 1.

Practice

1. What does collection slicing in Swift allow you to do?
let numbers = [10, 20, 30, 40, 50]
For example, numbers[1...3] returns which part of the array?

easy

A. A part of the collection from index 1 to 3 inclusive

B. The entire collection reversed

C. Only the first element of the collection

D. An error because slicing is not allowed

Solution

1. Step 1: Understand collection slicing syntax

   The syntax numbers[1...3] means take elements from index 1 to index 3, including both ends.

2. Step 2: Identify the elements in the slice

   Indexes 1, 2, and 3 correspond to values 20, 30, and 40 respectively.

3. Final Answer:

   A part of the collection from index 1 to 3 inclusive -> Option A

4. Quick Check:

   Slice [1...3] = [20, 30, 40] [OK]

Hint: Remember slicing uses inclusive ranges [OK]

Common Mistakes:

• Thinking slicing returns a single element
• Assuming slicing reverses the collection
• Believing slicing causes an error

2. Which of the following is the correct syntax to slice a Swift array named items from index 2 to 5 (inclusive)?

easy

A. items[2..<5]

B. items.slice(2, 5)

C. items[2...5]

D. items[2,5]

Solution

1. Step 1: Understand Swift range operators

   The closed range operator ... includes both start and end indexes, so 2...5 means indexes 2, 3, 4, and 5.

2. Step 2: Evaluate the slicing syntaxes

   items[2..<5] uses half-open range which excludes index 5. items.slice(2, 5) calls a non-existent method. items[2,5] is invalid syntax. items[2...5] is correct.

3. Final Answer:

   items[2...5] -> Option C

4. Quick Check:

   Closed range includes end index [OK]

Hint: Use ... for inclusive slicing in Swift [OK]

Common Mistakes:

• Using half-open range when inclusive is needed
• Using comma inside brackets incorrectly
• Trying to call slice() method which doesn't exist

3. Given the Swift array:
let fruits = ["apple", "banana", "cherry", "date", "fig"]
What is the output of this code?
let slice = fruits[1..<4]
print(slice)

medium

A. ["banana", "cherry", "date"]

B. ["banana", "cherry", "date", "fig"]

C. ["banana", "cherry"]

D. Runtime error

Solution

1. Step 1: Understand half-open range slicing

   The range 1..<4 includes indexes 1, 2, and 3 but excludes 4.

2. Step 2: Identify elements at these indexes

   Indexes 1, 2, 3 correspond to "banana", "cherry", and "date".

3. Final Answer:

   ["banana", "cherry", "date"] -> Option A

4. Quick Check:

   Half-open range excludes last index [OK]

Hint: ..< excludes the upper bound index [OK]

Common Mistakes:

• Including the element at index 4 mistakenly
• Confusing half-open with closed range
• Expecting a runtime error for valid slice

4. Consider this Swift code:
let letters = ["a", "b", "c", "d"]
let slice = letters[1...4]
What is the problem with this code?

medium

A. Slicing syntax is incorrect, should use parentheses

B. Index 4 is out of range causing a runtime error

C. Slice returns an empty array

D. No problem, code runs fine

Solution

1. Step 1: Check array bounds

   The array has indexes 0 to 3. Index 4 does not exist.

2. Step 2: Understand slicing with out-of-bounds index

   Using 1...4 tries to access index 4, which causes a runtime error.

3. Final Answer:

   Index 4 is out of range causing a runtime error -> Option B

4. Quick Check:

   Accessing invalid index causes error [OK]

Hint: Check array size before slicing [OK]

Common Mistakes:

• Assuming slicing auto-adjusts out-of-range indexes
• Thinking syntax is wrong instead of index error
• Expecting empty slice instead of error

5. You have a Swift array:
let data = [5, 10, 15, 20, 25, 30]
You want to create a slice that includes every element except the first and last. Which code correctly creates this slice?

hard

A. let slice = data[1..

B. let slice = data[1...data.count-1]

C. let slice = data[1...data.count]

D. let slice = data[1..

Solution

1. Step 1: Determine indexes to include

   Exclude first (index 0) and last (index 5) elements, so include indexes 1 to 4.

2. Step 2: Use half-open range for slicing

   Using 1.. means indexes 1 up to but not including 5, which is indexes 1 to 4.

3. Final Answer:

   let slice = data[1.. -> Option D

4. Quick Check:

   Half-open range excludes last index [OK]

Hint: Use half-open range to exclude last index [OK]

Common Mistakes:

• Using closed range which includes the last index
• Using count directly as upper bound causing out-of-range
• Confusing inclusive and exclusive ranges