Collection slicing and indices in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a collection slice in Swift?

A collection slice is a view into a portion of a collection, sharing the same indices as the original collection but representing only a subset of its elements.

beginner

How do indices behave in a Swift collection slice?

Indices in a collection slice are the same as in the original collection, meaning they are not zero-based but reflect the original collection's index positions.

intermediate

Why is it important that collection slices share indices with the original collection?

Sharing indices allows efficient access and avoids copying elements, but requires careful handling because indices may not start at zero and can cause confusion if assumed otherwise.

intermediate

How can you convert a collection slice back to a standalone collection in Swift?

You can create a new collection (like an Array) from a slice by initializing it with the slice, e.g., Array(slice), which resets indices to start at zero.

beginner

What happens if you try to access a collection slice using an index outside its range?

Accessing an index outside the slice's range will cause a runtime error because the slice only contains elements within its defined indices from the original collection.

In Swift, what does a collection slice represent?

A A copy of the entire collection

B A reversed version of the collection

C A view into a subset of the collection sharing original indices

D A collection with zero-based indices

A collection slice is a view into a subset of the collection and shares the original collection's indices.

If you slice an array from index 2 to 5, what will be the first index of the slice?

A 2

B 1

C 5

D 0

The slice keeps the original indices, so the first index of the slice is 2.

How do you create a new array from a collection slice to reset indices?

A Use Array(slice)

B Use slice.toArray()

C Use slice.copy()

D Use slice.resetIndices()

Initializing a new Array with the slice resets the indices to start at zero.

What error occurs if you access an index outside a slice's range?

A Compile-time error

B Runtime error

C No error, returns nil

D Warning only

Accessing an invalid index causes a runtime error in Swift.

Why might collection slices improve performance?

A They use less memory by deleting elements

B They copy all elements to a new array

C They compress the data

D They avoid copying by sharing storage with the original collection

Slices share storage with the original collection, avoiding copying and improving performance.

Explain how indices work in Swift collection slices and why this matters.

Think about how the slice relates to the original collection's positions.

Describe how to convert a collection slice into a standalone collection and why you might want to do this.

Consider when you want a fresh collection with simple indices.

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