Collection slicing and indices in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of slicing a Swift array with a range?

Consider the Swift array let numbers = [10, 20, 30, 40, 50]. What is the output of numbers[1..<4]?

Swift

let numbers = [10, 20, 30, 40, 50]
let slice = numbers[1..<4]
print(Array(slice))

A [10, 20, 30, 40]

B [20, 30, 40]

C [20, 30, 40, 50]

D [30, 40, 50]

💡 Hint

Remember that the range 1..<4 includes the start index but excludes the end index.

The slice numbers[1..<4] includes elements at indices 1, 2, and 3, which are 20, 30, and 40.

📝 Syntax

intermediate

Which option correctly slices a Swift dictionary's keys?

Given let dict = ["a": 1, "b": 2, "c": 3, "d": 4], which code correctly slices the keys from index 1 to 3?

Swift

let dict = ["a": 1, "b": 2, "c": 3, "d": 4]
let keysSlice = ???
print(Array(keysSlice))

A Array(dict.keys)[1..<3]

B dict.keys[1..<3]

C dict.keys[1...3]

D dict.keys[1,3]

💡 Hint

Dictionary keys are not directly indexable; you may need to convert them to an array first.

Dictionary keys are a collection but not indexable by integer ranges directly. Converting to an array allows slicing by integer indices.

❓ optimization

advanced

Which slicing method is most efficient for large Swift arrays?

You have a large array let bigArray = Array(1...1000000). Which slicing method avoids copying elements and is most efficient?

Swift

let bigArray = Array(1...1000000)
let slice = ???

A bigArray[100...199]

B Array(bigArray[100..<200])

C bigArray.prefix(200).suffix(100)

D bigArray[100..<200]

💡 Hint

Consider which methods return slices without copying the underlying elements.

Using bigArray[100..<200] returns an ArraySlice which is a view and does not copy elements, making it efficient.

🔧 Debug

advanced

Why does this Swift slice cause a runtime error?

Given let arr = [1, 2, 3], why does let slice = arr[1..<5] cause a runtime error?

Swift

let arr = [1, 2, 3]
let slice = arr[1..<5]
print(Array(slice))

A Slicing with range operator is not allowed on arrays.

B The lower bound 1 is invalid for slicing.

C The upper bound 5 is out of range for the array indices.

D The array must be converted to a slice before slicing.

💡 Hint

Check the valid index range for the array.

The array has indices 0 to 2. Using 5 as the upper bound exceeds the array's valid indices, causing a runtime error.

🧠 Conceptual

expert

How does Swift's Collection slicing preserve indices?

When slicing a Swift collection, how are the indices of the slice related to the original collection?

A The slice uses the same indices as the original collection, preserving index values.

B The slice resets indices starting from zero regardless of original indices.

C The slice uses random indices unrelated to the original collection.

D The slice uses string keys as indices instead of integers.

💡 Hint

Think about how Swift's ArraySlice works with indices.

Swift slices preserve the original collection's indices, so the slice's indices match the original collection's indices for the sliced elements.