Collection mutability tied to let/var in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Swift code snippet?

Consider the following Swift code:

let numbers = [1, 2, 3]
numbers.append(4)
print(numbers)

What will happen when this code runs?

Swift

let numbers = [1, 2, 3]
numbers.append(4)
print(numbers)

A Compile-time error: Cannot use mutating method on immutable value

B The code prints [1, 2, 3]

C The code prints [1, 2, 3, 4]

D Runtime error: Index out of range

💡 Hint

Think about whether let allows changing the array.

In Swift, arrays declared with let are immutable. You cannot add elements to them. Trying to call append causes a compile-time error.

❓ query_result

intermediate

What is the output of this Swift code snippet?

Consider this Swift code:

var fruits = ["apple", "banana"]
fruits.append("cherry")
print(fruits)

What will be printed?

Swift

var fruits = ["apple", "banana"]
fruits.append("cherry")
print(fruits)

A ["apple", "banana"]

B Compile-time error: Cannot append to array

C ["apple", "banana", "cherry"]

D Runtime error: Index out of range

💡 Hint

Think about whether var allows changing the array.

Arrays declared with var are mutable. You can add elements using append. So the array will include "cherry".

📝 Syntax

advanced

Which option causes a compile-time error due to collection mutability?

Given the following Swift code snippets, which one will cause a compile-time error?

A let dict = [String: Int]()

B

var dict = ["a": 1, "b": 2]
dict["c"] = 3

C

var dict = [String: Int]()
dict["a"] = 1

D

let dict = ["a": 1, "b": 2]
dict["c"] = 3

💡 Hint

Check which dictionary is declared with let and then mutated.

Dictionaries declared with let are immutable. Trying to add or change keys causes a compile-time error.

🔧 Debug

advanced

Why does this Swift code fail to compile?

Examine this Swift code:

let set: Set = [1, 2, 3]
set.insert(4)
print(set)

Why does it fail to compile?

Swift

let set: Set<Int> = [1, 2, 3]
set.insert(4)
print(set)

A Because 'set' is declared with 'let' and cannot be mutated

B Because 'insert' is not a method of Set

C Because Set cannot contain integers

D Because 'print' cannot print a Set

💡 Hint

Think about whether let allows changing the set.

Sets declared with let are immutable. You cannot add elements to them. Calling insert causes a compile-time error.

🧠 Conceptual

expert

How does Swift enforce collection mutability with let and var?

Which statement best describes how Swift handles mutability of collections declared with let and var?

A Collections declared with <code>let</code> can be modified if their elements are classes; <code>var</code> collections cannot be modified.

B Collections declared with <code>let</code> are immutable; their contents cannot be changed, while those declared with <code>var</code> are mutable and can be modified.

C Both <code>let</code> and <code>var</code> collections are mutable, but <code>let</code> collections cannot be reassigned.

D Collections declared with <code>var</code> are immutable; only <code>let</code> collections can be changed.

💡 Hint

Think about the difference between let and var in Swift.

In Swift, let makes a collection immutable, so you cannot change its contents. var allows modification of the collection's contents.