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.

Practice

1. In Swift, if you declare a collection with let, what does it mean for that collection?

easy

A. The collection cannot be changed after creation.

B. You can add or remove items anytime.

C. Only the items inside can change, but not the collection size.

D. The collection is deleted automatically.

Solution

1. Step 1: Understand let in Swift

   let creates a constant, meaning the value cannot be changed after it is set.

2. Step 2: Apply to collections

   If a collection is declared with let, you cannot add, remove, or modify its elements because the collection itself is constant.

3. Final Answer:

   The collection cannot be changed after creation. -> Option A

4. Quick Check:

   let means constant collection [OK]

Hint: Remember: let means no changes allowed to collection [OK]

Common Mistakes:

• Thinking you can add items to a let collection
• Confusing let with var for collections
• Assuming only items can change but not size

2. Which of the following is the correct way to declare a mutable array in Swift?

easy

A. mutable numbers = [1, 2, 3]

B. let numbers = [1, 2, 3]

C. const numbers = [1, 2, 3]

D. var numbers = [1, 2, 3]

Solution

1. Step 1: Recall Swift syntax for mutable collections

   Use var to declare variables that can change.

2. Step 2: Check options

   var numbers = [1, 2, 3] correctly declares a mutable array. let is immutable, and const or mutable are not Swift keywords.

3. Final Answer:

   var numbers = [1, 2, 3] -> Option D

4. Quick Check:

   Mutable array uses var [OK]

Hint: Use var for collections you want to change [OK]

Common Mistakes:

• Using let instead of var for mutable collections
• Using non-Swift keywords like const or mutable
• Confusing var with let

3. What will be the output of this Swift code?

let fruits = ["Apple", "Banana"]
fruits.append("Cherry")
print(fruits)

medium

A. Compile-time error

B. ["Apple", "Banana"]

C. ["Apple", "Banana", "Cherry"]

D. Runtime crash

Solution

1. Step 1: Identify collection mutability

   The array fruits is declared with let, making it immutable.

2. Step 2: Analyze append operation

   Trying to append to an immutable array causes a compile-time error because you cannot modify a constant collection.

3. Final Answer:

   Compile-time error -> Option A

4. Quick Check:

   Append on let collection = Compile-time error [OK]

Hint: Appending to let collection causes compile error [OK]

Common Mistakes:

• Assuming append works on let collections
• Thinking it crashes at runtime instead of compile-time
• Expecting original array unchanged

4. Consider this Swift code snippet:

let numbers = [1, 2, 3]
numbers[0] = 10
print(numbers)

What is the problem here?

medium

A. The array must be declared with var to print.

B. You cannot change elements of an array declared with let.

C. The print statement is incorrect.

D. Array index 0 does not exist.

Solution

1. Step 1: Check array declaration

   The array numbers is declared with let, so it is immutable.

2. Step 2: Understand element assignment

   Trying to assign a new value to an element of an immutable array is not allowed and causes a compile-time error.

3. Final Answer:

   You cannot change elements of an array declared with let. -> Option B

4. Quick Check:

   Changing element in let array = Not allowed [OK]

Hint: Elements of let arrays cannot be changed [OK]

Common Mistakes:

• Thinking only size is fixed but elements can change
• Assuming index 0 is invalid
• Believing print causes error

5. You want to create a dictionary in Swift that you can update later by adding or changing keys and values. Which declaration should you use and why?

hard

A. Use let and then create a new dictionary to update.

B. Use let because it allows changing keys and values.

C. Use var because it allows modifying the dictionary after creation.

D. Use var but you cannot change keys or values.

Solution

1. Step 1: Understand mutability of dictionaries

   To modify a dictionary after creation, it must be declared mutable.

2. Step 2: Choose correct keyword

   var declares a mutable dictionary allowing adding or changing keys and values. let makes it immutable.

3. Final Answer:

   Use var because it allows modifying the dictionary after creation. -> Option C

4. Quick Check:

   Mutable dictionary needs var [OK]

Hint: Use var for collections you want to update later [OK]

Common Mistakes:

• Using let and expecting to modify dictionary
• Thinking let allows changing keys or values
• Believing var prevents changing keys or values