Collection mutability tied to let/var in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What happens when you declare a collection with let in Swift?

Declaring a collection with let makes it immutable. You cannot add, remove, or change elements after initialization.

beginner

How does declaring a collection with var affect its mutability?

Declaring a collection with var makes it mutable. You can add, remove, or modify elements freely.

intermediate

Can you modify the elements of a collection declared with let if the elements themselves are reference types?

Yes, if the elements are reference types (like classes), you can modify their internal properties even if the collection is declared with let. But you cannot change the collection structure itself.

beginner

Why does Swift tie collection mutability to let and var?

Swift uses let and var to clearly express intent. let means the collection won't change, helping prevent bugs. var allows changes when needed.

beginner

What error occurs if you try to modify a let-declared collection?

You get a compile-time error saying you cannot use mutating methods on an immutable value.

What does declaring an array with let in Swift mean?

A The array can be changed anytime

B The array elements can be changed but not the array size

C The array is deleted automatically

D The array cannot be changed after creation

Declaring with let makes the array immutable, so you cannot add, remove, or change elements.

If you declare a dictionary with var, what can you do?

A Only read elements

B Only remove elements

C Add, remove, or modify elements

D Only add elements

Using var allows full mutability of the collection.

Can you modify the properties of class instances inside a let-declared array?

A Yes, properties of class instances can be changed

B No, nothing can be changed

C Only if the array is declared with <code>var</code>

D Only if the class is a struct

The collection is immutable, but the reference type elements can have their properties changed.

What keyword in Swift declares a mutable collection?

A let

B var

C const

D mutable

var declares a mutable collection.

What kind of error do you get if you try to append to a let-declared array?

A Compile-time error

B Runtime error

C No error

D Logical error

Swift prevents this at compile time to avoid mutating an immutable collection.

Explain how let and var affect collection mutability in Swift.

Think about whether you can change the collection after creating it.

Describe what happens when you try to modify a let-declared collection and why Swift enforces this.

Consider Swift's goal to avoid bugs by enforcing immutability.

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