Collection mutability tied to let/var in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a mutable array.

Swift

var numbers = [1]

Drag options to blanks, or click blank then click option'

A[1, 2, 3]

Blet [1, 2, 3]

C(1, 2, 3)

Dvar (1, 2, 3)

Attempts:

3 left

💡 Hint

Common Mistakes

Using let instead of var makes the array immutable.

Using parentheses instead of square brackets for arrays.

Using var with an array literal [1, 2, 3] creates a mutable array.

2fill in blank

medium

Complete the code to add an element to a mutable array.

Swift

var fruits = ["apple", "banana"]
fruits.[1]("orange")

Drag options to blanks, or click blank then click option'

Aremove

Binsert

Cappend

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using remove or pop which remove elements.

Using insert without specifying an index.

The append method adds a new element to the end of a mutable array.

3fill in blank

hard

Fix the error in the code to modify an immutable array.

Swift

[1] colors = ["red", "green", "blue"]
colors.append("yellow")

Drag options to blanks, or click blank then click option'

Avar

Blet

Cconst

Dfinal

Attempts:

3 left

💡 Hint

Common Mistakes

Using let which makes the array immutable.

Using keywords from other languages like const or final.

You cannot modify an array declared with let. To fix the error, declare the array with var instead of let.

4fill in blank

hard

Fill both blanks to create a mutable dictionary and add a new key-value pair.

Swift

var capitals = [1]
capitals["France"] = [2]

Drag options to blanks, or click blank then click option'

A["USA": "Washington"]

B"Paris"

C"London"

Dlet ["USA": "Washington"]

Attempts:

3 left

💡 Hint

Common Mistakes

Using let instead of var for mutable collections.

Using parentheses instead of square brackets for dictionaries.

Use var with a dictionary literal to create a mutable dictionary. Then assign a new value to a key to add it.

5fill in blank

hard

Fill all three blanks to declare an immutable set and check if it contains a value.

Swift

let primes: Set<Int> = [1]
let containsFive = primes.[2](5)
print(containsFive)  // [3]

Drag options to blanks, or click blank then click option'

A[2, 3, 5, 7]

Bcontains

Ctrue

Dvar

Attempts:

3 left

💡 Hint

Common Mistakes

Declaring the set with var instead of let.

Using incorrect methods like containsValue.

Expecting false when the value is actually in the set.

Use a set literal with square brackets to declare the set. Use the contains method to check membership. The result is true because 5 is in the set.

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