Why let and var distinction matters in Swift - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a constant in Swift.

Swift

let number = [1]

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

A10

Bvar

Clet

D"ten"

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'var' instead of a value

Assigning a string instead of a number

In Swift, let declares a constant, so you assign a value like 10 to it.

2fill in blank

medium

Complete the code to declare a variable in Swift.

Swift

var name = [1]

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

A10

Bvar

Clet

D"Alice"

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'let' instead of a value

Assigning a number instead of a string

Variables declared with var can change. Here, name is set to the string "Alice".

3fill in blank

hard

Fix the error in the code by choosing the correct keyword to allow changing the value.

Swift

[1] score = 5
score = 10

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

Ascore

B10

Cvar

Dlet

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'let'

Choosing a value like '10' instead of the keyword

Choosing 'score'

The error occurs because let creates a constant that cannot be reassigned. Use var to declare a variable that can be changed.

4fill in blank

hard

Choose the keyword that allows mutating the array by appending an element.

Swift

[1] scores = [85, 90]
scores.append(95)

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

Alet

Bvar

Cconstant

Dfinal

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'let'

Using non-Swift keywords like 'constant'

let makes the array a constant and prevents mutating methods like append. Use var to declare mutable arrays.

5fill in blank

hard

Choose the keyword that allows the function to modify the counter.

Swift

[1] counter = 0

func incrementCounter() {
  counter += 1
}

incrementCounter()
print(counter)

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

Avar

Bstatic let

Clet

Dprivate var

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'let'

Adding unnecessary modifiers like 'static' or 'private'

To modify counter from within the function, declare it with var. Constants declared with let cannot be changed.

Practice

1. What is the main difference between let and var in Swift?

easy

A. let declares a variable, var declares a constant.

B. let declares a constant, var declares a variable.

C. let is used for functions, var is used for classes.

D. let and var are interchangeable.

Solution

1. Step 1: Understand let usage

   let is used to declare constants, meaning their value cannot change after being set.

2. Step 2: Understand var usage

   var is used to declare variables, which can be changed after initialization.

3. Final Answer:

   let declares a constant, var declares a variable. -> Option B

4. Quick Check:

   let = constant, var = variable [OK]

Hint: Remember: let = fixed, var = changeable [OK]

Common Mistakes:

• Thinking let can be changed after assignment
• Confusing let and var roles
• Assuming both behave the same

2. Which of the following is the correct way to declare a variable that can change in Swift?

easy

A. var score = 10

B. const score = 10

C. let score = 10

D. variable score = 10

Solution

1. Step 1: Identify Swift syntax for variable declaration

   In Swift, var is used to declare variables that can change.

2. Step 2: Check each option

   let score = 10 declares a constant. const score = 10 and variable score = 10 use invalid keywords. var score = 10 is correct.

3. Final Answer:

   var score = 10 -> Option A

4. Quick Check:

   Use var for changeable values [OK]

Hint: Use var for changeable values, let for fixed ones [OK]

Common Mistakes:

• Using let instead of var for variables
• Using non-Swift keywords like const or variable
• Syntax errors from wrong keywords

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

var count = 5
count = 10
print(count)

medium

A. 5

B. Error: Cannot assign to 'count' because it is a let constant

C. 10

D. nil

Solution

1. Step 1: Analyze variable declaration and assignment

   count is declared with var, so it can be changed. Initially, it is 5, then reassigned to 10.

2. Step 2: Understand print output

   The print statement outputs the current value of count, which is 10 after reassignment.

3. Final Answer:

   10 -> Option C

4. Quick Check:

   var allows reassignment, so output is 10 [OK]

Hint: var allows change, so print latest value [OK]

Common Mistakes:

• Thinking count stays 5 after reassignment
• Confusing var with let causing error
• Expecting nil output

4. Identify the error in this Swift code:

let name = "Alice"
name = "Bob"

medium

A. Cannot assign to 'name' because it is a constant

B. Missing semicolon after first line

C. Variable 'name' not declared

D. No error, code runs fine

Solution

1. Step 1: Check declaration of name

   name is declared with let, making it a constant.

2. Step 2: Check reassignment attempt

   The code tries to assign a new value to name, which is not allowed for constants.

3. Final Answer:

   Cannot assign to 'name' because it is a constant -> Option A

4. Quick Check:

   let constants cannot be reassigned [OK]

Hint: let means no change allowed, reassignment causes error [OK]

Common Mistakes:

• Assuming let variables can be reassigned
• Thinking semicolon is required in Swift
• Ignoring error messages

5. You want to keep track of a user's score that changes during the game, but also store the user's ID which never changes. Which declarations are correct?

hard

A. var userID = 12345
let score = 0

B. let userID = 12345
let score = 0

C. var userID = 12345
var score = 0

D. let userID = 12345
var score = 0

Solution

1. Step 1: Identify which values change

   User ID never changes, so it should be a constant declared with let. Score changes, so it should be a variable declared with var.

2. Step 2: Match declarations to requirements

   let userID = 12345 var score = 0 correctly declares userID with let and score with var.

3. Final Answer:

   let userID = 12345 var score = 0 -> Option D

4. Quick Check:

   Constants for fixed data, variables for changing data [OK]

Hint: Use let for fixed ID, var for changing score [OK]

Common Mistakes:

• Using var for fixed values causing accidental changes
• Using let for changing values causing errors
• Mixing up which data changes