Let for constants (immutable) in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of let constant reassignment attempt

What is the output or result of running this Swift code?

Swift

let number = 10
number = 20
print(number)

A 10

B 20

C Compile-time error: Cannot assign to value: 'number' is a 'let' constant

D Runtime error

💡 Hint

Remember that 'let' declares a constant that cannot be changed after assignment.

In Swift, variables declared with 'let' are constants and cannot be reassigned. Trying to assign a new value to 'number' causes a compile-time error.

❓ Predict Output

intermediate

Value of a let constant after initialization

What will be printed by this Swift code?

Swift

let greeting = "Hello, world!"
print(greeting)

A greeting

B Hello, world!

C Compile-time error

D Runtime error

💡 Hint

Check what value is assigned to the constant 'greeting'.

The constant 'greeting' is assigned the string "Hello, world!" and printing it outputs that string.

🔧 Debug

advanced

Why does this let constant cause a compile error?

Consider this Swift code snippet. Why does it cause a compile-time error?

Swift

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

A Because 'numbers' is a constant array and cannot be modified after initialization

B Because 'append' is not a valid method for arrays

C Because the array is empty

D Because 'print' cannot print arrays

💡 Hint

Think about what 'let' means for collections like arrays.

Declaring an array with 'let' makes the array itself immutable. You cannot add or remove elements. To modify the array, declare it with 'var'.

📝 Syntax

advanced

Which option correctly declares an immutable constant?

Which of the following Swift declarations correctly creates an immutable constant?

A const pi = 3.14159

B var pi = 3.14159

C immutable pi = 3.14159

D let pi = 3.14159

💡 Hint

Swift uses a specific keyword for constants.

'let' is the correct keyword to declare constants in Swift. 'var' declares variables. 'const' and 'immutable' are not valid Swift keywords.

🚀 Application

expert

Number of items in a let constant dictionary after initialization

Given this Swift code, how many key-value pairs does the constant dictionary contain?

Swift

let capitals = ["France": "Paris", "Japan": "Tokyo", "India": "New Delhi"]
print(capitals.count)

A 3

B 0

C 1

D Compile-time error

💡 Hint

Count the number of key-value pairs inside the dictionary literal.

The dictionary 'capitals' has three key-value pairs, so its count is 3.