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.

Practice

1. What does the let keyword do in Swift?

easy

A. Defines a function

B. Creates a variable that can be changed anytime

C. Creates a constant value that cannot be changed

D. Declares a class

Solution

1. Step 1: Understand the purpose of let

   The let keyword is used to declare constants in Swift, meaning the value assigned cannot be changed later.

2. Step 2: Compare with other keywords

   Unlike var, which declares variables that can change, let ensures immutability.

3. Final Answer:

   Creates a constant value that cannot be changed -> Option C

4. Quick Check:

   let means constant = A [OK]

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

Common Mistakes:

• Confusing let with var
• Thinking let declares variables
• Assuming let can be reassigned

2. Which of the following is the correct way to declare a constant integer named age with value 30 in Swift?

easy

A. let age = 30

B. var age = 30

C. constant age = 30

D. const age = 30

Solution

1. Step 1: Identify correct Swift syntax for constants

   In Swift, let is used to declare constants, so let age = 30 is correct.

2. Step 2: Check other options

   var declares a variable, not a constant. constant and const are not valid Swift keywords.

3. Final Answer:

   let age = 30 -> Option A

4. Quick Check:

   Use let for constants = B [OK]

Hint: Use let for constants, var for variables [OK]

Common Mistakes:

• Using var instead of let for constants
• Using non-Swift keywords like const
• Syntax errors with declaration

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

let number = 10
// number = 20
print(number)

medium

A. 10

B. Compile-time error

C. 20

D. Runtime error

Solution

1. Step 1: Understand the code behavior

   The constant number is set to 10. The line trying to change it to 20 is commented out, so it does not run.

2. Step 2: Determine the printed value

   Since number remains 10, print(number) outputs 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Constant unchanged prints original value = 10 [OK]

Hint: Commented code does not run; constant stays same [OK]

Common Mistakes:

• Thinking commented code runs
• Expecting reassignment to work
• Confusing output with attempted change

4. Identify the error in this Swift code:

let pi = 3.14
pi = 3.14159
print(pi)

medium

A. No error, prints 3.14159

B. Compile-time error: cannot assign to value: 'pi' is a 'let' constant

C. Runtime error: variable not initialized

D. Syntax error: missing semicolon

Solution

1. Step 1: Check reassignment of constant

   The code tries to assign a new value to pi, which was declared with let and is immutable.

2. Step 2: Understand Swift's error

   Swift does not allow changing a constant's value, so this causes a compile-time error.

3. Final Answer:

   Compile-time error: cannot assign to value: 'pi' is a 'let' constant -> Option B

4. Quick Check:

   Reassigning let constant causes compile error = D [OK]

Hint: Constants cannot be changed after assignment [OK]

Common Mistakes:

• Trying to reassign let constants
• Expecting runtime error instead of compile error
• Ignoring error messages

5. You want to store the number of days in a week as a constant in Swift. Which code correctly declares this and prevents accidental changes later?

hard

A. let daysInWeek
// Assign value later

B. var daysInWeek = 7
// Later changed to 8 accidentally

C. constant daysInWeek = 7
// Prevents changes

D. let daysInWeek = 7
// daysInWeek cannot be changed later

Solution

1. Step 1: Choose correct keyword for constant

   Using let declares a constant that cannot be changed after assignment.

2. Step 2: Check code correctness

   let daysInWeek = 7 // daysInWeek cannot be changed later declares and assigns daysInWeek with 7 and prevents changes later. var daysInWeek = 7 // Later changed to 8 accidentally uses var which allows changes. constant daysInWeek = 7 // Prevents changes uses invalid keyword. let daysInWeek // Assign value later declares without assignment, which is invalid for constants.

3. Final Answer:

   let daysInWeek = 7 // daysInWeek cannot be changed later -> Option D

4. Quick Check:

   Use let with assignment to make constant = A [OK]

Hint: Use let with immediate value to make constant [OK]

Common Mistakes:

• Using var instead of let for constants
• Trying to declare let without initial value
• Using non-Swift keywords like constant