Let for constants (immutable) in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a constant named pi with the value 3.14.

Swift

let [1] = 3.14

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

Api

Bvar

Cradius

Darea

Attempts:

3 left

💡 Hint

Common Mistakes

Using var instead of let for constants.

Choosing a variable name instead of a constant name.

In Swift, let declares a constant. Here, pi is the constant name.

2fill in blank

medium

Complete the code to declare a constant maxScore with the integer value 100.

Swift

let [1] = 100

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

Ascore

BscoreMax

Cmax

DmaxScore

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different name than maxScore.

Using var instead of let.

The constant name should be maxScore as requested.

3fill in blank

hard

Fix the error by completing the code to declare a constant greeting with the value "Hello".

Swift

let [1] = "Hello"

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

Alet greeting

Bgreeting

Cvar greeting

Dgreet

Attempts:

3 left

💡 Hint

Common Mistakes

Writing let let greeting or var greeting instead of just the name.

Omitting the constant name.

When declaring a constant, use let once, then the name. Here, only the name greeting goes after let.

4fill in blank

hard

Fill both blanks to declare a constant speedLimit with the value 60 and print it.

Swift

let [1] = 60
print([2])

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

AspeedLimit

Bspeed

Dlimit

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for declaration and print.

Using var instead of let.

The constant name is speedLimit. Use it both to declare and to print.

5fill in blank

hard

Fill all three blanks to declare constants width and height with values 10 and 20, then print their sum.

Swift

let [1] = 10
let [2] = 20
print([3] + [2])

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

Awidth

Bheight

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong names like length.

Mixing variable and constant declarations.

Declare width and height constants, then print their sum using width + height.

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