Let for constants (immutable) in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the let keyword do in Swift?

The let keyword declares a constant, which means its value cannot be changed after it is set.

beginner

What happens if you try to change a let constant after it is set?

Swift will give a compile-time error because constants cannot be modified once assigned.

beginner

How do let and var differ in Swift?

let declares a constant (immutable), while var declares a variable (mutable).

beginner

Example: <br><code>let pi = 3.14</code><br>Can you change <code>pi</code> later in the code?

No, because pi is declared with let, it is a constant and cannot be changed.

beginner

Why use let instead of var when possible?

Using let helps prevent accidental changes and makes your code safer and easier to understand.

What keyword declares an immutable value in Swift?

A let

B var

C const

D immutable

let declares a constant that cannot be changed.

What happens if you try to assign a new value to a let constant?

A The program runs normally

B The value changes silently

C Runtime error

D Compile-time error

Swift prevents changing constants by giving a compile-time error.

Which keyword allows you to change the value after declaration?

A var

B const

C let

D final

var declares a variable that can be changed.

Why is it good to use let when possible?

A To make code faster

B To prevent accidental changes

C To allow changes later

D To save memory

Using let helps keep values safe from accidental changes.

Which of these is a correct constant declaration in Swift?

A immutable name = "Alice"

B var name = "Alice"

C let name = "Alice"

D const name = "Alice"

let name = "Alice" correctly declares a constant.

Explain what let does in Swift and why it is useful.

Think about how constants help avoid mistakes.

Describe the difference between let and var in Swift.

One can change, the other cannot.

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