Let for constants (immutable) in Swift - Step-by-Step Execution

Concept Flow - Let for constants (immutable)

Declare constant with let

↓

Assign value once

↓

Use constant in code

↓

Attempt to change value?

Yes→Error: Cannot modify constant

No↓

Program continues safely

This flow shows how a constant is declared once with let, assigned a value, used, and cannot be changed later.

Execution Sample

Swift

let pi = 3.14
print(pi)
// pi = 3.14159 // Error: cannot assign to let constant

Declare a constant pi with value 3.14, print it, then show that changing pi causes an error.

Execution Table

Step	Action	Code Line	Result	Notes
1	Declare constant pi	let pi = 3.14	pi = 3.14	pi is set once and cannot change
2	Print pi	print(pi)	Output: 3.14	Prints the value of pi
3	Attempt to change pi	// pi = 3.14159	Error	Cannot assign to constant pi

💡 Execution stops at error because pi is immutable and cannot be reassigned

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
pi	undefined	3.14	3.14	Error on reassignment

Key Moments - 2 Insights

Why can't we change the value of pi after declaring it with let?

Because let creates a constant that is immutable. The execution_table row 3 shows an error when trying to assign a new value.

What happens if we try to assign a value to pi before declaring it?

Swift requires constants to be declared and initialized before use. The execution_table row 1 shows pi is assigned at declaration.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of pi after step 1?

A 3.14

B undefined

C Error

D 3.14159

💡 Hint

Check the 'Result' column in row 1 of execution_table

At which step does the program produce an error?

A Step 1

B Step 2

C Step 3

D No error

💡 Hint

Look at the 'Result' column in execution_table row 3

If we replace let with var, what would happen at step 3?

A Error still occurs

B Value of pi changes successfully

C Program crashes

D pi becomes undefined

💡 Hint

Constants (let) are immutable, but variables (var) can be changed

Concept Snapshot

let constantName = value
- Declares an immutable constant
- Value assigned once, cannot change
- Trying to reassign causes error
- Use for values that must stay fixed

Full Transcript

In Swift, 'let' declares a constant that cannot be changed after it is set. First, you declare the constant with 'let' and assign a value. Then you can use it safely in your code. If you try to change the value later, Swift gives an error. This helps keep important values fixed and prevents accidental changes. The example shows declaring pi as 3.14, printing it, and then an error when trying to change it. Remember, use 'let' for values that should never change.

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