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.