Why let and var distinction matters in Swift - Challenge Your Understanding

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of code using let and var

What is the output of this Swift code?

Swift

let x = 10
var y = 20
y = y + 5
print(x, y)

A 15 20

B 10 25

C 10 20

D Compilation error

💡 Hint

Remember that let creates a constant and var creates a variable.

x is a constant with value 10 and cannot change. y is a variable initially 20, then increased by 5 to 25. So the output is 10 25.

❓ Predict Output

intermediate

What error occurs when modifying a let constant?

What error does this Swift code produce?

Swift

let a = 5
a = 10
print(a)

A Compilation error: Cannot assign to value: 'a' is a 'let' constant

B Runtime error

C Output: 10

D No error, output: 5

💡 Hint

Think about whether constants can be changed after assignment.

You cannot assign a new value to a constant declared with let. The compiler will stop with an error.

🧠 Conceptual

advanced

Why use let instead of var?

Which reason best explains why using let is preferred when possible?

A Because <code>let</code> variables can be changed later if needed

B Because <code>var</code> variables use less memory

C Because <code>let</code> makes the code safer by preventing accidental changes

D Because <code>var</code> variables are slower to access

💡 Hint

Think about safety and bugs in your code.

Using let prevents accidental changes to values, making code safer and easier to understand.

❓ Predict Output

advanced

Value after modifying a var inside a function

What is the output of this Swift code?

Swift

var count = 1
func increment() {
    count += 1
}
increment()
print(count)

A Runtime error

B 1

C Compilation error

D 2

💡 Hint

Consider how var variables can be changed inside functions.

The variable count starts at 1, the function adds 1, so the output is 2.

🔧 Debug

expert

Why does this code fail to compile?

Why does this Swift code fail to compile?

Swift

func updateValue() {
    let number = 10
    number += 5
    print(number)
}
updateValue()

A Because you cannot change a constant declared with let

B Because the function is missing a return type

C Because print cannot be used inside functions

D Because the variable number is not initialized

💡 Hint

Look at how number is declared and then changed.

The constant number declared with let cannot be changed with +=. This causes a compile error.