Variables (let, var) and type inference in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

What is the output of this Swift code snippet?

Consider the following Swift code:

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

What will be printed?

iOS Swift

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

A 10

B 15

C 5

D Compilation error

💡 Hint

Remember that 'let' defines a constant and 'var' defines a variable that can change.

The variable y starts at 5, then we add x (which is 10) to y, so y becomes 15. The print statement outputs 15.

❓ ui_behavior

intermediate

What happens if you try to change a 'let' constant in Swift?

Given this code:

let name = "Alice"
name = "Bob"

What will happen when you try to compile this?

iOS Swift

let name = "Alice"
name = "Bob"

A Runtime error when assigning "Bob"

B The value of 'name' changes to "Bob"

C Compilation error because 'name' is a constant

D The code runs but prints "Alice"

💡 Hint

Think about what 'let' means in Swift.

'let' declares a constant, so you cannot assign a new value after initialization. The compiler will show an error.

🧠 Conceptual

advanced

What type does Swift infer for this variable?

Look at this Swift code:

var score = 42

What type does Swift assign to 'score'?

iOS Swift

var score = 42

A Int

B Double

C String

D Any

💡 Hint

Swift infers types based on the assigned value.

Since 42 is an integer literal, Swift infers the type of 'score' as Int.

❓ lifecycle

advanced

What is the value of 'message' after this code runs?

Consider this Swift code:

var message = "Hello"
let greeting = message
message = "Hi"
print(greeting)

What will be printed?

iOS Swift

var message = "Hello"
let greeting = message
message = "Hi"
print(greeting)

A Hello

B Hi

C Compilation error

D Runtime error

💡 Hint

Think about how assignment copies values for strings in Swift.

The constant 'greeting' copies the value of 'message' at assignment time, so it remains "Hello" even after 'message' changes.

🔧 Debug

expert

Which option causes a compile-time error?

Which of these Swift code snippets will NOT compile?

A

var text = "Hello"
text = "World"

B let pi = 3.14

C

var flag: Bool
flag = true

D

let number: Int = 10
number = 20

💡 Hint

Remember that 'let' constants cannot be reassigned.

Option D tries to assign a new value to a constant 'number', which causes a compile-time error.

Practice

1. What does the keyword let do in Swift?

easy

A. Creates a constant that cannot be changed

B. Creates a variable that can be changed

C. Declares a function

D. Defines a class

Solution

1. Step 1: Understand let usage

   The keyword let is used to declare constants in Swift, which means their value cannot be changed after assignment.

2. Step 2: Compare with var

   Unlike var, which declares variables that can change, let makes the value fixed.

3. Final Answer:

   Creates a constant that cannot be changed -> Option A

4. Quick Check:

   let = constant [OK]

Hint: Remember: let = fixed value, var = changeable value [OK]

Common Mistakes:

• Confusing let with var
• Thinking let allows value changes
• Mixing let with function declaration

2. Which of the following is the correct way to declare a variable named score with initial value 10 in Swift?

easy

A. let score = 10

B. var score = 10

C. score var = 10

D. variable score = 10

Solution

1. Step 1: Identify variable declaration syntax

   In Swift, var is used to declare variables that can change, and the syntax is var name = value.

2. Step 2: Check each option

   var score = 10 uses correct syntax. let score = 10 declares a constant, not a variable. score var = 10 and variable score = 10 are invalid syntax.

3. Final Answer:

   var score = 10 -> Option B

4. Quick Check:

   Variable declaration = var name = value [OK]

Hint: Use var for variables, let for constants [OK]

Common Mistakes:

• Using let instead of var for variables
• Incorrect keyword order
• Using non-existent keywords like variable

3. What will be the output of this Swift code?

var name = "Alice"
name = "Bob"
print(name)

medium

A. Error: Cannot assign to let constant

B. Alice

C. Bob

D. nil

Solution

1. Step 1: Understand variable reassignment

   The variable name is declared with var, so it can be changed after initial assignment.

2. Step 2: Trace the code execution

   Initially, name is "Alice". Then it is reassigned to "Bob". The print statement outputs the current value, "Bob".

3. Final Answer:

   Bob -> Option C

4. Quick Check:

   var allows change, so output is reassigned value [OK]

Hint: var means value can change, so print latest value [OK]

Common Mistakes:

• Thinking let was used and reassignment causes error
• Expecting original value to print
• Confusing variable with constant

4. What is wrong with this Swift code?

let age = 25
age = 30

medium

A. Cannot assign to value: 'age' is a constant

B. Missing semicolon after 25

C. Variable 'age' not declared

D. Syntax error: let cannot be used

Solution

1. Step 1: Identify declaration type

   age is declared with let, making it a constant.

2. Step 2: Understand reassignment error

   Trying to assign a new value to a constant causes a compile-time error: "Cannot assign to value: 'age' is a constant".

3. Final Answer:

   Cannot assign to value: 'age' is a constant -> Option A

4. Quick Check:

   let constants cannot be changed [OK]

Hint: let means constant; reassignment causes error [OK]

Common Mistakes:

• Thinking semicolon is required
• Confusing let with var
• Ignoring reassignment rules

5. Consider this Swift code:

var x = 5
let y = x
x = 10
print(y)

What will be printed and why?

hard

A. 10, because y tracks x's value

B. nil, because y is not initialized

C. Error, cannot assign var to let

D. 5, because y is a constant copy of x's initial value

Solution

1. Step 1: Understand value assignment

   Variable x is 5 initially. y is a constant assigned the current value of x, which is 5.

2. Step 2: Reassign x and check y

   Changing x to 10 does not affect y, because y holds its own copy of the value 5.

3. Final Answer:

   5, because y is a constant copy of x's initial value -> Option D

4. Quick Check:

   let copies value; later changes to var don't affect it [OK]

Hint: let copies value at assignment; later var changes don't affect it [OK]

Common Mistakes:

• Thinking y updates when x changes
• Expecting runtime error on assignment
• Confusing reference and value types