Variables (let, var) and type inference in iOS Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the difference between let and var in Swift?

let declares a constant, which means its value cannot change after it is set. var declares a variable, which means its value can change.

beginner

What does type inference mean in Swift?

Type inference means Swift automatically figures out the type of a variable or constant based on the value you assign to it, so you don't always have to write the type explicitly.

beginner

How do you declare a variable named score with an initial value of 10 using type inference?

You write: var score = 10. Swift infers that score is an Int because 10 is an integer.

beginner

Can you change the value of a constant declared with let after it is set?

No, once a constant is set with let, its value cannot be changed. Trying to do so causes a compile-time error.

intermediate

Why is it good to use let instead of var when possible?

Using let helps keep your code safe and predictable because constants cannot change unexpectedly. It also helps the compiler optimize your code.

Which keyword declares a variable whose value can change in Swift?

A let

B var

C const

D static

var declares a variable that can change. let declares a constant.

What type will Swift infer for let name = "Alice"?

A String

B Int

C Double

D Bool

Because "Alice" is text, Swift infers the type String.

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

A The value changes successfully

B The app crashes at runtime

C You get a compile-time error

D The value resets to default

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

Which is a correct way to declare a variable with type inference?

A var age: Int = 25

B var age = 25

C let age = 25

D All of the above

All are correct. var age = 25 uses type inference. var age: Int = 25 explicitly sets type. let age = 25 declares a constant.

Why might you prefer let over var when possible?

A Because constants prevent accidental changes

B Because <code>var</code> is deprecated

C Because <code>let</code> variables use less memory

D Because <code>let</code> variables run faster on all devices

Using let helps avoid bugs by preventing accidental changes to values.

Explain the difference between let and var in Swift and why you would choose one over the other.

Think about whether the value should stay the same or can change.

Describe what type inference is and how it helps when declaring variables in Swift.

Imagine Swift as a helpful friend who figures out the type for you.

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