Variables (let, var) and type inference in iOS Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a constant named name with value "Alice".

iOS Swift

let [1] = "Alice"

Drag options to blanks, or click blank then click option'

Aname

Bvar

Clet

DName

Attempts:

3 left

💡 Hint

Common Mistakes

Using var instead of let for a constant.

Capitalizing variable names incorrectly.

Use let to declare a constant and name as the variable name.

2fill in blank

medium

Complete the code to declare a variable age with initial value 30.

iOS Swift

var [1] = 30

Drag options to blanks, or click blank then click option'

Aage

Blet

CAge

Dvar

Attempts:

3 left

💡 Hint

Common Mistakes

Using let instead of var for a variable.

Capitalizing variable names incorrectly.

Use var to declare a variable and age as the variable name.

3fill in blank

hard

Fix the error in the code by choosing the correct keyword to declare a variable that can change.

iOS Swift

[1] score = 100
score = 150

Drag options to blanks, or click blank then click option'

Afinal

Blet

Cconst

Dvar

Attempts:

3 left

💡 Hint

Common Mistakes

Using let for variables that need to change.

Using JavaScript keywords like const or final.

Use var to declare a variable that can be changed later.

4fill in blank

hard

Fill both blanks to declare a variable height of type Double with value 1.75.

iOS Swift

var [1]: [2] = 1.75

Drag options to blanks, or click blank then click option'

Aheight

BInt

CDouble

Dvar

Attempts:

3 left

💡 Hint

Common Mistakes

Using Int type for decimal numbers.

Putting the type before the variable name.

Use height as the variable name and Double as the type for decimal numbers.

5fill in blank

hard

Fill all three blanks to declare a constant greeting with inferred type and value "Hello".

iOS Swift

[1] [2] = [3]

Drag options to blanks, or click blank then click option'

Alet

Bgreeting

C"Hello"

Dvar

Attempts:

3 left

💡 Hint

Common Mistakes

Using var instead of let for constants.

Forgetting quotes around string values.

Use let for constant, greeting as name, and "Hello" as value with type inferred.

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