Structs are value types (copy on assign) in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a struct named Point.

Swift

struct [1] {
    var x: Int
    var y: Int
}

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

APoint

BClass

CRectangle

DCircle

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Class' instead of a struct name.

Choosing a shape name unrelated to the example.

The keyword struct defines a value type named Point.

2fill in blank

medium

Complete the code to assign one struct instance to another.

Swift

var p1 = Point(x: 5, y: 10)
var p2 = [1]

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

Ap1

BPoint(x: 0, y: 0)

CPoint()

Dp2

Attempts:

3 left

💡 Hint

Common Mistakes

Creating a new instance instead of copying.

Assigning the variable to itself.

Assigning p1 to p2 copies the value because structs are value types.

3fill in blank

hard

Fix the error in copying and modifying a struct instance.

Swift

var p1 = Point(x: 3, y: 4)
var p2 = p1
p2.[1] = 10

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

Az

Bwidth

Cx

Dheight

Attempts:

3 left

💡 Hint

Common Mistakes

Using a property name not defined in the struct.

Trying to modify the original instance instead of the copy.

The struct Point has properties x and y. Modifying x on the copy is valid.

4fill in blank

hard

Fill both blanks to create a copy of a struct and change its property.

Swift

var original = Point(x: 1, y: 2)
var copy = [1]
copy.[2] = 5

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

Aoriginal

Bcopy

Cx

Dy

Attempts:

3 left

💡 Hint

Common Mistakes

Modifying the original instead of the copy.

Using the wrong property name.

Assigning original to copy creates a value copy. Then modifying x changes only the copy.

5fill in blank

hard

Fill all three blanks to demonstrate that modifying a copy does not affect the original struct.

Swift

var a = Point(x: 7, y: 8)
var b = [1]
b.[2] = 0
print(a.[3])

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

Aa

Bx

Dy

Attempts:

3 left

💡 Hint

Common Mistakes

Modifying the original struct instead of the copy.

Printing the wrong property.

Copy a into b. Change b.x. Printing a.y shows the original value unchanged.

Practice

1. What happens when you assign one struct variable to another in Swift?

easy

A. A new copy of the struct data is created.

B. Both variables point to the same data in memory.

C. The original struct is deleted.

D. The struct becomes a class automatically.

Solution

1. Step 1: Understand struct assignment behavior

   Structs in Swift are value types, so assigning one struct to another copies the data.

2. Step 2: Compare with reference types

   Unlike classes, structs do not share references; each variable has its own copy.

3. Final Answer:

   A new copy of the struct data is created. -> Option A

4. Quick Check:

   Struct assignment = copy data [OK]

Hint: Structs copy data on assignment, not references [OK]

Common Mistakes:

• Thinking structs share data like classes
• Assuming assignment links variables
• Confusing structs with reference types

2. Which of the following is the correct way to define a struct in Swift?

easy

A. struct Person = { var name: String }

B. struct Person { var name: String }

C. class Person { var name: String }

D. struct Person() { var name: String }

Solution

1. Step 1: Recall Swift struct syntax

   Structs are defined using the keyword struct followed by the name and braces containing properties.

2. Step 2: Identify correct syntax

   The correct syntax is struct Person { var name: String }. Using = after the name is invalid, using class defines a class instead of a struct, and adding parentheses like Person() is incorrect.

3. Final Answer:

   struct Person { var name: String } -> Option B

4. Quick Check:

   Struct syntax = struct Name { properties } [OK]

Hint: Use 'struct Name { }' to define structs [OK]

Common Mistakes:

• Using class keyword instead of struct
• Adding parentheses after struct name
• Using equals sign in struct definition

3. Consider this Swift code:

struct Point { var x: Int; var y: Int }
var p1 = Point(x: 5, y: 10)
var p2 = p1
p2.x = 20
print(p1.x)

What is the output?

medium

A. 20

B. Compilation error

C. 5

D. 10

Solution

1. Step 1: Understand struct copying on assignment

   Assigning p1 to p2 copies the data, so p2 is a separate copy.

2. Step 2: Analyze mutation effect

   Changing p2.x to 20 does not affect p1.x, which remains 5.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   Struct copy means original unchanged [OK]

Hint: Changing copy doesn't affect original struct [OK]

Common Mistakes:

• Assuming p1.x changes to 20
• Confusing structs with reference types
• Expecting runtime error on assignment

4. This Swift code tries to update a struct inside a function:

struct Counter { var count: Int }
func increment(counter: Counter) {
    var counter = counter
    counter.count += 1
}
var c = Counter(count: 0)
increment(counter: c)
print(c.count)

What is the output and why?

medium

A. 0, because the function modifies a copy, not the original

B. 1, because the function updates the original struct

C. Compilation error due to modifying parameter

D. Runtime error due to immutable struct

Solution

1. Step 1: Understand parameter passing for structs

   Structs are passed by value, so the function receives a copy of c.

2. Step 2: Analyze mutation inside function

   The function modifies its local copy, but c outside remains unchanged.

3. Final Answer:

   0, because the function modifies a copy, not the original -> Option A

4. Quick Check:

   Structs passed by value = original unchanged [OK]

Hint: Structs passed to functions are copies, not originals [OK]

Common Mistakes:

• Expecting original struct to change
• Thinking function parameter is reference
• Assuming compilation or runtime error

5. You want to create a Swift struct that tracks a score and allows updating it inside a method. Which approach correctly updates the struct's property?

hard

A. Use a global variable to hold the struct and update it.

B. Use a non-mutating method and assign a new struct to self.

C. Declare the struct as a class to allow property changes.

D. Use a mutating method inside the struct to change the property.

Solution

1. Step 1: Recall how to modify struct properties

   Structs are value types; to modify properties inside methods, the method must be marked mutating.

2. Step 2: Evaluate options

   Use a mutating method inside the struct to change the property. correctly uses a mutating method. Use a non-mutating method and assign a new struct to self. is incorrect because non-mutating methods cannot change properties. Declare the struct as a class to allow property changes. changes the type to class, which is different. Use a global variable to hold the struct and update it. uses a global variable, which is not ideal.

3. Final Answer:

   Use a mutating method inside the struct to change the property. -> Option D

4. Quick Check:

   Mutating methods update struct properties [OK]

Hint: Mark methods mutating to change struct properties [OK]

Common Mistakes:

• Trying to modify properties in non-mutating methods
• Confusing structs with classes for mutability
• Using global variables instead of methods