Structs are value types (copy on assign) in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift code?

Consider this Swift code using a struct. What will be printed?

Swift

struct Point {
    var x: Int
    var y: Int
}

var p1 = Point(x: 5, y: 10)
var p2 = p1
p2.x = 20
print(p1.x, p2.x)

A 5 20

B 20 20

C 5 5

D 20 5

💡 Hint

Remember that structs are value types and are copied when assigned.

When p2 is assigned from p1, it creates a copy. Changing p2.x does not affect p1.x. So p1.x remains 5, p2.x becomes 20.

❓ Predict Output

intermediate

What is the value of 'a.count' after this code runs?

Look at this Swift code. What is the value of a.count after execution?

Swift

struct Counter {
    var count: Int
}

var a = Counter(count: 1)
var b = a
b.count += 1

A Compilation error

B 2

C 0

D 1

💡 Hint

Think about whether a and b share the same data or not.

Since Counter is a struct, b is a copy of a. Modifying b.count does not change a.count. So a.count remains 1 (while b.count becomes 2).

🔧 Debug

advanced

Why does this code print different values?

Examine this Swift code. Why do the two print statements output different values?

Swift

struct Score {
    var points: Int
}

var s1 = Score(points: 10)
var s2 = s1
s2.points = 20
print(s1.points)
print(s2.points)

A Because s2 was not properly assigned, it still references s1, so both print 10.

B Because structs are value types, s1 and s2 are independent copies, so s1.points remains 10.

C Because structs are reference types, s1 and s2 point to the same data, so both print 20.

D Because the print statements are inside a closure capturing s1.points, both print 10.

💡 Hint

Recall how structs behave when assigned to new variables.

Structs in Swift are value types, so assigning s1 to s2 copies the data. Changing s2.points does not affect s1.points. Therefore, s1.points prints 10, s2.points prints 20.

📝 Syntax

advanced

Which option causes a compile-time error?

Given this struct, which code snippet will cause a compile-time error?

Swift

struct Person {
    var name: String
    var age: Int
}

let p1 = Person(name: "Alice", age: 30)
var p2 = p1

A p1.name.append(" Smith")

B p2 = Person(name: "Charlie", age: 25)

C p2.age += 1

D p2.name = "Bob"

💡 Hint

Consider mutability of struct instances and their properties.

p1 is a let constant, so its properties cannot be mutated. p1.name.append(" Smith") calls a mutating method on p1.name, which is not allowed and causes a compile-time error.

The other options only affect the mutable p2.

🚀 Application

expert

How many items are in the array after this code runs?

Consider this Swift code using structs and arrays. How many elements does arr2 contain after execution?

Swift

struct Item {
    var id: Int
}

var arr1 = [Item(id: 1), Item(id: 2)]
var arr2 = arr1
arr2.append(Item(id: 3))

A 1

B 2

C 3

D Compilation error

💡 Hint

Think about how arrays and structs behave when assigned and modified.

Arrays in Swift are value types (structs). Assigning arr1 to arr2 copies the array. Appending to arr2 does not affect arr1. So arr2 has 3 elements, arr1 remains with 2.