Programonic

Before you start

Swift functions.

Model data with a struct, which is copied when assigned.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

A struct groups stored properties and methods into a value type. Swift generates a memberwise initializer, so Point(x: 3, y: 4) works without writing one.

Structs are copied on assignment, which makes them predictable: changing one copy never changes another. A struct held in a var can have its properties changed, as with point.x = 6.

Code example

Swift

struct Point {
    var x: Int
    var y: Int

    func distanceSquared() -> Int {
        return x * x + y * y
    }
}

var point = Point(x: 3, y: 4)
print(point.distanceSquared())
point.x = 6
print(point.distanceSquared())

Output

25
52

Example details

Follow the code

Open the walkthrough to inspect the example one step at a time.

Show guided walkthrough

Guided walkthroughReady

1struct Point {

2 var x: Int

3 var y: Int

5 func distanceSquared() -> Int {

6 return x * x + y * y

7 }

8}

10var point = Point(x: 3, y: 4)

11print(point.distanceSquared())

12point.x = 6

13print(point.distanceSquared())

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
A value type with two Int properties.	`—`	—
Use the generated initializer.	`point = (3, 4)`	—
9 + 16.	`point = (3, 4)`	25
Change one property in place.	`point = (6, 4)`	25
36 + 16.	`point = (6, 4)`	25 52

Behind the scenes

How Swift processes this code

Swift compiles the program; its top-level statements run in source order.

Cost and common mistakes

O(1) time and space per call.

A struct held in a let constant cannot have its properties changed.
Because structs are copied, a copy made earlier does not see later changes.

Try it yourself

Predict the second result if y were set to 0 instead of x to 6.

Reveal answer and reasoning

The second call returns 9 + 0 = 9.

Swift Structs