An Example Program Using Structs

To understand when we might want to use structs, let’s write a program that calculates the area of a rectangle. We’ll start by using single variables, and then refactor the program until we’re using structs instead.

Let’s make a new project with Scarb called rectangles that will take the width and height of a rectangle specified in pixels and calculate the area of the rectangle. Listing 5-6 shows a short program with one way of doing exactly that in our project’s src/lib.cairo.

Filename: src/lib.cairo

fn main() {
    let width1 = 30;
    let height1 = 10;
    let area = area(width1, height1);
    println!("Area is {}", area);
}

fn area(width: u64, height: u64) -> u64 {
    width * height
}

Listing 5-6: Calculating the area of a rectangle specified by separate width and height variables.

Now run the program with scarb cairo-run:

$ scarb cairo-run 
   Compiling listing_04_06_no_struct v0.1.0 (listings/ch05-using-structs-to-structure-related-data/listing_03_no_struct/Scarb.toml)
    Finished release target(s) in 1 second
     Running listing_04_06_no_struct
Area is 300
Run completed successfully, returning []
Remaining gas: 19913250

This code succeeds in figuring out the area of the rectangle by calling the area function with each dimension, but we can do more to make this code clear and readable.

The issue with this code is evident in the signature of area:

fn area(width: u64, height: u64) -> u64 {

The area function is supposed to calculate the area of one rectangle, but the function we wrote has two parameters, and it’s not clear anywhere in our program that the parameters are related. It would be more readable and more manageable to group width and height together. We’ve already discussed one way we might do that in the Tuple Section of Chapter 2.

Refactoring with Tuples

Listing 5-7 shows another version of our program that uses tuples.

Filename: src/lib.cairo

fn main() {
    let rectangle = (30, 10);
    let area = area(rectangle);
    println!("Area is {}", area);
}

fn area(dimension: (u64, u64)) -> u64 {
    let (x, y) = dimension;
    x * y
}

Listing 5-7: Specifying the width and height of the rectangle with a tuple.

In one way, this program is better. Tuples let us add a bit of structure, and we’re now passing just one argument. But in another way, this version is less clear: tuples don’t name their elements, so we have to index into the parts of the tuple, making our calculation less obvious.

Mixing up the width and height wouldn’t matter for the area calculation, but if we want to calculate the difference, it would matter! We would have to keep in mind that width is the tuple index 0 and height is the tuple index 1. This would be even harder for someone else to figure out and keep in mind if they were to use our code. Because we haven’t conveyed the meaning of our data in our code, it’s now easier to introduce errors.

Refactoring with Structs: Adding More Meaning

We use structs to add meaning by labeling the data. We can transform the tuple we’re using into a struct with a name for the whole as well as names for the parts.

Filename: src/lib.cairo

struct Rectangle {
    width: u64,
    height: u64,
}

fn main() {
    let rectangle = Rectangle { width: 30, height: 10, };
    let area = area(rectangle);
    println!("Area is {}", area);
}

fn area(rectangle: Rectangle) -> u64 {
    rectangle.width * rectangle.height
}

Listing 5-8: Defining a Rectangle struct.

Here we’ve defined a struct and named it Rectangle. Inside the curly brackets, we defined the fields as width and height, both of which have type u64. Then, in main, we created a particular instance of Rectangle that has a width of 30 and a height of 10. Our area function is now defined with one parameter, which we’ve named rectangle which is of type Rectangle struct. We can then access the fields of the instance with dot notation, and it gives descriptive names to the values rather than using the tuple index values of 0 and 1.