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Think Julia by Allen B. Downey, Ben Lauwens

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Chapter 15. Structs and Objects

At this point you know how to use functions to organize code and built-in types to organize data. The next step is to learn how to build your own types to organize both code and data. This is a big topic; it will take a few chapters to get there.

Composite Types

We have used many of Julia’s built-in types; now we are going to define a new type. As an example, we will create a type called Point that represents a point in two-dimensional space.

In mathematical notation, points are often written in parentheses with a comma separating the coordinates. For example, 0 , 0 represents the origin, and x , y represents the point x units to the right and y units up from the origin.

There are several ways we might represent points in Julia:

  • We could store the coordinates separately in two variables, x and y.

  • We could store the coordinates as elements in an array or tuple.

  • We could create a new type to represent points as objects.

Creating a new type is more complicated than the other options, but it has advantages that will be apparent soon.

A programmer-defined composite type is also called a struct. The struct definition for a point looks like this:

struct Point
    x
    y
end

The header indicates that the new struct is called Point. The body defines the attributes or fields of the struct. The Point struct has two fields: x and y.

A struct is like a factory for creating objects. To create a point, you call Point as if it were a function having ...

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