Think Julia

Chapter 16. Structs and Functions

Now that we know how to create new composite types, the next step is to write functions that take programmer-defined objects as parameters and return them as results. In this chapter I also present the “functional programming style” and two new program development plans.

Time

As another example of a composite type, we’ll define a struct called MyTime that records the time of day. The struct definition looks like this:

"""
Represents the time of day.

fields: hour, minute, second
"""
struct MyTime
    hour
    minute
    second
end

The name Time is already used in Julia, so I’ve chosen this name to avoid a name clash. We can create a new MyTime object as follows:

julia> time = MyTime(11, 59, 30)
MyTime(11, 59, 30)

The object diagram for the MyTime object looks like Figure 16-1.

Figure 16-1. Object diagram

Exercise 16-1

Write a function called printtime that takes a MyTime object and prints it in the form hour:minute:second. The @printf macro of the standard library module Printf prints an integer with the format sequence "%02d" using at least two digits, including a leading zero if necessary.

Exercise 16-2

Write a Boolean function called isafter that takes two MyTime objects, t1 and t2, and returns true if t1 follows t2 chronologically and false otherwise. Challenge: don’t use an if statement.

Pure Functions

In the next few sections, we’ll ...