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- In this lesson,

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we're going to investigate
a complete Rust application

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that parses a CSV file.

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So this will give us a chance

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to consolidate the understanding

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of application structure in Rust in terms

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of how to break the application
down into separate modules

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and a closer look at the syntax of Rust.

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We've already covered that in some detail

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but to see it applied

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in a larger application will will be nice.

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And also a closer look at some

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of the libraries available
in Rust and some techniques.

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We've had a touch on some of these topics

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but we're gonna delve deeper

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into some particular libraries here,

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such as file handling and date handling,

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common requirements in
lots of applications.

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So I'm going to just lead you

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through the application functionality.

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I'm going to run the application.

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It's in this project, lesson16_csv_parser.

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I'm just gonna show a
couple of files initially.

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The application is gonna be reading data

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from a file called data.txt.

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It's a CSV file, comma-separated value.

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And I'm gonna show you
the structure of the file

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so you can understand the
scenario, the application

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and then I'll just show you the main code.

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It doesn't do much, it just
kind of kickstarts lots

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of other parts of the application

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but it'll give you an
idea of the overall shape.

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I'll run the application then

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and I'll go through the functionality

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so you can understand what
it is we're going to achieve

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when we look at the code in detail

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for the rest of the lesson.

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Okay, so let's take a look at the code.

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Okay, so in the src folder, note this.

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I've got the main file,
which is the entry point

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and then I've got four other
modules, four other files

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to split the application
functionality into chunks.

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Fh.rs contains all the file
handling functionality.

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Menu.rs displays a menu
of options to the user.

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You could have guessed that I suppose.

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Types.rs define some of the structures

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that I'm gonna use across the application.

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And then util.rs, I mean I've
chosen these names myself.

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You can call 'em anything you like.

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Util.rs contains utility functions
that are generally useful

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across the whole application

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and obviously, main is the entry point.

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So in an application
that has multiple files,

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the first thing you do in
main is to declare submodules.

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Remember the mod keyword declares a module

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and Rust then has to find
the code for that module.

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And by default, it'll look
for a file called types.rs.

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So this statement will
kind of include the code

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for types.rs to define my data types

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and then I define a module for util,

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which will obviously pull in util.rs

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with my utility code and file
handling and menu handling.

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Now, probably the first new note here

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is that the main function's very small.

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This is quite common in
professional applications.

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The main function doesn't really do much.

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It kickstarts other
functionality located elsewhere.

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So generally, our main function
would be a couple of lines,

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literally something like this.

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So let me give you like a broad overview

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of the intent before we
look at the detail later on.

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In my util module,

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the util module that I declared there,

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in my util module,

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there's a function, parse_command_line,

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which we'll have a look at later on.

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The idea when you run the
application, optionally,

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you can run the application
and you can specify cargo run

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and you can specify the name

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of the text file that
you'd like to load in.

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If the user hasn't specified the text file

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on the command line, then
it defaults to data.txt.

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So let's take a look at that now.

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Data.txt this here.

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Let me tell you a story
about a country that decided

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to leave the European Union in 2016.

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Brexit.

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I live in the UK, and before Brexit,

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we could travel to Europe
completely unimpinged,

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complete freedom of movement.

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Well, those days have
gone, we've Brexited now.

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One of the consequences

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is that you can only travel to a European,

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Schengen country for a maximum of 90 days

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in any 180-day period.

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And I travel away to Europe a lot

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with my work and holiday.

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Living in the UK, Europe is
our main holiday location.

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So I started to feel a bit anxious

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that I might accidentally be away

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for more than 90 days in 180-day period.

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And I'm not sure what would happen then.

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I don't know if I wanna find out.

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So I wrote myself a Rust program

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and here it is to keep track
of all my visits to Europe

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so that I can keep track of
how many days I've been away.

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It's a rolling thing.

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So for any day, you have to look back

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at the previous 180 days and
you must not have traveled more

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than 90 in that window.

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Tomorrow, the window moves on a day, okay?

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So it's always looking back
at the previous 180 days.

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So this actually, this date here,

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it was my first visit
overseas after COVID lockdown.

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I was amazed to find out

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that this course was actually
going ahead in person.

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I remember it so clearly.

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I was overjoyed to actually
get the chance to travel again,

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like a lot of people.

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It was on the 30th of August 2021.

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And so the start date is
the date when I arrived

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in the country.

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It was in Norway, and the
end date is exclusive.

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So I was in Norway on the 7th of September

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but this date is the day after.

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And like a lot of things in programming,

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the lower index or the
lower bound is inclusive

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and the upper bound is exclusive.

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So if you know the date

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after you came back and
the date when you went,

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you can calculate how
many days you were there.

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I've got a description of
what I was doing as well.

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I was teaching C++ in Kongsberg.

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So each line is basically
a different visit

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into the Schengen zone
and it's mainly Norway.

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I do most of my work in
Norway and I love Norway.

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I really do.

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But we went to a trip, a holiday
to Malta in November 2021,

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which is our first holiday,

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like a holiday holiday for
about two years abroad.

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And it was magical,

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even though it was starting
to get a bit chilly

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in November in Malta, we went to Valletta,

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I'd really recommend it.

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So anyway, there we go.

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Various different...

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Now you know my calendar.

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Up to and including a course
that's currently scheduled.

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Oh, that's my birthday, the
3rd of December in Copenhagen.

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That's another beautiful
city, a city of spires.

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So that's my file, data.txt.

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And when I run the application,
it looks to see if the user

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from the command line has
specified a different file name.

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And if not, it'll just default
to reading in data.txt.

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So that's the file name
that's gonna get read in.

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And all my file handling functionality

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I've put into my Fh module.

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So in the Fh module,

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I've got a function called
read_visits_from_file.

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I give it the file name,

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the file name that I specified up here

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and it'll give me back
a collection of visits.

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I've got a structure called
visit defined in types.rs.

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We'll have a look at that later.

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It says what date you visited,
what date you came back

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and what were you doing there?

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Teaching C++ in plus plus in Kongsberg.

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So that's like a vector of visits.

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And then I displayed a menu.

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My menu module has a do_menu function.

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And you'll see how this
works in the moment.

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It displays a list of options
like calculate dates in visit.

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It gives you a chance to add another visit

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if you've just booked
another trip or you can quit.

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In fact, the do_menu
function, it loops around,

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allowing you to choose items
repeatedly from the menu.

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But when you say quit,
the function returns.

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And in fact, there are two
different ways of quitting

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in my example.

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You can quit without saving any changes

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or you can quit asking for
those changes to be saved.

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And this function, as we'll
see, returns a true or false.

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If it returns true, it
means the user chose

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to quit the application and
save the changes they've made

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to the itinerary.

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So if this function returns
true, then I save the visits

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that have been updated back to the file.

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And again, write_visits_to_file
is defined in my Fh module.

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So over the next few sections,

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we're gonna delve into the file handling.

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We talk about utility handling as well

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and dates, times, and
command line parsing.

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So you'll see how it all
works, but you can kind

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of get a framework for what
we're trying to achieve here.

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So I'm gonna run the application,
I'm gonna play with it

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for a few minutes and you can as well

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just to get a flavor for what's going on.