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- As applications get larger,
and Rust is no exception,

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it becomes important
to start thinking about

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how you're gonna organize your application

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into separate files and
separate data types.

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So things to consider.

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How are you gonna split your application

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into different files or modules in Rust?

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What data types do we
need in the application

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in order to, you know,
congeal data cohesively

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and do proper structures?

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Are there any additional
libraries, or crates,

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as Rust would say, that we need
to add into our application?

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So let's take a look at our application

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with those things in mind.

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We're going to take a
bit of a closer look now

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at the source files in the application.

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So we're gonna have a
look at main.rs first.

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We've already had a quick look at that.

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Then we'll have a look at types.rs,

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where we define structures that we need

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to represent the data in our application,

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like visit, for example, to Europe.

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Then we'll have a look at util.

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I'm not gonna go into the details yet,

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I'm just gonna discuss the broad intent.

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Util.rs defines some
miscellaneous utility functions,

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getting stuff in the keyboard,
handling dates and times,

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miscellaneous utility, things like that.

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File handling in fh.rs.

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And then the menu.rs,

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which displays our menu

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and keeps on redisplaying the menu

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until the user chooses to quit.

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Okay, so we'll have a look
at all of these files.

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I will also have a quick
look at the toml file

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to see what other crates
our application requires.

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Okay, so here we are then.

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In main.rs first of all,

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we have already seen this in
the last code walkthrough.

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So it defines four modules,

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which are defined in separate files,

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types.rs and it defines util.rs

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and module and main.

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So let's have a look
first of all, in types,

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that's the first module that we define.

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It defines common structures
and maybe other types,

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like enums and traits that
are used in application.

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Okay, so here we are.

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Interesting, lots of
different details here.

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Obviously we're gonna dive
into the details later on.

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So I'm just gonna give
you the big picture now.

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So you've got this kind of
image of where we're heading to.

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A visit.

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The visit structure contains information

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about a visit to Europe,

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is public so I can use it in other files.

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It defines the data for
a visit, the start date

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the end date, and the description.

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I've used a class or structure
called naive date, okay?

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Naive date is defined in the chrono crate.

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Naive date represents,

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it's not that naive really.

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You'd think with a name like naive date

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you wouldn't wanna use
it because it's naive,

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but it's naive in the sense

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that it doesn't have a time zone.

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Okay, but apart from that,
it's perfectly usable.

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So if you're not interested in time zones

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in your date handling, use naive date.

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It's by far the simplest way.

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So the visit structure
represents, as read from the file,

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a visit to Europe.

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What date did you visit?

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What was the end date, or
the day after you came back,

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and the description of the visit?

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And then I've got some functionality

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associated with the visit
structure as well, okay?

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And we'll have a look at
these functions later on,

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but you can see that it has,

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it's quite a simple structure really.

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So we'll have a look at the
details of that in due course.

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Oh, also the visit structure
implements the display trait.

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Remember the display trait?

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If a structure implements display

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then you can output it
using the default formatter

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in println.

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You can say println, curly brackets, visit

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and it'll call the display
formatting to output.

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It's like a two sprint basically
for the visit structure.

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I've also got another
structure called record.

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The reason for that would
become apparent later on.

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It defines some data.

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Oh, it automatically supports the eq

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and the partial eq traits.

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Remember those from before?

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That gives you the ability to
say less than or greater than,

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less than or equal to,
greater than or equal to.

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So there's implementation

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and then it implements the
display trait to be displayed.

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And it implements a couple
of other traits as well.

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I've gone to really like the way

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that Rust splits trait
implementations into separate slices.

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If this is Java or C# or C++,

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then you would define
a single block of code

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that contained all of this,

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the data and the functions.

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And the functions which
implement one interface

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and the functions which implement another,

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all in one massive class.

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Whereas in Rust, it's much more obvious.

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This is the implementation of
the display trait for record

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and this is the implementation
of the ord trait

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and the partial ord trait.

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So it's much more segmented,
much more discreet.

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So it's quite nice, I think.

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So obviously we look at
the details of that later,

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but that's our types module.

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Next up we're gonna look at util.

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Here's our util module

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and it contains some miscellaneous
utility functions, okay?

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Nothing strategic, just tactically useful

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to do some tricky code, hide
it into this util module

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to parse the command line.

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It grabs the parameters
in the command line

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and it gives me either
a string, or nothing,

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depending on whether there
was a command line or not.

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In many cases in the application

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I ask the user, please
enter a string, okay?

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So I've encapsulated that

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in a prompt for string function.

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There's a message that
it displays to the user,

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a prompt, please enter
something, something, something,

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and it returns a string
as entered by the user.

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There were other times
when I asked the user

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to enter a date, okay?

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So like, what start date
would you like to specify?

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So it displays a message
and it returns a date,

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or rather a naive date without time zone,

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like the 31st of December.

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And then a conversion function.

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I pass in a string or a string slice,

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and it'll convert it or
parse it into a date.

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It's like a date parser, really.

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You parse in a string and it parses it

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and returns the date that it contains.

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So I didn't plan on
having these functions,

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I just started writing
the main body of the code

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and often just something fiddly

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ends up being five lines of code

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just to parse a string for a date.

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So at that point, it's
useful to push it down

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into a function to hide it
from the main body of the code.

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And also potentially be
able to reuse that function

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elsewhere.

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Okay, so that's util.

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The next module we're gonna
look at is FH, file handling.

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So just two functions in here,

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notice they're both public, obviously.

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There's a function which
will read visits from a file.

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This is the function.

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It gets (indistinct) called at
the start of the application.

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Remember, in main when
the application starts

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from the file module fh, it
reads the visits from the file

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whatever file name it is, like data.text

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and it returns a
collection of visit objects

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as read from the data file.

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Okay, so it takes a file name

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and it returns a vector of visit,

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where visit is defined in
the types module in my crate.

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Oh, and then at the end of the application

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remember if the user
chose to save the data

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then at the end of the
application in my FH module

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it'll write those visits to the file.

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It takes the name of the file,
it takes the visits data,

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as a vector, basically will
reference to the vector.

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It borrows the vector
and writes it to a file.

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So inside here, it's got
code to take that vector,

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it doesn't change the vector,

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it just writes it to a file.

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And we're gonna look at
that later in detail.

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Right, so the last thing to
look at is the menu module.

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And the menu module, let's
just collapse this down.

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I'm gonna go through this
as well in more detail.

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I just wanna kinda give
you the structure for now

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so you kinda get a framework in your mind.

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So at the moment, the rule with Schengen

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is that the number of
days that matter is 180.

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You need to go back over
the previous 180 days

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to count the number of visits.

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So I defined that as a constant.

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I could've just put the number
180 in my code down below,

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but it's much better to
declare it as a constant.

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It's much more readable and
potentially maintainable.

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So this is the main do menu function.

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That's what I call from main.

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Basically when the application
starts, it says menu do menu.

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So in my menu module,
the do menu function,

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here is the menu that's
gonna be displayed, okay?

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I'm not sure if I've
mentioned r strings before,

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Raw strings.

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A raw string is basically,

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it's a string that can
span multiple lines, okay?

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So I don't have to kind of end one line

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and then display another
line, that's just one string.

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And in the string I've
got soft count returns.

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Those were actually displayed

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as count returns in the output.

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So my application loops forever

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until the user says to quit.

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So that's a good example of loop.

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Loop forever, until the user says, stop.

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In my util module,

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there's a prompt to string
function that we saw just now.

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The message that it displays is this menu,

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it'll display that menu
on the screen, okay?

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And it returns the option
selected by the user

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which will hopefully be one or
two or three or four or five.

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So I take that selection as a string

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and trim any white space,

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and then see what option
the user has chosen.

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I mean, there were lots of different ways

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I could have done this.

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If the user chose option
one, call this function.

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Now these functions are
actually here in my menu code.

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So if the user chose option
two, then that function is here.

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If the user chose option three,
then that option is here.

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Okay, so the code to enforce
or to enact those options

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is actually here in the same module.

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If the user chose option
four, then a return true

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back to the main code,
which means save the data.

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If I return false, then it
returns back to the main code

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and it says, don't save the data.

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If the user chooses any other option,

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I say, "What?

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Okay, and that loops forever.

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Right, so there we are.

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And then obviously at the end,

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it'll choose, if you've returned true,

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to save the data back to the file.

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So we start to get a feel now

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for how the code structure works.

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The next steps obviously
is to dive into the details

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of each of those modules,

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and have a closer look
at the coding techniques

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and the library functions

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that I've used to achieve the effects.