ios 11 Developer Course Tutorial - Optionals Masterclass Part 17 Video Lecture | iOS 11 and Swift 4 App Development: A Complete Tutorial

now if classes and objects is something
that often confuses new programmers then
it competes with optionals as the most
confusing topic for new iOS developers
but it doesn't have to be this way so
let's spend just a couple of minutes to
get really clear on what an optional is
and how it works so we've seen that it's
possible to create a variable that
doesn't have a value so if I create for
example a variable called a number and
give it a type of int I don't have to
set a value for number however if I try
and use that variable when it doesn't
have a value I'll get a nasty red error
message saying that the number variable
is used before being initialized so if I
want to create a variable that doesn't
have a value then what I do is add a
question mark onto the end of the
definition and now that nasty error
message goes away and as we've seen
before I just see that it has a nil
value when I try and look at it
so optionals allow us to use variables
without values without crashing our code
but then you might be wondering why on
earth what I want to define a variable
like this in the first place all the
odds are you don't necessarily want to
define a variable like that but you
might be doing something in your code
that results in an outcome that you
don't know is going to have a value or
not so let's say for example that the
user has entered some text and that is
the number 3 in a string so remember if
the user enters some text it will almost
certainly be a string even if they've
entered it has a number then what we
might want to do is convert it to an
integer
so that we can do something with it and
we might do that with int and then user
enter text and we can see here that
that's worked out fine it's converted it
correctly to an integer but what if the
user enters mistakenly the number three
spelled out like that then this is going
to have a nil value because we can't
convert T hree
to the number three so then let's say we
wanted to take the user entered integer
and multiply it by seven as we did for
the Caviar's app so we might call it
something like at age and then take the
user entered integer and multiply it by
seven so you can see straight off we've
got an error and this is the error that
you'll likely see when you first
encounter optional so value of optional
type int not unwrapped did you mean to
use exclamation mark or question mark so
this is saying that user entered integer
is an optional because it might be nil
it might not work so what we have to do
is unwrap it to say no it's okay I know
for sure that user entered integer does
have a value and you can multiply by
seven so here of course that works fine
but if the user enters something like
three written out in characters then
we'll get a horrible error because we
couldn't convert it to an integer and so
we get this nasty error unexpectedly
found nil while unwrapping an optional
value so it's taking the optional value
we've said nope it's definitely has a
value that's fine but it actually
doesn't so what's a good way round that
then well there's a construction that
we'll be using over and over again that
works like this essentially we put
and if in front of the let command and
then we don't need to use the
exclamation mark and essentially this is
saying if user entered integer has a
value then set that value equal to cat
age and then we can say print cat age x
7 and it will only attempt to do this if
cat age has a value so if we put a 4
there it works fine
but if we go back to our 3 we don't get
a crash it just doesn't attempt to
multiply by 7 so if this would be we
could put an else statement in there
show an error message to the user so
really this is the best way to deal with
optionals is to have a line like this
which tests whether or not they have a
value and then if they do then do
whatever you want with them if they
don't then deal with that usually by
showing some kind of error message or
changing the behavior of the app
accordingly so that's really all you
need to know about optionals they're
variables that may or may not have
values and if you're going to use those
values you need to be very careful and
test in advance to make sure that they
do in fact have a value and the best way
to do that is usually to use this if let
construction and that's what we'll be
doing throughout the course so it's
definitely good to get clear on how it
works right now ok so now we're in a
position finally make out is it prime
app which is the final app in this
section we'll start doing that in the
next video