Flutter 3 - IF ELSE and Switch Video Lecture | Flutter: Build beautiful native apps in record time - App Development

everybody this is Brian welcome to the
third flutter tutorial just posted the
first to the other day and as you can
see I've already got a few hundred on
each one in comments and some thumbs up
so everybody seems to be really
interested in flutters so we're just
going to continue on down the path here
now it's back up we have to learn dart
before we learn flutter looking at the
tour of the dart language you can see
how they kind of if you go in they go to
cover variables keywords variables
built-in types which we've covered then
they jump to functions which if you're
new to programming really doesn't make a
whole lot of sense if you remember a
variable is something that'll change
where a Const or a final is something
that will not change a function is a way
of organizing that into a logical group
but you haven't yet used variables you
don't really know what to do with them
so we're gonna actually skip down to
control flow statements so control flow
statements are exactly that they control
the flow of a program and the first one
we're going to cover is if and else so
let's just crack this open here we're
gonna make a new dart console
application and we're gonna call this
flow one and while that is thinking
we're gonna skip back here so some of
you may be confused and go why are you
jump into control flow statements you're
not talking about functions we're going
to talk about functions after we deal
with control flow statements the reason
being you know what a variable is like I
said now you need to know what to do
with the variable it's something that
will change but how do you handle that
change what do you do when the change
occurs so that's what we're gonna really
cover today so let's just get rid of
this and get rid of that once again the
structure of our application we have got
our main dart in here and if you're
wondering what this lib is this is just
that part of that demonstration code
that the let me undo all this it's part
of that demonstration code that comes
with the template really they're just
importing this which we're going to
cover in a future tutorial but for right
now you can safely ignore it now if you
go back to our previous tutorial you'll
see that tada
there is a variable here called
arguments so this main function already
has a variable so something's going in
here an argument is something that is
to your application when it is first
started something we're not gonna really
be dealing with but just be aware that
that's there that's how you can tell a
program you know start just normally or
start and open this file that kind of
thing so if and else
woopsie cannot type so let's make a
variable here let's say bull is on equal
false so we have a variable and we're
going to say let me actually just jump
down a little bit so it's easier to read
if and then you have these parentheses
is on equal notice there's two of those
now there's a bit of a heated debate in
the programming community and I'll get
to this in just a second as I can't
multitask today it's has to do with the
brackets here this is what's called a C
style language C is probably the
grandfather of all languages some not
all language but all C style languages C
was meant to mate assembly easier C's
actually kind of challenging but so you
have these brackets and this denotes a
scope or a block of code you know so
there's two of them here you notice how
there's here in here and actually in
this IDE if you click on one it shows
you where the ending one is so they're
in there they're in there that's your
scope so what we're really going to
learn right now is two quick things
first is variable scope and what is the
if and else statements but the religious
battle if you will is where to put this
little bracket some people like putting
it down here some people like putting it
up there now when I say up there what I
really mean is you take this and you put
it on the same line like that it doesn't
really matter where you do it some
companies have coding guidelines and
standards that you should really follow
but just know that you need a start and
end bracket to denote scope so what is
scope I'm glad you asked okay so let's
make another very let's call this bear
Kitty equal I don't know what to name a
cat this just call it cat why not so
let's run this see what happens
it is off so we've got our variable bull
is on equal notice there's only one
false if so this reads like a statement
here if and then we have our parentheses
this goes back into arithmetic where you
have to multiply and add things so you'd
say 3 plus 2 plus 6 equals you know
whatever right what's ever done in the
parentheses is going to be
calculated first that's why it's in
parentheses to begin with so we know
that this is going to be evaluated now
this is an evaluator actually this is a
drumroll please
operator and it is an equality operator
and you can read a lot more here so you
see how there's two of them now why
wouldn't you put one some of you old
programmers know what I'm talking about
if you do that what you're really doing
is you're assigning the value much like
you're doing up here is on equals false
so when you do the double you're saying
is on is equal to you know if is on is
equal to whatever the value so that's
why it says it is off now if we change
this to true and run this it is on so
you can see how you have control flow
here this statement is being executed
because it is true if we flip it back to
false then this statement will be
executed along with this variable being
created so let me add some comments here
interscope outer scope I misspelled
outer that'll be embarrassing if I did I
don't think that's outer that's odder
anyways it's not even hotter me and cold
medicine I'm telling you
so scope anyways scope goes in a logical
order or you should say top-down so for
example is on notice how we can actually
access it here let's actually do it like
this
and let's actually just copy this here
run this so is unequals false so we can
access this variable in this scope or in
this block of code what happens if we
try to access kitty
notice how intellisense isn't helping us
out and it's saying object object
meaning it doesn't know what the hell
we're trying to do here and it's
complaining and it's saying hey on an
unidentified named kitty but we have it
here that's called scope and true enough
if we try to run this it's just going to
say blah know top-level getter for kitty
what's a getter we'll cover that in a
future tutorial however just be aware
Kitty is out of scope so where is Kitty
right here where's the scope right here
we can access Kitty just fine as long as
we're in that scope once again is on
this variable probably horrible variable
name is in the outer scope so it's
available to anything in a sub scope sub
scope being a scope declared within this
larger scope that gets confusing so
let's go over that one more time we have
our outer scope here in that outer scope
we have sub scopes Kitty is defined
within a sub scope so it is not
accessible in the outer scope however
variables described in the outer scope
are available on the inner scopes is
that even the right way to say inner
scope somebody correct me if I'm wrong
but if we simply move this print
statement up here we can see it runs
just fine
so that is a very quick lesson on if and
else and is on so what's the difference
between if-else and scroll down here
whoa we went way too far else if you'll
see this a lot what's really the
difference here
I just stuttered that was embarrassing
to the and it was
the different ween if else if and else
let's let's figure this out let's just
do the magic of copy and paste change
this around a bit actually no I want to
keep this as an example let's say it's
mmm test roll one why not
why are you indented I don't like you
all right so you have test equal I'll
say two now if you're going to choose a
bracket style you should be consistent
in that style and you should probably
put it in the print statement Brian
alright so we've got our little else
here and we can even break that down
further like this I'm kind of like an
indentation snob if you will I like it a
particular way else and we're going to
just say now from our scope conversation
we know that we can access this variable
because it's in the outer scope so let's
run this so test is one so this got
executed not this so let's just copy
this again let's see what happens
notice how we're getting an error
already expected a statement what's
going on here eight line 30 position
three so we're at line 30 position three
because we're three spaces in unexpected
token else but if we say else if up here
what happens hmm it breaks so let's look
at this and see what we're doing wrong
here we have our if we have our else--if
now this is what we're missing here so
we have to notice how all the air is
suddenly clear up and let's just so we
know this run that so test is one
because test is one confused yet don't
be it's just a way of chaining if
statements you haven't if statement here
and if statement here and if neither of
these actually apply then do this that's
really all that does now there is an
easier way of doing that or at least a
more elegant way of doing that
but before we cover that I want to talk
about comparisons in the if statement
it's tough if you read their disclaimer
it says remember unlike JavaScript dart
treats all values other than true as
false see Bolling's for more information
sometimes people will say things like
test equals zero and then instead of
that they'll just say test if test you
notice how it must be a type of bool so
it's got to go into a true or false sort
of statement it's pretty easy to do that
though so now we're going to cover
what's called the switch statement
it's called switching case and I think
that'll prior wrap up for this tutorial
so this looks just horrendous what is
this thing okay let's break it down a
little bit
you've got switch now think of this as a
giant switch board on/off right you've
got a dozen switches in front of you and
you can flip each one on and off
so here's the variable so we've got very
command and then we've got our evaluator
the parentheses and then we've got a
scope so within this scope we've got a
bunch of cases now think of these cases
as shorthand for if-else we're gonna
rewrite this whole thing so using the
test switch test and there's our scope
notice how what we're not doing is
testing for equality or unequality like
we're not saying you know not equals or
is equals we're just saying we're going
to switch on this test now we've got a
bunch of switches and we'll say case 0
break case let's actually back out here
in case one break what is going on here
well let's find out the test is not 1
ties to 0 derp I hate having a cold I
feel so just out of it when I have a
cold it's finally going away but man
what a long road this has been alright
so you can see up here our variable is 0
so what's gonna happen and we'll do
let's actually define this out here
switch that way we can really see what's
going on when we print these out run
this bad boy you can see switch test is
0 so really what we're doing is pretty
much the same thing it's just we don't
have all this if-else gobbly gook so
this monstrosity right here is the same
thing as saying case equal 1 inside of a
switch so here's our switch here's our
switch now one thing you should know is
there is an invisible scope here that
you don't really realize so let's just
say bear
dog equal Sam and then we're going to
print Sam
notice how Sam is going to say undefined
object and if we try to run this of
course it's going to break no it
actually didn't break interesting
probably because it didn't execute let's
do this yeah now it's going to break so
that's how you'll find some errors in
your code like if it's working just find
one time but then not working another
time check your scope the invisible
scope is between the case and the break
think of the invisible brackets being
here in here each one of these cases is
a little switch like a light switch
right so we've got two of them here
we're declaring this variable inside of
this switch or this case it's not
declared here therefore this code gets
executed when it's zero this code gets
executed when it's one so if this code
is executed those variables never
created you see the catch-22 now so if
you wanted that you would either have to
make that in each one or better to do it
up above now let's get rid of that and
get rid of that and let's see if this
works no no top level get it for Sam
declared so you'll have to understand
that scope here is a little wonky and a
little crazy sometimes
so you just definitely want to make sure
that you declare your variables in the
proper place now why isn't this working
here that's a good question and honestly
I don't know in other languages this
works just fine so why is this not
working probably because I have the
wrong variable name altogether so let's
retest all of that that was embarrassing
so let's print a dog I may be wrong I'll
be flabbergasted if I'm totally wrong
here all right so back here test is 1 so
this will execute yeah all right so
and if we put this here it suddenly
works but it would be much better to
actually put it up above right here all
right sorry for that little confusion
that's also a good conversation we
should have as variable names if your
variable doesn't exist you're gonna get
an undefined error as you just saw so
that in a nutshell is the switch
statement but there is one little
question we have one naggy little
question what is default you notice how
it's Ches Ches Ches Ches case default
and what does break mean well let's try
something here let's actually make a
case three print I'm actually trying to
hurry because I'm supposed to take my
daughter out to lunch and I am starving
which is probably why I'm making a lot
of mistakes here and then we're going to
say default no idea wow I can't even
spell no today I'm gonna go get some
food here right after this video because
I am just dying I'm gonna go get some
hot wings man I'm hungry alright so
we've got our our cases here we're just
going to put harder turn so it's easier
to read here so we've got our case here
our case here our case here and default
which is actually a case so let's change
our test to a different number or
something that we're not accounting for
and run this CEO says no idea so default
is basically a giant else it's this else
statement here we're gonna switch this
back to let's say did we totally skip to
we did that's just odd somebody's
probably yelling at their monitor going
there's no two
get a dog here all right so we've got to
let's just get those in the other way
they should be put that in as a to run
it just to double-check test is to now
let's talk about these brakes let's
actually comment these out and see what
happens
notice how last statement of case should
be break continue wreaths wrote returned
or throughout what does that mean we're
gonna get into that a little more in
depth in future tutorials but right now
you need to understand what's happening
break is like hitting the brakes on your
car it stops execution of the code if we
change brake to continue continue
statements which page must have a label
or as a target what that's telling us is
that we could continue the flow of
execution if we wanted to go somewhere
else I actually don't know how this
works but let's say continue three no
continue case three I'm sure there's a
way to do this I just don't know
continue case is not a word yeah I'll
figure that out in a future tutorial but
basically just understand what brake is
doing is it is actually breaking so
let's get rid of this again and see what
it's saying the last statement of case
should be brick continued wreaths row
return or throw so let's try return
because I know that'll actually work
what is the difference here see how case
three decay this the last statement of
the case blah blah blah let's turn that
there we go now it's Berger so what's
going to happen here
all right pop quiz everybody based off
the current code test is equal to two
what do you think's gonna happen is it
gonna print done or is it not based off
this code let's find out
there's no done but if we switch this
oops and rerun it see how it done
suddenly appears so that's what's going
on break we'll stop execution but not
return return means it actually exits
the entire function return is actually
returning out of this entire function
actually out of this function right here
switch isn't really a function but I'm
going to call it a function Shh don't
tell anybody so it's kind of exit out of
the switch statement and then hit print
so when in doubt use break but just
understand that that break is there for
a reason some languages have what's
called follow through so you could
actually say this and then this so if
it's case one break case two we wanted
to do this and this and this so just
understand that is a thing and we need
to prepare for it which is why the break
statement is there well that is a lot of
talking and typing and I'm gonna go get
some hot wings with my daughter thank
you for watching everybody the source
code for this and other tutorials will
be out on my website boy drums calm
there is a link to github and let's just
go out there you can click on
repositories' and what I do is I have
individual repositories but then I have
a master repository that has the
individual lessons in there these are
called sub modules so you may have to
use it may not actually download with
the full zip you may have to actually
use get and pull the whole thing down
that way people can contribute to the
code if I make a glaring mistake and it
is out in the world forever
somebody else can you know submit it if
you found this even remotely helpful my
website and all this code is actually
run purely off donations you don't have
to obviously but if
you are a company and this is helping
your employees learn definitely donate
anything that goes above and beyond what
I used to run the website actually gets
donated to charities
other than that thank you for watching