Flutter 4 - Do While and For Video Lecture | Flutter: Build beautiful native apps in record time - App Development

everybody its Brian welcome to our
drumroll I think this is the fourth I
lost count
yep it's the fourth flutter tutorial
today is you spend nuts so forgive me if
I'm a little scatterbrained but we are
going to pick up where we left off on
control flow statements and we're going
to be looking at the while and do along
with the break and continue and we're
going to touch on the for loop so if
you're reading along in the official
documentation you can see there is this
while and do-while while loop evaluates
the condition before the loop and you
can see this nice little example versus
a do-while loop evaluates condition
after the loop that's confusing so let's
look at these a little bit here so let's
say mm-hmm let's make a variable just
call it bool because I like things
indented while running we're going to
say print
now if you're new to programming one
thing you should be aware of is that a a
loop needs a break if you will or it
needs to be able to stop execution on
that loop or what happens is something
like this where it just goes boom and it
just runs forever you can see down here
it just runs and runs and runs and to
kind of illustrate that what we're going
to do is we're going to make a variable
here we're going to set counter if you
don't know what that is it's the same
thing as the encounter equal counter
plus one it's actually shorthand for
that and we're going to say we'll call
this the wild counter so if we were to
just run this right now as is it's just
going to count until eventually against
the stack overflow and dies or until the
underlying framework resets that into a
zero sometimes I'll actually flip it
back to a zero for you I actually don't
know what it does in dart but every
framework kind of handles that a little
bit differently stack overflow would
basically be like the whole program just
crashes and you get an error message
where it'll say a value out of range or
something like that so what we really
need to be able to do here is say if
counter greater than or equal to 10
actually for say 5 just because I'm lazy
then we will save running equal false so
what we're really doing is we're saying
hey stop running but we tell it hey set
this variable to stop running let's run
that and you can see it says while
counter equal blah blah blah down to 5
so what's going on into the hood here is
it saying while evaluates this
expression what's equal true and because
we're setting it to false here every
time it reiterates through this loop
it's reading so when it gets to here and
that's false it just stops so we can say
print
to the next and you'll see kind of
what's going on here and it says BAE BAE
BAE BAE bang on to the next and then it
just stops now the do loop is very
similar and there's our evaluator and we
need to reset that actually running
equal true I'll see counter zero and
we're going to actually put in some
print statement here let's not see
waiting let's say done with while and
then down here we're going to say done
with do loop we can actually just take
this
and you'll see that it's pretty much the
same thing they probably help if we put
this as a do instead of a wow so it's
less confusing you see it's pretty much
the same thing so why do we need two of
them it's actually pretty simple it
depends on when you want your statement
evaluated do you want it before or do
you want it after
typically what this will do is it'll
take action then evaluate the statement
presence it typically that's exactly
what it does it'll execute the scope
code then evaluate the statement so I
just screwed that all up and the
contrast is that the while statement
will actually evaluate and then if it's
true then it'll run it it's a unique
difference you can get into some pretty
special situations you can actually get
into trouble a little bit like if you
have like if you're doing like socket
programming and you're not ready to
receive data yet or you're not ready to
send the data but you do a do loop
instead of a while loop you can get kind
of messed up so just understand the
basic differences between that
essentially they do the same thing it's
just the order in which your statement
is evaluated now this could also be
rewritten like this
what is that actually doing well it's
doing a few things first it's ignoring
our evaluator and it's actually breaking
out of the scope you ever play that game
break out
that's what break does remember how we
said it kind of hits the breaks so it's
going to run this scope but when it hits
this break it's gonna say nope I'm done
and it's just going to break right out
so which one should you actually use
well it depends on your situation what
you're trying to do
you could also say
my counter is less than five I'll
actually say less than equal to five and
let's do this we're just going to ignore
that whole running variable clear buffer
now you can see how things change a
little bit here now we have up to six
where we really wanted to you know
because it has this counter in here we
only wanted five now it's going up to
six so you would say you know less than
five less than five run it again
bang so you see how it really depends on
how you structure your code how you
write it
there's really no correct way of doing
it that's the beautiful thing about
programming personally I don't like to
put this evaluator here I like to use a
actual variable that way I can check
that variable state at any time and then
I like to actually set the variable I
don't like to do the breaks
personal preference that's just how I
like to do it let me make sure this
still runs the way we expect it for the
tutorials and now we're gonna look at
the for loop for loops a little bit
different actually let's cover continue
here real quick why not we'll say if
counter equal 4 then continue this is
the difference between break and
continue
remember how break will break out of the
scope of execution and what we're gonna
do here is we're gonna say I don't like
doing that but we'll do it anyways let's
run this so you see exactly what's
happening here 1 2 3 5 we totally
skipped over 4 so what was this is doing
is we're saying if and then we're
evaluating here the counter is equal to
4 continue what continue does is it says
ok I've heard everything I need to hear
this is a lot like arguing with your
wife I've heard everything I need to
know and I don't need to hear the rest
of this let's just go back to the
beginning so what this does is it
actually says go back to the next
iteration and starts completely over let
me go back up here so you can see it so
in this case it'll go 1 and it'll go
through 2 it'll go through 3 it'll go
through 4 it'll go through hit this and
say no I've heard everything I need to
know and it goes back up here so this
never gets executed and on five it runs
all the way through this and says hey
counter is greater than or equal to 5
and we can actually just say equal to
probably a better or streamlined way of
doing this counter is equal 5 then break
you know just hit the brakes I'm done
with this whole thing I'm getting a
divorce and I'm leaving you and it just
jumps out and it says done with the wild
loop so there you go now back to what we
were going to talk about which is the
for loop which is a little bit
interesting for loop if you're new to
programming you're looking at this going
oh my gosh this looks insane I don't
want to type this out it's actually very
simple and it's very easy to get and the
first two times you do it you're going
to get
the rest of your life and it's in almost
every major language so here we go I
equals zero say I equals zero notice
that little guy right there and then
we're going to say I less than 5 I plus
plus so what are we doing here what is
all of this let me put this back up here
for people on YouTube yell at me and
hurt my feelings
let's just print this little guy out so
we can see what's going on here
and we're seeing eyes 0 1 2 3 4 so we've
done this 5 times so let's kind of
examine this the four statement actually
takes three separate parameters and I
almost wish those four commas but it's
just not how it is and that's a holdover
from the old c c++ days so this first
one is actually setting a variable we're
gonna say we want a variable named I and
set it to the value of whatever in this
case is zero then we're gonna say while
because there's actually a hidden while
in here while I is and then whatever and
we're going to say less than five
increment I so here's our increment or
is back here so basically we have what
we want when we want to do it and what
we want to do to it and then while that
whole thing evaluates to true we're
going to execute plus we have our
variable from the top confusing but why
do we need this imagine this list and
we're gonna say string will call this
list equal new list I have a bit of a
sore throat and it's kind of bugging me
a little bit here so that's how we make
a new list of strings now whereas a list
I add everybody always asked me why do
you always put your own name in these I
don't notice it because I'm on the spot
and I just can't think of names I've
come up with names like Slartibartfast
like I did in the previous tutorial I
just on the spot can't think of really
anything other than like myself and
maybe immediate family if I could spell
immediate family
all right so let's say we've got a list
of people and in this list we've got
three people so hmm
how would we go about doing this
remember this is a zero based index
right so we're going to say a list and
we're going to say I is less than length
and then we're going to increment so
let's just run this and see what happens
here so we know that we have three items
first one starts with zero so what we
can actually do here is we can say we're
gonna try this I'm not sure if we can do
this but we're gonna try it it might
blow up and I may have to put it outside
but it would be really cool if we could
do this boom
didn't do it the way I wanted it to do
it
mmm yeah let's just put it outside that
was easier for people to follow along so
for the sake of the tutorial this is
what we're going to do here we've got
our string and in there we have our I
variable equals and it's not actually
signing this is just a string and then
we're going to catenate is that right
catenate that string with the value of
that item so you can see that zero is
Brian one is Heather two is Chris I kind
of want to figure out how to get that to
work element add let's try element at I
and see if this actually works here I'm
sure there's a way to do this and that's
why I'm kind of wanting to try it here
forgive me for just a minute see if that
actually I don't think it's going to
work fingers crossed
nope didn't do it all right
ah dear dart developers if you are our
dart yeah dart developers would be right
if you're watching these videos please
make that work that was awesome I'm
probably not doing it right there's
probably a better way of doing it but
that in a nutshell is how you would do
it personally I prefer the shorthand get
rid of that scope operator there same
thing so why do you need the for loop
what's really the difference I mean
couldn't you just say while we are not
at the end we could but it's very ugly
so what a for loop replacement would
look like we would say mmm counter zero
let's say hey while counter is less than
list dot oops this dot length have a
nasty habit of putting that on the other
line counter plus plus and say print and
here we're going to say
oops we want it to beat counter actually
let's make it that way it's more
intelligent here and I my hand i and
it's around this what do we got here
what's going on with us invalid value
not in range inconclusive three hmm so
what we're saying here is we have it
starts at zero and mmm I is less than
the length so let's try this you see why
this gets a little hairy here let's do
this and see if it just runs okay so
that does run let's get rid of this guy
because that's what's actually causing
the error here let's just see what we
got here
boom Oh see one two three so we got in
the wrong spot that's really what's
going on here so we need to actually
increment this afterwards
now it'll run there we go so this kind
of illustrates some pretty good point
and let's actually put this so we can
see the difference between the two so
we've got our four and then our four
replacement they do exactly the same
thing it's just one is a little bit more
complex a little bit more error-prone
while the other is a little more
streamlined elegant and kind of if you
will industry standard so if you ever
get like a professional job writing code
and you do something like this your
quality control agent or your managers
probably going to sit you down and say
why didn't you use the for loop because
this is exactly what the for loop is
designed for
so anyways those are the distinct
differences between them just to recap
the while loop evaluates before the
do-while loop evaluates after and the
for loop is perfect for going over lists
and enumerations whoo that was a
mouthful thank you for watching if you
found this educational entertaining be
sure to go out visit my website all the
code is out on github and i've got a
link out to the repository right now if
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