Flutter 2 - Variables Video Lecture | Flutter: Build beautiful native apps in record time - App Development

everybody this is Brian welcome to the
second flutter tutorial I know it says
dart here but before we really dive into
flutter if you remember from our
previous video we actually have to learn
the dart language first don't worry it's
pretty painless so we're gonna just
click on language
we're gonna go to tour and this is what
I'll really be working from although I
won't be following it verbatim but I
want to definitely point you to where
you can find additional information here
so tour of the dart language they walk
you through what is a basic program and
we're just going to actually open
IntelliJ create a new project we're
gonna make a new dart console
application and let's call this a ver
test
oops just because I'm horrible at naming
projects and this will take just a
second to load actually don't know why
this is so slow i have just a ungodly
amount of ram and two solid-state drives
this is actually my pc i use for video
gaming so i'm not sure why this takes a
while but while that's sitting there
churning in the background what we're
gonna really cover in this tutorial is
what is a variable what is a constant
what is a final there's a distinction
there it's a little tricky and then some
of the built-in types you can actually
click on that built-in type and you'll
see there's numbers strings boolean's
lists maps runes and symbols not going
to really cover these last two but you
can definitely look them up in the
official dart documentation alright so
now that we have got our brand new shiny
program let's just run this and make
sure everything build correctly if you
followed along and you've got the same
setup as I do you'll see hello world 42
that's actually kind of funny it's funny
because of a book by Douglas Adams
called Hitchhiker's Guide to the galaxy
where the ultimate answer to everything
was 42 so we're going to remove that
import statement and remove the print
statement just save it run it again and
you should have absolutely nothing there
that'll be a good starting point for us
now structure of a dart program you can
see how the oops
the main dot dart file is in the bend
and this is our main statement or our
main function
this
the entry point to the application so
we're just gonna say a print hello world
what we're doing here is we're calling a
function you've probably seen this in
other videos and if we just save and run
you'll see sure enough it says hello
world but what's really going on here
under the hood we have a function which
we're going to cover in a future
tutorial that works with something
called a variable which is right here
hello world is actually a string
variable so what is a variable a
variable simply something that will
change over time so variable is actually
a scientific term something that will
change over time so we're just gonna say
very something whoops packet spell hello
world so what's really going on under
the hood is dart is creating a new
variable with that information in it so
this and this are exactly the same thing
you can save run and it does a literally
the same thing to kind of drive that
point home you can actually press dot
after hello world and you see it brings
up all of the other functions in the
string class which if all of that sounds
like voodoo nonsense don't worry we're
going to cover all this in future
tutorials but you just need to
understand that a variable is something
that will change to drive that point
home you can say oops help I spilled
something right
cats rule now if your dog lover you but
dogs rule I won't come find you but
point is we've changed this we've made
changes to it now on the flip side of
that a constant is a scientific term
that means it is constant it will never
change so I will say Const uh what do we
want to put here I will just say nope
equal one so if we try to take our nope
variable here and I should say nope
constant and make this a two notice how
there's got that little red underline
and it starts complaining and if we even
try to run it it's going to crash it's
gonna say kena a sign to final variable
nope
now what's going on here we call this a
Const but it says final variable so
there's a little bit of information you
need to know and I found this really
great article I'm just gonna put this
right in the code but the difference
between final and Const is it's tricky
to understand and you won't really get
it until you really get into the
language but I wanted you know right up
front to tell you there is a difference
if you're writing your code and you're
following along say in another book and
you say final no / 1 no / equal to
essentially you're doing the same thing
quote-unquote you have something that
you cannot change and it's going to
complain every time and if you try to
run it let's comment that out you'll see
that yes it still crashes cannot assign
to final variable no / ok let's comment
that out by the way if you don't know
what a comment is a comment is just
something that the computer completely
ignores it's there for your enjoyment if
you will a lot of times programmers will
comment code as they go so you can
understand it not just from a machine
perspective but from a very readable
human perspective so anyways back to
this variable if something will change
final cannot be changed so what's the
difference between constant final this
article sums it up the best ignore
static for now but final means a single
assignment a final variable or field
must have an initializer what does that
mean basically it means that you have to
give it a value so for example if we
just comment that out you can see how it
must be initialized you must actually
give it an initial value let's try the
same thing with nope here notice how
nope is also complaining must be
initialized so you understand what an
initializer is and because it's a
constant or a final it has to have a
value starting up once assigned a value
a final variables value cannot be
changed
final modifies variables basically it
takes the variable and set it in stone
it says this will never change for
example a constant would be the Sun
unless it explodes and is a constant it
will never change or I should say it's a
final it'll never change a constant
however has meaning that's a bit more
complex and subtle in dart Const
modifies values notice the distinction
here values final modifies the variable
where constitute or quick here this is
the variable this is the value so let's
go back here a final modifies the
variable so final modifies this the
variable where Const modifies the value
so you can pass that constant around to
other constants it will always remain a
constant but you can pass it around with
a different variable name whereas a
final is final that is the only time
that can ever exist in that so if we try
to do a Const no per see how we're
getting an argument upper is already
defined you're gonna get that anyways
but that's really the distinction and it
gets really deep and like I said you're
not going to truly understand
until you get really good with dart and
you end up in those cases where you need
to understand the difference between the
two but I'm going to take this particle
and throw it in here just so we kind of
have that as a reference whoa what is
this cancel all right so which one
should you use I use Const just because
it suits my needs and until I get into
complex applications I won't really need
to know the difference now if that's not
confusing enough there is that other one
called static which we'll get into in
future tutorials which really deals with
classes and that's why we're gonna cover
in future tutorials
so onward now that your brain is
completely numb just understand there is
a variable if something will change and
a constant or a final will not change
change of molars is not changeable in
this tutorial we're going to be working
with things that change or variables
there's different variable types
notice how in the front I put vert or
Const or final that's the type well
there's a lot more types here so we're
gonna get into numbers and they are well
numbers yeah exactly what you think they
are so we'll say num and we'll say age
equal 12 why not now
if we type a GE and hit the period you
see that we go into that class of scope
and we get a whole lot of functions this
is a lot of built-in functionality in
dart everything is an object much like
in Java or other languages so because
everything is an object you can actually
go in and get reusable code something
somebody else has already written like
we can round we can double we can
convert it floor ceiling ABS to string
is it not a number etc etc now notice
here on the right you see some words
like int double string what are these
well that's the return type so that
function is going to return a double
where this function will return an
integer so you know automatically what
you're going to get
so as you would expect because this is a
variable well let's add 20 years to this
looks like 30 years to this why not
we're gonna just print the age out here
so you can see how now it is 42 and I'm
kind of doing this on the fly here so
you see how it says age is 42 if you put
the dollar sign inside you can actually
reference a variable outside if that
makes no sense let me repeat it what
we've done here is we've created a
string variable but inside that string
variable we're referencing an external
variable called age see how that works
otherwise what you would need to do is
actually go whoops plus age now notice
how the argument type num cannot be
assigned to the parameters type string
because this right here is actually a
string if you hit the period or the
scope operator you will see that you
have options or functions from that
class that you can choose from so we
actually got two variables we're working
with here this variable and this
variable so it's kind of a matter of
preference
some people like doing this some people
like doing that do what's comfortable to
you is really what you should do and
just in case you were curious if you're
going to do it like this you just say
age dot two string and it converts it to
a string for you automatically so you
can actually let's do like this and
we'll run it just so you can see age is
42 I'm going to put it back the other
way
just for the purpose of this tutorial
though so I should actually got a little
ahead of myself here so abilene we're
gonna actually jump backwards in time
here and we're gonna say bull is working
equal false a bullying is like a light
switch it's either on or off true or
false and we're gonna say is working
equal true boolean's are very helpful
for certain things like for example
let's go back to our age example here if
we want to say is infinite is age
infinite it's going to return a bowling
value and it's going to tell us whether
or not that number is infinite or
negative or finite alright let me check
one somebody messaged me that's the
problem with doing videos real-time I
don't edit my videos so you'll see a lot
of seeing here a lot of craziness going
on here one thing we should really be
aware of is that there is more than one
type of number for example an int would
be say cats the number of cats I have I
actually don't have any cats right now
so I'm lying a little bit and then
there's a double let's say PI equal 3.14
so what's the difference between an int
and a cat these are big big big big
numbers you can actually go out to the
dart documentation you'll see integer
values generally be between negative two
to the 53rd power to two to the 53rd
power that is a huge number we're a
double is a 64-bit double-precision
floating-point numbers as specified by
the I Triple E 754 standard that's a
mouthful basically what you need to
understand is almost all numbers that
you create are going to be converted
either into an integer or a double
depending on what they are but both
integer and double actually inherit from
the number class what does that mean
exactly well what it means is that if
you say
age dot you'll see all these things like
round to double to n to Florida blah
blah blah blah blah what inheritance
does is if we say cat dot you'll see
that it's inheriting all of those plus
it adds a few little extra so an
inheritance is think of back to
scientific terminology you inherit your
genes from your mother and your father
everybody does unless you were born in
the laboratory but anyways we won't get
into that so you have parts of both your
mother and your father so for example an
integer has parts of num a double has
parts of num pretty simple we'll cover
that more when we get into our classes
tutorial but just understand that
there's essentially two different types
of numbers there's integers and doubles
and you can use the generic num if you
don't know what type of number it
actually is see it won't break anything
if we do that but if we try doing that
with an integer it gets a little angry
at you it says it has to be a double all
right now we get into the really fun
stuff strings I don't know if you guys
can hear that it's actually it's I'm in
Michigan and it's just starting to get
cold and snow and my neighbor is now
just trying to get their leaves up and I
can hear them like yelling at each other
it's kind of funny so so I will say
string title equal hello world so what
is a string a string is a sequence of
characters what is a character a
character would be these individual
letters now you should know that I think
there's 200 and we're on the internet so
somebody will correct me if I'm wrong
255 different characters not all of them
are actually human readable for example
the space is a character 8 tabs a
character 0 not the numbers there but
just nothing is a character things like
that so a string is a sequence of
characters now if you want to work with
Unicode spoiler alert you'll need to
work with something called a rune which
we're not going to cover specifically in
this tutorial we
covered in future tutorials if there's
like a huge public outcry for it alright
so you get now that it's a sequence of
characters what can you really do with
this I mean what does a string will
really let you do strings let you do a
whole lot of really really cool stuff
for example you can say title equal
whoops my kittens just why not I have no
idea where I was going with that and
we're just gonna say print title so you
can actually build strings there is a
class out there you should use called a
string builder if you're going to
actually start building strings it's
much more efficient than working with a
string directly but for this tutorial
we're going to just cover strings
alright so back to this what do we want
to do let's say we want to play around
with this thing and let's actually so
we're just gonna have this say hello
world right here let's say we want to
know some values out of this thing let's
say we want the we want to extract part
of that string substring so we want to
start index let's say we're gonna start
at 0 and we want the first 4 out of this
thing
let's run this and you'll see that tada
nothing happens because we're not
actually changing this variable we're
actually calling a function that's
returning a variable so let's grab this
and you'll see the first four is well
that's kind of dark I didn't mean to do
that the first floor is hell sorry
ladies and gentlemen but uh what we're
doing here is we're grabbing a substring
or the first four characters now you can
see how the four characters are outlined
there what does this is zero well under
the surface this is really a list of
characters we're going to cover lists
here in a few minutes but lists are zero
based indexes meaning it starts with a
zero instead of a one so this position
right here before that age is zero zero
one two three four that's how you get
those four characters so substring
starts with zero and then we tell it how
many characters we want or you could
start it a different one so for example
let's say we start at one and now we're
going to get some gibberish Ile with a
space that's not a good example let's do
let's do two that's really not a good
example either
let's grab five characters why not
hello so you could see oh you know what
hmm we should cover other things as well
let's put this back the way it was just
so I can really freak people out when
they download this code and let's say we
want to I'm really doing this video on
the fly here so bear with me here we
want to replace all so we're gonna say
title equal title that replace all and
we want to get rid of the ELLs the
uppercase ELLs anyways
then replace those with Z's
as you can see we've got these two Z's
in there now pretty neat
now one thing you can really do with a
string that's kind of interesting is you
can play around with it so we're gonna
say string mm book
my world's issues in a day what have we
done here you notice how instead of
using single quotes we're using double
quotes you can use those interchangeably
it doesn't really matter and you would
use those for specific instances like
this where you want to put a single
quote in there this could also be
written as let's say string book 2
notice how it's got that slash in front
of the single quote that's called an
escape sequence you would use that for
specific things like for example this is
a single quote so anything after the
escape is the sequence we're going to
inject into the string for example let's
say we want to do a /n which is a new
line Terminator so let's print these two
out got a print book and then we're
going to print out book - just so you
can see the difference here what we've
done you can see how my world's issues
in a day and then my world's issues
harder turn or actually new line in a
day now on some platforms and like
windows I think it's our n instead of
just n so it gets into some
platform-specific stuff there but you
can see that really it doesn't matter
whether you single or double quotes and
it's kind of personal style and
preference on which one you use all
right so now let's get a little
different here and let's say string
names equal just going to make some
names there
and we're going to assert something now
assert is a special function in dart
that will say this must return true
whatever we put in here has to return
true so we just say names dot whoops dot
contains and we're looking for Heather
probably help if I put that at the end
there notice how each line has to end
with a semicolon that tells us that we
are at the end of the line very funny
huh so when we run this nothing happens
but we've done this assertion so let's
say just random junk in here and we try
to run this now notice how it'll air out
names contains whatever is not true so
that'd be a good way of testing your
variables they put this here so we're
going to talk about lists next lists are
a list of objects so of course a list
and you notice how the intellisense pops
up and it says there's this weird
bracket with an e in there what was that
well this is called generic programming
and what it allows us to do is reuse
code and we're going to get into this in
future tutorials just right now
understand that we have to give it a
value type or I should say a variable
type sorry
in this case string so we're going to
say list equal names dot split splits
going to take a pattern and we're gonna
split that based on the comma so what
are we really doing here if we examine
name dot splits
I'm sorry names dot split you can see
how it's returning a list with string so
a list is going to be a list of objects
in this case we want a list of strings
we where it could easily have said
integers or doubles or even you know ver
if we don't know what type it is
actually I don't think it would allow us
to do that but
in this case we're just going to use a
string so think of a list like a it's a
good example here think of a list like a
shopping cart shopping cart you just
throw anything you want in there but
we're seeing a specific type of thing so
in our shopping cart you can only put
strings we don't want to put integers or
doubles or anything else we only want
strings in there because we're going
with string shopping if there is such a
thing so from our list we can then pull
items back out or we can just examine
the list so let's say we want to know
what's in our list save this and run it
in our list we have Brian Heather at
Chris Karl let's say we want to get the
the item at or I'm sorry the element at
the second index because remember a list
we talked about this as zero base so the
second one 0 1 2 is Chris now you can
also shorthand this as most people do
let's say list bracket 2 that way you
don't have to do the element that does
the exact same thing under the hood you
see how we got Chris there twice now now
from here let's say we want to add to
our cart we want to add a string here so
we're going to add Bob Bob's been a good
boy he can go into our little shopping
cart of strings and let's print our list
out again
and you see we now have Bob at the very
end now let's say we want to add
somebody but we want to add them in the
list not at the very end so we're gonna
actually say insert notice how it's
wanting an index and a string element so
insert we're going to insert this at the
first position and we're gonna call this
hmm I need a name I need a good name hmm
back to our Hitchhiker's Guide to the
galaxy let's just say we're gonna add
forward I should have added
Slartibartfast but yeah let's actually
do that he's a character in the book I
love that book it's funny so now we
gonna print our list out and you'll see
how in the first position because it's a
zero based zero one we have now added or
inserted Slartibartfast so what if we
want to know where something is I'll say
while I print and I can't type say we
want to know the index of let's say Karl
ooh why did that return a negative one
let me play around with us for a minute
did I misspell something there we go so
Bob's at the 5th index why did that say
zero or negative one for Karl that's
weird hmm obviously I have a boo-boo
somewhere mm-hmm that's interesting I'll
have to figure that one out but
basically index AB gives you the
position of where they are hmm
let's actually play with this a little
bit here figure this out so we're going
to say there's a little function here
call for each and what for each will do
is it will list out every single item in
the list and we're going to say for each
mm give me just a second here
this looks like voodoo magic I will
explain this in just a minute
oh semicolon expect outdo derp derp derp
all right that's what's going on here so
if you look at our list here we've got
Bryant's learning Bart fast space
Heather Space Chris Space Carl and Bob
that's the problem that's what's going
on here
remember how we split it based off of
this so what we should try to do here
what is the name of that was it trim
let's just do this easy way I'll have to
look it up but I swear there was a
function there you can do that let's
remove those the spaces in there right
what I was looking for here is name dot
I'm sorry names dot trim trim will
remove the blank spaces I was hoping to
do that per item but I think it's gonna
be more of an advanced topic I've also
got a cold so I'm not gonna really fight
through the cold medicine to do it so
when we run this now you'll see how
there's no spaces in there and if we go
back to putting Carl in there I think
Carl was throwing an air and so it was
Heather you'll see now that it is at
position 4 so back to this list for each
what in the world are we doing here so
we're taking our list which is this
thing here and we're saying for each
item in the list and then we're doing a
anonymous function and we want a V or a
variable and we're gonna print that I
know this is a bit of an advanced topic
for people that are brand new to the
language but those of you you know
really gridiron veterans who have worked
with multiple languages are probably
loving me for this right now just know
that you can do these kind of things
with dart language that was makes it so
elegant so nice and it makes
troubleshooting things a little bit
easier too as you can see so anyways
finally Maps
maps are a little bit different I want
to say map and we want to give it once
again we're doing generic programming
here we're gonna say string and an int
I'm going to call this ages equal and
what a map is is you have what's called
a key value pair so we're gonna say
Brian and 43 man am I 43 I'm getting old
I was just playing call of duty that
explains some things why I was just
getting completely smoked on Call of
Duty by some of these players literally
like I shot a rocket at some guy and he
just laughed at me and like stabbed me
in the face of the shovel so it was not
fun I actually thought about just
uninstalling the game at that point
fortunately for me though Carl also
plays video games and he is older than
me so Thank You Carl for being my friend
anyways so what we're doing here is
we're actually building a map or a key
value pair Oh somebody's texting me this
looks important it can wait just a
minute
so the key value pair is going to align
up we have the key and we have the value
and in here is our actual map now what
it's called a map because you actually
have two lists under the hood these
names are lists or the keys and these
values are a list called values so let's
just say print ages dot keys
you'll see Heather I'm sorry Brian
Heather Karl
Brian Heather Karl those are the keys
you notice how it's a list now let's say
Prince whoa did I hit caps lock sorry
for yelling at you guys
print ages dot values so you can see how
we have two distinct lists here so what
a map does is map those are correlate
them this one maps to that one this one
maps to that one that's why it's called
a map all right so how do we work with
this how do we really extrapolate data
out of this thing well let's say we want
to get the value of one of these keys so
we're gonna say print pages and then
we're going to say Brian my throat's get
a little sore I've had this nasty cold
that's just really driving me nuts
so there's 43 so you can see how Brian
maps to 43 and we've got the value of
Brian using that key we could also do
something like this and we could say
ages dot keys at element 0 so we want to
get the first one whoops
maybe that isn't really working well
really I thought I could do that element
at let's do it this way because I said
so and which does the same thing
basically but now I really want to know
why I couldn't do this you think you
could do this did I screw something up
no it's probably because it's got these
double blocks
hmm which leads me to my next point it's
just a fairly new language so things
will evolve over time I expect something
like that would actually function at
some point all right so how do we
actually add in here so we're gonna say
ages dot whoops actually let's do let's
say ages give you a name mark equal
marks in his 50s so we'll say that then
we can print ages mark let's save and
run this bad boy and you can see there's
mark in his 50s now just for giggles
let's say we wanted to do something like
we did with this list here where we do a
for each so we're gonna say ages for
each notice how it has string int fat
arrow into void if you have no idea what
I just said we will cover all of that I
promise you in future tutorials and for
this we're going to say K and B for key
and value and we of course because this
is all one statement need to end that
with a semicolon that looks crazy nuts
doesn't it if you're used to other
languages so what we're going to do here
is we're gonna say print
and then do this so you can see that for
each item in our map we're saying key is
value years old so Brian's 43 Heather's
24 on and on and on pretty neat pretty
simple so that in a nutshell is working
with variables and I realize we kind of
got off base on a few areas I apologize
I've got this bad cold cold medicine
going on but if you ever want to know
more you can definitely go out to the
dart lang
age dartlang.org and look at the dart
tor language excuse me dart language tor
and definitely check these out one other
thing you should note is you can click
on libraries tour of dart libraries and
you can see the built in libraries dart
core is it's called and you can get much
much more information out of all these
things you can get some really good
examples this is something I really have
grown to love about dartlang.org is it
has all this information baked right
into the web page so I don't have to
spend five ten minutes you know looking
up on Stack Overflow or expert exchange
or anything like that you can get a lot
of great info out here so definitely
check this out
that being said thank you for watching I
hope you found this educational
entertainment entertaining and before
you go the source code for this will be
out on void robes comm also on github
actually there's a link to it on my
website and be sure to check out the
void robes Facebook group there's 1,700
other developers out there that's it