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5.4: Boolean Variables - Processing Tutorial



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hi, so by now if you're here

you've probably watched some videos about variables

and some videos about conditional statements

in this video we are going to somehow combine those two in an interesting new way

and we're going to look at

what it means to have a variable of type boolean, right?

what is a variable? a variable is a type like an integer,

it is a name, like we might make up the name 'x',

and we might give it an initial value

so this is a totally reasonable variable:

the integer x, that you can't exactly see

cause [inintelligible]

x equals 100

let's, this is crazy, I'm totally out of sync here

but let's make a variable now that has

a type boolean, 'going' equals false

this is the type of the variable

it is not a number, it is not a string,

it is not other types of variables,

that you may or may not have learned about,

it is a boolean. that means it only has one

possible, two possible values: true or false

you could use an integer in many ways

and have the integer either be 0, for false, or 1, for true

but there are many other numbers

that piece of data could be

in this case the piece of data can only be false or true

and if you remember, a conditional statement

is an expression : if, some expression, evaluates to true, execute some code

guess what? a boolean variable evaluates

to true or false, right?

this boolean 'going' is false, so 'if going', meaning it's not, we're not going to

execute this code because 'going' is false.

but if I change its value to true

then this code will become executed.

so, let's look at a way that we might use this in an example.

here is a very simple example,

it has a circle moving across the screen

size(), background(), an ellipse drawing at x,

x = x+2. what if I don't want this circle to start

until I click the mouse?

well, you might think I would do something like

ah, if mousePressed and then run this

and now, ah, the circle is not moving

if I click the mouse it is moving

if I let go of the mouse it stopped

but what if I just want the event

of clicking the mouse to trigger the circle

to move from now on and forever going forward?

so, to do that, what if I made up my own variable

like 'going'?

and we say: boolean going equals false

and run this code

it's not moving!

now if I change the value of 'going' to true

it's moving, because 'going' is true,

so this code is being executed.

so now all you need to do is change the value of 'going'

from false to true the moment that I want the circle start moving

when I click the mouse.

so in this case right here

let's start with 'going' as false, and then when in mousePressed, this is an event

that's triggered as soon as the user clicks the mouse

let's set: going = true, and it bothers me that you can't see all of the code here

so I'm just gonna make this fit here.

now we can see: ok, going is false

so when we run it, it's not moving

it's not moving because going is false

but now when I click the mouse

going becomes true and it moves.

ah, I'm gonna stop it! click the mouse!

ah, no matter what, when I click the mouse now going is true

what if want going to become false?

here's a neat little trick:

we have time for this,

you have time for this, I have somewhere I need to be

but I'm making this video anyway

so, in mousePressed we are saying: going equals true

but what if I want going to only become true

if it wasn't true, and if going was already true I want going now to be false

in other words I want to say:

if going ... then going equals false, right?

if going is true then set going equal to false,

otherwise set going equal to true

so: if going is true, set going to false

otherwise, if going is false, set it equal to true.

let's go implement that!

ok, so I'm down here, and I'm going to implement this.

if going, and by the way, you could have written it like this: if going == true

but that's really long-winded, going is just true or false,

and a boolean expression evaluates to true or false

so I can really just say: if (going == true) I can just say if (going)

by the way, if (going == false) is the same as saying: if (!going)

that exclamation point is NOT, is not going, my ... a little bigger here

hopefully you can read this.

ok, so remember: if it's going then going should be false

otherwise let's say going is true

let's run this now, right?

so going is false right now, so when I click the mouse

[inintelligible]

now if I click the mouse again, going is true, ah, going is set to false

oh, what a magical thing just happened!

ready? start ... stop ... start ... stop

oh, it's gonna leave the screen.

ok, but, maybe you thought this was all

there's one more piece of this,

this is fine and good and perfectly reasonable

it is rather long-winded

we as programmers, sometimes our whole life is just trying to make things shorter,

although that's not really any point of that,

you might also just make things longer,

it's good for things to be clear, readable, but anyway,

if we want to make this shorter,

let's examine this statement

going = ! going ;

this seems like it must be some sort of illegal code

that you could not possibly write

and it's self-referential and circular in some way

it's not gonna work, but in fact

this is exactly what we want in this particular case

let's think about this again

let's say x = y + z

if we have a statement like this,

remember we evaluate this side of the equation

and store the result over here

so we're going to do the same thing here

we evaluate this side of the equation

and store the result here

it's kind of like how, when you say

x = x + 1 , this seems a little strange

it equals itself plus 1?

but it's not big deal, if it were 5 then 5+1=6

and now x equals to 6, this is an operation

that increments x over and over again

5 becomes 6 becomes 7, I'm talking way too fast in this video,

but here, let's say going is true

what is not true? not true is false

let's say going is false, what is not false? true!

so going equals not itself just takes the value of going,

if it's true it turns it into false, if it's false it turns it into true!

that might give you a little bit of a headache

but I think that as you start to think about it

and write it down and think about it again

it will start to make more and more sense

so let's go back over here and let's look at what he can write here now

going equals not going, ok?

going is currently false, so as soon as I click the mouse

going is going to equal not false, which is true

and when I click the mouse again, going is going to equal not true

which is false.

so this is just a little bit of an inkling

of what it means to use a variable

with a type of boolean

it's just a little bit of a helper, it's kind of like we can store

the value of a switch in our program

we can always know is that switch on or is that switch off?

and at any point we could act upon that

if the switch is on, do this code, if it's off do this other code.

and we can also change the value of that switch

by turning it on, setting it to true, or turning it off, setting it to false.

usually at the end of these videos I give some sort of exercise

but I can't think of one right now

but what you might do is find some program that you wrote

and see where you might store a value of true or false

and act upon that. that could be something that you could do.

I'll think of something better later!

maybe put it in the notes

ok, I'm gonna stop this video

and you're going to enjoy the rest of your day

I'm over here and I'm going now.

my going is true!

ok, bye