5. QUESTIONS

Welcome back!

We've got one more tutorial here

in this hour of learning to program with Processing.

So, one of the things you might have noticed

is that our programs, while they animate now,

they kind of just do the same thing always.

But what if, for example, you wanted your program

to sometimes draw circles on the screen

and other times draw squares on the screen.

Computer programs can take a different path.

They can sometimes do one thing

and they can sometimes do other things.

And the way that they do this is by answering a question.

If the answer to this question is "yes"

draw a square.

If the answer to this question is "no"

draw a circle.

A yes or no question is often sometimes

referred to as a true or false question. Right?

We can think of like taking a true or false test.

And if the answer is true, draw the square.

If the answer is false, draw the circle.

So we need to figure out how to make our programs

take different paths.

What might be a question that we would ask,

for example? We could say, "Hey! Is the mouse pressed?"

If the mouse is pressed, do draw that square.

If the mouse is not pressed, draw that circle.

And this can get more and more advanced

in more advanced programs.

And you can think of a game --

If these two objects run into each other,

make a nice little pretty heart appear.

I don't know. That might be a nice thing.

Whatever.

The point is, we can have,

based on certain things that happen,

based on questions that have the answer

yes or no,

other things can happen.

And our program can take a different path.

So let's look at how we do this.

By the way, this is known as

a conditional statement.

This is one of the fundamental

building blocks of programming.

Writing conditional statements.

And the syntax of what we're actually going to write,

it looks like this.

If, parentheses, some empty space,

another curly bracket,

and another closed curly bracket.

If, we could ask a question.

Is it raining outside?

If the answer is yes, execute some code

that goes here.

What is the code that might go here?

Carry your umbrella.

If the answer to that question is no,

we're not going to do this code.

We're going to go and find out

whatever code is there --

drawing rectangles, ellipses.

So, this is how we can have a program

answer a yes or no question.

A true or false question.

And what, one of the things we're going to look at,

as our example of this

is saying, if mousePressed.

mousePressed is a word that Processing knows.

It's like asking the question

"Is the mouse pressed?"

This word is true when the mouse is pressed

and it is false when the mouse is not pressed.

So, if I were to write

rectangle here

then only when the mouse is pressed

would I draw a rectangle.

Okay. Let's take a look at that below.

So I'm going to make a basic example below

where there is a circle on the screen.

It's not doing much but there's a

circle on the screen.

Now, I'm going to add to this -- the code below

this "if" statement, this conditional statement

with a rectangle drawn when

the mouse is pressed.

Now I'm going to run that code.

Go and click in the window over here.

What happens when you click?

Only when you hold that mouse down

the rectangle appears.

As soon as you let the mouse up,

the rectangle disappears.

This is the power of conditional statements.

You allow your program to sometimes

do something one way and

sometimes do something another way.

Okay. There's a bit more.

We're almost done but there's a bit more

to that than this.

What if we want the program to draw nothing?

But draw a rectangle when the mouse is pressed.

Otherwise draw an ellipse.

The word we use in computer programming

is not otherwise, it's else.

Else, and another curly bracket,

and another curly bracket

and we can put ellipse here.

So when the answer to the question

mousePressed is yes,

or true,

we're going to execute this code.

When the answer is no,

or false,

we're going to execute this code.

This allows us to have multiple permutations

of different scenarios.

And we can add as many of these

if statements to our program.

We can have a series of three or four of them.

Let's revise that example now and add an else.

So the ellipse is going to be drawn --

or the rectangle is going to be drawn

when the mouse is pressed, else,

otherwise an ellipse is going to be drawn.

So now try clicking in the window over there.

If you're clicking I see a rectangle, right?

If you let go I see an ellipse.

It's one or the other.

I've already forgotten.

But I think you get the idea.

So I'm going to load our painting program

that we finished with at the end of our

third video tutorial, below.

Now, if you paint in the window,

you can paint as long as you want.

Make all sorts of wonderful patterns

and draw your name,

and smiley faces and rainbows

whatever you want. It's never going to erase.

What if we said,

If the mouse is pressed, draw the background.

So only when you're holding down the mouse,

the background is drawn,

effectively erasing everything that you've painted.

We now have a program

that as you move the mouse in the window,

it draws.

When you click the mouse, it erases.

We could also do the opposite.

What if you only draw when you're

moving the mouse?

So you have to hold down the mouse,

draw, you could let go, move over here,

draw some more.

That might be something you choose

to try to implement as an exercise.

Okay. So this wraps up this last video tutorial

about learning to program with Processing.

We've done actually quite a bit of stuff.

We've learned about coordinate systems.

shapes. We've learned about color, gray-scale,

and RGB. We've learned about the flow of a program.

How it has a set-up and a draw

and loops over time and things can animate

and move with the mouse.

And now we've learned about how a program

can create different paths and do something

different based on whether the mouse

is clicked or not.

So this is a time where you can spend

a little moment to explore this idea of

the conditional statement.

If you go back to your painting program

you could try adding something where

it's painting one way when you're

holding down the mouse,

and painting a different way when you're

not holding down the mouse.

You could try clearing the background

in the conditional statement,

all sorts of possible things you could try.

So work with conditional statements

in this exercise.

I'm going to leave you with a simple

painting program that uses

a conditional, below.

But you can always revert back to your code

from the last time,

just as you always have.

So, take a little time to do this exercise,

when you're done, click next

and it will be one last wrap up video.

We'll kind of say goodbye, and I'll also

show you how you can share your code

and save it so that you can come back

and look at it again later,

as well as look for resources,

for how you might, hopefully

if you're excited and interested in this,

there's lots more to learn. Right?

This is just the beginning.

We've just scratched the surface.

And there's more resources online,

through books, and other websites

that will help you continue this process.

Okay? So, see you after you

complete this exercise.