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[MUSIC PLAYING]

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SPEAKER: Welcome back,
everyone, to an introduction

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to Programming with Scratch.

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And so far we've seen a variety of
different types of Scratch blocks.

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We've seen functions that
perform some sort of task.

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We've seen values that hold information,
like what direction the sprite is

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facing, or what position it is, or how
big or how small it is, for example.

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And last time we took a look at
conditions, the ability for our Scratch

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programs to ask questions
and then make decisions based

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on the answers to those questions.

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But through all of
this, every time we run

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the code, every time
we press the green flag

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or press the space bar inside
of our Scratch program,

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everything, every block of
code, is just running one time.

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We're running through the
code, top to bottom, one time.

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And if we want to see
the program run again,

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we have to press the green flag
again, or press the space bar again,

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or make something else to trigger
the rest of the blocks to run.

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This time what we're
going to take a look at

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is the ability of
Scratch programs to loop,

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to repeat some blocks multiple times,
as computers will do quite often.

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And so let's take a simple example.

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Right now we have our usual Scratch cat.

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And if I say, when the
green flag is clicked,

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let's have the Scratch
cat move 10 steps.

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This program we've seen before.

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When I press the green
flag the cat moves.

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And if I want the cat
to move again, I have

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to press the green flag again, and
then move again, and then move again,

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and then move again, and I
have to be the one controlling

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every single time, the cat's movement.

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So if I want the cat to
keep moving continuously,

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I have to keep pressing the
green flag again and again.

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Ideally, I'd like for the cat to be
able to do all of this on its own.

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I'll move the cat back
to its original position.

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And I'm going to
introduce a new block now.

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It's also under Control, where
we saw the if-thens last time,

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and this time the block we're going
to use is this block called "forever".

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The forever block looks
similar, in terms of its shape,

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to the if-then block.

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The forever block also has
a container inside of it

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where I could add another stack
of blocks inside of the forever,

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but there's no question to ask.

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The forever block's role is just
to run the blocks of code that

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are inside of it and then, as
this little arrow suggests,

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go back to the beginning of the
forever block and run it all again.

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And then run it all again, over and
over forever, until I stop the program.

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This is an example of
what, in computer science,

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we would call a loop, something
that allows for multiple blocks

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to repeat some number of
times, in this case, forever.

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And so what I'm going to do is I'm
going to take the move 10 steps block,

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and drag it inside of the
forever block, and connect that

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to when the green flag is clicked.

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Now the behavior of
this program is going

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to be when I press the green flag, we're
going to see the cat move 10 steps,

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but then the loop is
going to happen again.

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It's going to move 10
steps again, and then

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move 10 steps again, and then again
and again, and you'll see what happens.

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I press the green
flag, and the cat moves

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until it hits the edge,
at which point it's stuck,

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and it can't move any further than that.

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I can drag it back to the beginning,
and we can watch it go again.

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I'll press the flag, the cat moves,
and I only had to press the flag once,

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and the cat went all the way
to the edge of the stage.

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It appears to be stopping.

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It's not really stopping.

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What's happening is that
it's trying to move 10 steps,

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but because it's already
at the edge of the stage,

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there's nowhere left
for the cat to move.

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And so I could fix this by
adding yet another block.

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I stop the program by
pressing the stop sign,

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otherwise the cat would keep moving.

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I'm dragging the cat back out.

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And under the Motion section of
blocks there is this block here,

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"if on edge, bounce".

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And the idea of "if on edge, bounce"
is exactly as the name might suggest.

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If the cat reaches the edge, it's
going to bounce off the edge.

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It's going to turn around
and face the other direction.

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So I'll press the green flag now,
and now notice what's happening.

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The cat's moving continuously.

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I only press the green
flag once, but we're

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running this loop where the
loop is saying move 10 steps,

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and if it's at the edge of the stage,
then bounce and go the other way.

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And now you might recall when we were
talking about directions that sprites

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can be facing.

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By default, sprites in Scratch
have all-around rotation,

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meaning that if you tell
it to face the other way,

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it's going to end up upside
down as if it had just been

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rotated to face the other direction.

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There is a fix to that
though, and the fix to that

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is to change the rotation style of the
cat, that we've seen a couple of times

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now.

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I can do that by going to direction
here, changing the rotation

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from all-around, which
is the first option,

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to just left and right,
which is the second option.

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And now when I run this program, you'll
notice the cat bounce a little bit

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more nicely, going
back and forth forever,

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at least until I press the red stop
sign to stop the program from running.

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And now if you're really
paying attention to this,

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trying to see if it
looks at all realistic,

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you'll notice the Scratch
cat, its legs aren't moving.

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Its legs are just straight, and it's
effectively gliding along the stage.

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Doesn't really look like it's walking or
running across the stage, for example.

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And we can fix that with
a little bit of animation.

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Animation the same way you might see a
television show or a movie be animated.

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And how does animation work?

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Well, animation, when you're
watching anything on TV,

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or watching anything on a
movie or on your screen,

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it's really just a whole bunch of
images one image after another image.

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And the images are moving
so quickly by your eyes

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that your eyes just process that
and think of it as actual movement.

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And that's what we can try and do here.

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Have a couple different images that
look very similar to each other,

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but are slightly different, and by
moving back and forth between them

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very quickly we can create the
illusion of motion the same way

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you might do so if you're
animating something like a movie.

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So if we go into Costumes here,
you'll notice that the Scratch

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cat does have two costumes.

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It has one costume here
where the legs are straight.

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And it has a second
costume here, costume2,

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where the cat's legs are bent.

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And if we switch back
and forth between them,

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you notice it kind of looks
like the cat's walking.

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And so we can achieve this this way.

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I'll go into Looks, and I'll
just add a "next costume" block

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into the forever loop.

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We're going to move 10 steps, if we're
on the edge, we're going to bounce,

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and then we're going to switch
the cat to the next costume.

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Now watch what happens when
I press the green flag.

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Now the cat's legs appear to be moving.

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They're moving very quickly because the
forever loop is happening very quickly,

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going very quickly through the
movement, through the bouncing,

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and then through the
switching of the costumes.

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But we've animated this cat
just by moving it back and forth

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between these two different costumes.

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Now its legs are moving rather quickly.

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Maybe I want to slow
this down a little bit.

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And I can do that just by going into
Control and waiting a little bit.

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One second is maybe a long time to wait.

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Let's wait 0.1 seconds,
just a fraction of a second,

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to slow down the cat just a little bit.

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And now the cat's walking in a
little more of a leisurely pace.

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It's repeating this
process over and over,

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moving 10 steps, if it's on
the edge, it's going to bounce,

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switching to the next costume between
whether its legs are straight or bent,

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and then waiting just a
fraction of a second so that we

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can slow the cat down a little bit.

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And you could play around
with this value determining

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how long you want the cat to
wait for to determine how quickly

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or how slowly the cat's going
to move across the screen.

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But using that loop we
now have the ability

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to create some effect that's happening
inside of our program forever,

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that keeps going on and on.

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So what's another example of this?

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What else could we do with this idea?

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Well, let's get rid of the cat for now.

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And let's bring back
our underwater backdrop.

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Here's Underwater 1.

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And let's add the fish
back into our program.

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Here's that fish.

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And let's try something else.

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When the green flag is clicked, I
also want something to happen forever.

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What do I want to have happen?

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Well, let's go to Motion, and let's
have it point in a particular direction.

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Now if it were always pointing
in the same direction,

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then it wouldn't really matter if
it was inside of a loop or not.

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It's just always going to face the
right, face the right, face the right,

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face the right, again and again.

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But I could have it point
in a particular direction,

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and I'll go ahead and choose
a block like this instead.

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Instead of pointing in a
direction with a number,

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let's have the fish
point towards something.

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Let's have it point towards the mouse.

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It's pointing towards the mouse pointer.

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So now watch what happens
when I press the flag.

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You'll notice that immediately when
I press the flag the fish turned.

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It's now facing my cursor, which is up
in the upper left corner of the stage.

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But watch what happens
as I move the cursor.

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Every time I move the cursor you'll
notice the fish is moving, as well.

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Because it's constantly, over and over
forever, following this instruction

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to point towards the mouse pointer.

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So wherever the cursor goes, the fish
is going to be looking in that direction

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because it's pointing
towards the mouse pointer.

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Now it's looking at the cursor,
though it's not really moving yet.

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So let's add one more
thing to this program.

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In addition to pointing
towards the mouse pointer,

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let's then have the
fish move five steps.

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So now what's going
to happen is wherever

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the cursor is, the fish is
going to look at the cursor

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and then move a little
bit closer to that cursor.

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So using the cursor will
effectively be able to control

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the movement of the fish.

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I'll press the green flag again,
and you'll notice the fish start

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to move after the cursor.

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Wherever I move the cursor, the
fish is pointing in that direction

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and then swimming in my direction.

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And we can use "forever"
for other things, too.

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Any time I want something to
continue to happen during my program,

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a forever loop is often the way
that I want to achieve that.

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Right now, my backdrop, I
can click on the backdrop,

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is just showing me
this underwater scene.

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But I can add some
blocks to the backdrop,

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as well, telling the
backdrop what to do.

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And maybe I'll have the backdrop
say, when the green flag is clicked,

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let's forever play a sound.

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The default sound is "pop",
but I can change those sounds.

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Let's find a sound that's
appropriate for being underwater.

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And if I scroll down through this long
list of sounds, I can try them out,

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but I think there's one called--

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Let's search for water maybe?

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"Ocean Wave".

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Here's "Ocean Wave", related to water.

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I can sample hear what that sounds like.

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I like that one.

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And so let's have the backdrop play
this "Ocean Wave" sound forever.

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It's going to play the sound until
it's done, and then play it again.

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And so now when I run the program,
notice what you see and hear.

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I have the fish following the cursor,
and I just stopped the program.

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But because I had multiple forever
loops, I had a loop on the backdrop,

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as well, we were just playing this
"Ocean Wave" sound over and over again.

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And you could use this if you
wanted a sound to play forever.

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You could add music to
your programs, as well.

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If you wanted music to be playing in
the background of your Scratch program,

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you might have that music
sound play until it's done,

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and then the song starts right back
up again as soon as the song is over.

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So you can continue to have music in
the background of your Scratch program

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00:12:02,070 --> 00:12:04,170
going over and over forever.

233
00:12:04,170 --> 00:12:06,660
So these forever loops can
be quite useful for helping

234
00:12:06,660 --> 00:12:10,590
to create this sense of
continuity, this continued

235
00:12:10,590 --> 00:12:16,770
presence of sounds or movement that are
happening inside of our applications.

236
00:12:16,770 --> 00:12:20,630
And so let's try now to
create another program.

237
00:12:20,630 --> 00:12:24,860
I'll get rid of the fish, and we'll
get rid of the underwater backdrop.

238
00:12:24,860 --> 00:12:28,160
Let's go back to this plain backdrop.

239
00:12:28,160 --> 00:12:30,760
And I'll get rid of this code, too.

240
00:12:30,760 --> 00:12:32,140
Let's bring back the cat.

241
00:12:32,140 --> 00:12:34,870


242
00:12:34,870 --> 00:12:38,620
And what I want the cat
to do now, just for fun,

243
00:12:38,620 --> 00:12:41,290
is to not let me get
too close to the cat.

244
00:12:41,290 --> 00:12:44,380
If my cursor gets too close to
the cat, I would like for the cat

245
00:12:44,380 --> 00:12:47,380
to meow or something, or
otherwise indicate that it doesn't

246
00:12:47,380 --> 00:12:50,330
want me to get too close, for example.

247
00:12:50,330 --> 00:12:52,130
And so how might this work?

248
00:12:52,130 --> 00:12:53,260
Well, let's try this.

249
00:12:53,260 --> 00:12:55,300
When the green flag is
clicked, my instinct

250
00:12:55,300 --> 00:12:57,220
might be to say, well,
if I want to check

251
00:12:57,220 --> 00:13:01,420
to make sure that I'm not too close to
the cat, that sounds like a condition.

252
00:13:01,420 --> 00:13:02,710
It sounds like an if-then.

253
00:13:02,710 --> 00:13:05,710
If I'm getting too close to
the cat, then have the cat meow

254
00:13:05,710 --> 00:13:07,760
because it's unhappy, for example.

255
00:13:07,760 --> 00:13:11,040
And so I might add an if-then.

256
00:13:11,040 --> 00:13:13,080
And here I can use an operator.

257
00:13:13,080 --> 00:13:15,180
I can use the less than
operator, because I

258
00:13:15,180 --> 00:13:19,230
want to check if the distance
between the cat and the mouse pointer

259
00:13:19,230 --> 00:13:22,230
is less than some value, we'll say 100.

260
00:13:22,230 --> 00:13:23,550
And how do I find the distance?

261
00:13:23,550 --> 00:13:25,410
That's under the Sensing category.

262
00:13:25,410 --> 00:13:29,220
It turns out there's a block here
called "distance to mouse pointer",

263
00:13:29,220 --> 00:13:29,970
so I can check.

264
00:13:29,970 --> 00:13:35,000
Is the distance between the cat
and the mouse less than 100?

265
00:13:35,000 --> 00:13:42,630
And if so, let's go ahead and play
the meow sound until it's done.

266
00:13:42,630 --> 00:13:45,950
So the idea of this code now, what
I would like for this code to do,

267
00:13:45,950 --> 00:13:51,260
is to say if I'm less than
100 spaces away from the cat,

268
00:13:51,260 --> 00:13:53,480
then go ahead and play the meow sound.

269
00:13:53,480 --> 00:13:55,760
And that's what I want the
cat to do, but watch what

270
00:13:55,760 --> 00:13:58,880
happens if I try and run this program.

271
00:13:58,880 --> 00:14:00,200
I'll run the program.

272
00:14:00,200 --> 00:14:04,890
And my cursor starts out pretty far away
from the cat, but as I bring the cursor

273
00:14:04,890 --> 00:14:08,800
closer nothing seems to be happening.

274
00:14:08,800 --> 00:14:11,980
I'm pretty close to the cat right
now, I'm even touching the cat now,

275
00:14:11,980 --> 00:14:15,640
and yet still, I'm not
hearing the cat meow.

276
00:14:15,640 --> 00:14:16,360
Why is that?

277
00:14:16,360 --> 00:14:18,430
What's going on?

278
00:14:18,430 --> 00:14:20,530
Well, it has to do with
the fact that by default,

279
00:14:20,530 --> 00:14:22,900
when you're running code
inside of a program,

280
00:14:22,900 --> 00:14:25,810
that code is only going to run once.

281
00:14:25,810 --> 00:14:27,730
What happened when I
pressed the green flag?

282
00:14:27,730 --> 00:14:31,870
When I took my cursor, and I dragged my
cursor over the green flag right there,

283
00:14:31,870 --> 00:14:34,810
and then I clicked on it, what was
happening inside of the program?

284
00:14:34,810 --> 00:14:37,240
Well, immediately, Scratch checks.

285
00:14:37,240 --> 00:14:38,560
It checks this question.

286
00:14:38,560 --> 00:14:41,680
Is the distance to the
mouse pointer less than 100?

287
00:14:41,680 --> 00:14:42,430
Well, no it's not.

288
00:14:42,430 --> 00:14:46,810
This is more than 100 spaces in the
world of Scatch's coordinate system,

289
00:14:46,810 --> 00:14:48,740
so we didn't play the meow sound.

290
00:14:48,740 --> 00:14:51,880
And at that point, this
stack of blocks was over.

291
00:14:51,880 --> 00:14:55,150
Nothing else happened because we
ran through all of these blocks

292
00:14:55,150 --> 00:14:59,320
from top to bottom, and that was
the end of the stack of blocks.

293
00:14:59,320 --> 00:15:02,950
What I really wanted to have happen
is for this check for this question

294
00:15:02,950 --> 00:15:05,590
to be asked not once,
but for the question

295
00:15:05,590 --> 00:15:08,020
to constantly be asked all the time.

296
00:15:08,020 --> 00:15:10,660
I want the cat to be
checking, is the distance

297
00:15:10,660 --> 00:15:12,640
of the mouse pointer less than 100?

298
00:15:12,640 --> 00:15:15,370
And any time it turns
out that I get to close,

299
00:15:15,370 --> 00:15:18,100
then I want for the cat
to play the meow sound.

300
00:15:18,100 --> 00:15:21,050
And that's another case where a
forever loop is quite helpful.

301
00:15:21,050 --> 00:15:23,620
I don't just want to ask
this question one time.

302
00:15:23,620 --> 00:15:28,100
I want to ask the question forever
as long as this program's running.

303
00:15:28,100 --> 00:15:31,120
And so I can go into
Control, grab a forever block

304
00:15:31,120 --> 00:15:34,420
and put it around the
condition just like that.

305
00:15:34,420 --> 00:15:37,720
And now this question, am I
getting too close to the cat,

306
00:15:37,720 --> 00:15:40,120
it's going to be asked not
once, but over and over again.

307
00:15:40,120 --> 00:15:43,550
Every time we ask the question,
we'll then ask it again.

308
00:15:43,550 --> 00:15:44,950
I'll click on the flag.

309
00:15:44,950 --> 00:15:46,070
The program's now started.

310
00:15:46,070 --> 00:15:48,243
We still don't hear
anything, but now watch

311
00:15:48,243 --> 00:15:50,410
what happens as my cursor
gets too close to the cat.

312
00:15:50,410 --> 00:15:55,920


313
00:15:55,920 --> 00:15:57,810
So it's further away, we hear nothing.

314
00:15:57,810 --> 00:16:03,540
But when I get too close, then
you start to hear the cat meow.

315
00:16:03,540 --> 00:16:07,920
And you hear that happen because this
condition is now inside of a loop.

316
00:16:07,920 --> 00:16:10,350
It's repeating over
and over again forever,

317
00:16:10,350 --> 00:16:15,620
checking to see if the
distance is less than 100.

318
00:16:15,620 --> 00:16:17,470
And so that then is a
forever loop, a loop

319
00:16:17,470 --> 00:16:20,110
that's going to run some code
over and over again, forever,

320
00:16:20,110 --> 00:16:21,357
until the end of the program.

321
00:16:21,357 --> 00:16:24,190
And you'll often hear this also
called an infinite loop, a loop that

322
00:16:24,190 --> 00:16:26,990
repeats infinitely again and again.

323
00:16:26,990 --> 00:16:29,050
But when we repeat code
inside of our programs

324
00:16:29,050 --> 00:16:31,880
we don't necessarily always want
for it to happen infinitely.

325
00:16:31,880 --> 00:16:34,810
We don't always want
for code to run forever.

326
00:16:34,810 --> 00:16:38,740
Sometimes we do want the code to
repeat, but not infinitely, many times.

327
00:16:38,740 --> 00:16:42,680
Just five times, or 10 times,
or 20 times, for example.

328
00:16:42,680 --> 00:16:45,400
And so how could we do that instead?

329
00:16:45,400 --> 00:16:47,620
Well, for that we're going
to need another block.

330
00:16:47,620 --> 00:16:53,330
Not the forever block, which I'll
get rid of now, but this block here.

331
00:16:53,330 --> 00:16:57,310
This block is "repeat" and then
10, and this is also a loop.

332
00:16:57,310 --> 00:16:59,060
It's going to take
some code and repeat it

333
00:16:59,060 --> 00:17:03,020
multiple times, which is implied by
this arrow here at the bottom right.

334
00:17:03,020 --> 00:17:07,640
But we get to specify, by
typing in here, how many times I

335
00:17:07,640 --> 00:17:10,287
would like for that code to repeat.

336
00:17:10,287 --> 00:17:11,579
And so when can this be useful?

337
00:17:11,579 --> 00:17:14,180
When might I want to use a repeat block?

338
00:17:14,180 --> 00:17:18,060
Well, imagine I wanted the cat
to meow three times, for example.

339
00:17:18,060 --> 00:17:21,230
I could go to Sounds, and just
play sound meow until done,

340
00:17:21,230 --> 00:17:24,740
and then play sound meow until done,
and then play sound meow until done.

341
00:17:24,740 --> 00:17:31,640
And now when I press the
flag I do hear three meows,

342
00:17:31,640 --> 00:17:33,830
but this is repetitive code.

343
00:17:33,830 --> 00:17:37,340
I find myself repeating the same
blocks again and again and again.

344
00:17:37,340 --> 00:17:42,500
And I could arguably improve the design
of my program by using a loop instead.

345
00:17:42,500 --> 00:17:46,370
Instead of repeating myself again
and again, I'll just have a loop.

346
00:17:46,370 --> 00:17:49,160
Not repeat 10, but repeat three.

347
00:17:49,160 --> 00:17:51,650
And what am I repeating three times?

348
00:17:51,650 --> 00:17:54,560
Let's play the sound meow until done.

349
00:17:54,560 --> 00:18:00,870
Now I press the flag, and
it still meows three times,

350
00:18:00,870 --> 00:18:03,420
but this is better than
the program I had before.

351
00:18:03,420 --> 00:18:05,320
It's better for a couple of reasons.

352
00:18:05,320 --> 00:18:08,070
One, I'm being more efficient
with my use of blocks.

353
00:18:08,070 --> 00:18:11,310
Before, I needed three blocks
to say meow three times.

354
00:18:11,310 --> 00:18:13,830
Now I just have one
block that says meow,

355
00:18:13,830 --> 00:18:16,540
plus one block that's
handling the repeating.

356
00:18:16,540 --> 00:18:18,900
So I'm being more efficient
with my use of blocks.

357
00:18:18,900 --> 00:18:21,600
And also, it's going to
be easier later for me

358
00:18:21,600 --> 00:18:25,110
to modify this program if I want
to change the way it behaves.

359
00:18:25,110 --> 00:18:29,670
Say that I later wanted this to say
meow not three times, but 30 times.

360
00:18:29,670 --> 00:18:32,010
In my old version I would
have to just drag out

361
00:18:32,010 --> 00:18:34,890
an additional 27
additional play meow blocks

362
00:18:34,890 --> 00:18:36,750
until we were able to get to that point.

363
00:18:36,750 --> 00:18:40,560
Now I just change the number
that appears inside of that oval.

364
00:18:40,560 --> 00:18:43,860
And by changing that value
I change the number of times

365
00:18:43,860 --> 00:18:46,290
that the cat is going to meow.

366
00:18:46,290 --> 00:18:49,110
And this doesn't just have to
be a number that I type in.

367
00:18:49,110 --> 00:18:52,360
I can drag in any value
into that oval, as well,

368
00:18:52,360 --> 00:18:55,740
which means I could do something
like ask the user how many times

369
00:18:55,740 --> 00:18:58,450
the user wants something to happen.

370
00:18:58,450 --> 00:19:00,010
So let's give that a try.

371
00:19:00,010 --> 00:19:04,300
I can go into the Sensing category
of blocks and ask a question.

372
00:19:04,300 --> 00:19:07,810
The question I'll ask
is, type in a number.

373
00:19:07,810 --> 00:19:13,780
And then instead of repeating three
times, let's repeat answer times.

374
00:19:13,780 --> 00:19:17,650
Whatever the answer is that's how
many times I want the cat to meow.

375
00:19:17,650 --> 00:19:20,150
I press the flag and
I type 3, for example.

376
00:19:20,150 --> 00:19:25,270
And then you'll hear three meows.

377
00:19:25,270 --> 00:19:35,220
But if I press the flag and
this time try 5, for example,

378
00:19:35,220 --> 00:19:36,820
then you hear it five times.

379
00:19:36,820 --> 00:19:39,600
So the user controls how
many times this happens

380
00:19:39,600 --> 00:19:43,770
because the answer block I've
plugged into that open space

381
00:19:43,770 --> 00:19:46,372
inside of the repeat block.

382
00:19:46,372 --> 00:19:47,830
And so the repeat block is helpful.

383
00:19:47,830 --> 00:19:50,970
Now, any time I want something
not to happen forever,

384
00:19:50,970 --> 00:19:53,790
but still to happen multiple times.

385
00:19:53,790 --> 00:19:55,390
What's another example of that?

386
00:19:55,390 --> 00:19:57,840
Well, way back when we
first introduced motion

387
00:19:57,840 --> 00:20:00,750
I showed you how you can get
the cat to move in a circle.

388
00:20:00,750 --> 00:20:05,220
I said you could do something
like move, let's say, 30 steps.

389
00:20:05,220 --> 00:20:08,370
And then turn 15 degrees,
just turn a little bit.

390
00:20:08,370 --> 00:20:10,920
And that had the effect, if I
bring the cat up to the top,

391
00:20:10,920 --> 00:20:14,430
where whenever I press the
flag you'll see the cat move

392
00:20:14,430 --> 00:20:19,620
and rotate a little bit, and
the cat moves in a circle.

393
00:20:19,620 --> 00:20:24,840
But in order for that to work I had to
keep pressing the green flag every time

394
00:20:24,840 --> 00:20:26,960
I want the cat to move.

395
00:20:26,960 --> 00:20:30,950
And every time it moved 30 steps
and then turned 15 degrees.

396
00:20:30,950 --> 00:20:33,560
This is a great opportunity
to use a loop so that I don't

397
00:20:33,560 --> 00:20:35,120
have to keep doing all the clicking.

398
00:20:35,120 --> 00:20:38,630
The loop will just take care
of repeating this process again

399
00:20:38,630 --> 00:20:39,710
and again and again.

400
00:20:39,710 --> 00:20:43,130
So I can go to Control, repeat
a certain number of times

401
00:20:43,130 --> 00:20:47,190
and, I counted, to get all the way
around I had to click 24 times.

402
00:20:47,190 --> 00:20:50,270
So go ahead and repeat
this code 24 times

403
00:20:50,270 --> 00:20:54,230
where now when I press the
green flag, you'll see the cat

404
00:20:54,230 --> 00:20:57,120
move all the way around in a circle.

405
00:20:57,120 --> 00:20:59,480
Very quickly, it's moving all
the way around in a circle

406
00:20:59,480 --> 00:21:01,010
every time I click on the flag.

407
00:21:01,010 --> 00:21:03,110
And the reason for that
is because it's taking

408
00:21:03,110 --> 00:21:07,310
every small movement, that movement of
just moving 30 steps and then rotating,

409
00:21:07,310 --> 00:21:09,560
and it's repeating that again
and again and again very

410
00:21:09,560 --> 00:21:13,790
quickly, faster than I could ever click
on buttons on the Scratch interface.

411
00:21:13,790 --> 00:21:16,580
And the effect of that is that
you're able to see this cat now

412
00:21:16,580 --> 00:21:20,890
move in a circle, as well.

413
00:21:20,890 --> 00:21:24,840
And so with that in mind,
let's revisit one final program

414
00:21:24,840 --> 00:21:26,040
that we've seen before.

415
00:21:26,040 --> 00:21:28,830
And that's the balloon program
we've seen a couple of times now.

416
00:21:28,830 --> 00:21:31,170
Where before, last time
we saw the balloon,

417
00:21:31,170 --> 00:21:33,900
I was pressing the space
bar to try and inflate

418
00:21:33,900 --> 00:21:36,960
the balloon until eventually it popped.

419
00:21:36,960 --> 00:21:40,740
But now that we have these loops to
repeat something again and again,

420
00:21:40,740 --> 00:21:43,500
I can have the balloon
inflate itself without me

421
00:21:43,500 --> 00:21:47,900
having to press buttons again and
again to get the balloon to inflate.

422
00:21:47,900 --> 00:21:49,590
I'll center the balloon.

423
00:21:49,590 --> 00:21:51,820
And let's try this.

424
00:21:51,820 --> 00:21:55,920
When the flag is clicked,
let's go ahead and repeat

425
00:21:55,920 --> 00:22:00,230
something some number of times.

426
00:22:00,230 --> 00:22:03,670
And what I want to do is
I want to change the size

427
00:22:03,670 --> 00:22:05,740
by 10, which is what we did before.

428
00:22:05,740 --> 00:22:08,830
And then just to make this happen
a little more slowly, let's

429
00:22:08,830 --> 00:22:14,870
wait, not a full second, but maybe 0.2
seconds, just a fraction of a second.

430
00:22:14,870 --> 00:22:17,620
So it's going to grow a little
then wait, grow a little

431
00:22:17,620 --> 00:22:20,110
and then wait, and then
after we repeat 10 times,

432
00:22:20,110 --> 00:22:27,250
after we're done with all of that
looping, let's hide the balloon.

433
00:22:27,250 --> 00:22:29,380
And we'll play a sound.

434
00:22:29,380 --> 00:22:32,750
We'll play this pop sound.

435
00:22:32,750 --> 00:22:35,630
So now I'll press the flag.

436
00:22:35,630 --> 00:22:41,048
It gets bigger and bigger, and
then it disappears with a pop.

437
00:22:41,048 --> 00:22:41,840
Let's try it again.

438
00:22:41,840 --> 00:22:44,380
I'll press the flag.

439
00:22:44,380 --> 00:22:45,610
OK, nothing happened.

440
00:22:45,610 --> 00:22:49,750
The balloon didn't come back,
which is a little bit strange.

441
00:22:49,750 --> 00:22:52,720
But if you pay attention to my code,
you'll see why that's happening.

442
00:22:52,720 --> 00:22:56,860
I repeated 10 times this
process of change the size by 10

443
00:22:56,860 --> 00:22:58,820
then wait 0.2 seconds.

444
00:22:58,820 --> 00:23:02,620
And then after that repeated
we decided to hide the balloon

445
00:23:02,620 --> 00:23:04,510
and play the pop sound.

446
00:23:04,510 --> 00:23:07,450
But because we use this hide
block to hide the balloon,

447
00:23:07,450 --> 00:23:11,630
the balloon's disappeared, and
then it never really comes back.

448
00:23:11,630 --> 00:23:15,280
So if ever I want something to reset
whenever I start the program again,

449
00:23:15,280 --> 00:23:19,600
I often need to add blocks immediately
after the green flag is clicked to say,

450
00:23:19,600 --> 00:23:23,170
when you press the green flag
let's be sure to show the balloon.

451
00:23:23,170 --> 00:23:26,770
Because otherwise, the balloon is
going to stay hidden from the last time

452
00:23:26,770 --> 00:23:29,720
I hid the balloon when
I ran the program.

453
00:23:29,720 --> 00:23:32,410
And so I can use the
"show" block for that.

454
00:23:32,410 --> 00:23:38,480
Going into Looks, dragging out
this block that says "show",

455
00:23:38,480 --> 00:23:45,640
and because I've been changing the size
I better change the size back to 100%.

456
00:23:45,640 --> 00:23:47,890
So now I see the balloon.

457
00:23:47,890 --> 00:23:50,420
It gets bigger and then it disappears.

458
00:23:50,420 --> 00:23:55,030
And I can press the green flag, and
I see the same thing happen again.

459
00:23:55,030 --> 00:23:57,820
Every time I click the green
flag, we show the balloon again,

460
00:23:57,820 --> 00:24:01,750
set its size back to 100%, then we
let it repeat, growing and growing

461
00:24:01,750 --> 00:24:05,410
and growing before we hide the
balloon and play the sound "pop"

462
00:24:05,410 --> 00:24:08,320
until that sound is done.

463
00:24:08,320 --> 00:24:12,170
And this allows me to repeat the
process of growing the balloon.

464
00:24:12,170 --> 00:24:14,230
And to what size does it grow?

465
00:24:14,230 --> 00:24:17,350
Well, we're changing the
size by 10 every time,

466
00:24:17,350 --> 00:24:20,013
and we're repeating
that process 10 times,

467
00:24:20,013 --> 00:24:22,180
so I could do the math in
my head and say, OK, we're

468
00:24:22,180 --> 00:24:24,220
going to get up to a size of 200.

469
00:24:24,220 --> 00:24:26,110
What if I wanted the size to be bigger?

470
00:24:26,110 --> 00:24:31,000
What if I wanted to let the balloon
grow up to size 250, for example?

471
00:24:31,000 --> 00:24:34,720
Well, I could do the math again and say,
all right, we're starting at size 100.

472
00:24:34,720 --> 00:24:36,970
I want to get up to 250.

473
00:24:36,970 --> 00:24:39,700
And if we're changing the
size by 10 every time,

474
00:24:39,700 --> 00:24:43,240
then I'm going to calculate that's
15 repetitions to get it up there.

475
00:24:43,240 --> 00:24:44,050
And I can try that.

476
00:24:44,050 --> 00:24:47,750


477
00:24:47,750 --> 00:24:52,220
And yeah, it does seem to get to 250,
but I had to do a lot of math on my own

478
00:24:52,220 --> 00:24:54,860
to figure that out, to
figure out, OK, 15, that's

479
00:24:54,860 --> 00:24:57,890
the number of times I would
like for this code to repeat.

480
00:24:57,890 --> 00:25:00,620
There is one other type of loop
that I'll introduce finally

481
00:25:00,620 --> 00:25:03,440
as one final block for
today that can sometimes

482
00:25:03,440 --> 00:25:05,330
make our logic a little bit easier.

483
00:25:05,330 --> 00:25:08,750
And that's this block
here, "repeat until",

484
00:25:08,750 --> 00:25:14,090
and this block effectively tries
to combine a loop and a condition.

485
00:25:14,090 --> 00:25:16,850
We're going to repeat some blocks,
not a fixed number of times,

486
00:25:16,850 --> 00:25:22,850
not five times, 10 times, 20 times, but
until some question is answered true.

487
00:25:22,850 --> 00:25:26,600
And notice this hexagon-shaped opening
is the same as the shape that we saw

488
00:25:26,600 --> 00:25:29,820
in those "if" blocks and the
"if-else" blocks, for example.

489
00:25:29,820 --> 00:25:32,750
So what this is going to
do is repeat some code

490
00:25:32,750 --> 00:25:36,660
until the answer to
some question is true.

491
00:25:36,660 --> 00:25:41,480
So if I want the balloon to keep
growing until it gets to a size of 250,

492
00:25:41,480 --> 00:25:44,630
then I don't have to do the math
and say, OK, that's going to be 15.

493
00:25:44,630 --> 00:25:49,280
I can just say repeat until,
drag the equals sign out,

494
00:25:49,280 --> 00:25:58,950
and I'm going to repeat until
the size is equal to 250.

495
00:25:58,950 --> 00:26:02,200
And now I'll take this code that was in
the old loop, bring it into the repeat

496
00:26:02,200 --> 00:26:06,660
until block, get rid of my old repeat.

497
00:26:06,660 --> 00:26:09,170
And this is my new code.

498
00:26:09,170 --> 00:26:12,680
I show the balloon,
set its size to 100%,

499
00:26:12,680 --> 00:26:15,470
we're going to keep repeating
until the size is 250.

500
00:26:15,470 --> 00:26:18,620
I don't know in advance necessarily
how long that's going to take,

501
00:26:18,620 --> 00:26:21,950
but we're going to just keep
repeating the code until that's true.

502
00:26:21,950 --> 00:26:26,300
We're going to change the size, wait
0.2 seconds, and then at the very end

503
00:26:26,300 --> 00:26:30,390
hide our balloon and
play that pop sound.

504
00:26:30,390 --> 00:26:37,660
And so I can press the flag, and
you see the balloon grow and grow,

505
00:26:37,660 --> 00:26:42,080
and once it reaches 250, at that
point the balloon disappears.

506
00:26:42,080 --> 00:26:45,890
So three different types of loops
that we've been able to explore today.

507
00:26:45,890 --> 00:26:49,510
One is the forever loop, that takes some
blocks and repeats them over and over,

508
00:26:49,510 --> 00:26:51,940
infinitely, until we stop the program.

509
00:26:51,940 --> 00:26:55,150
One is the repeat block, which
repeats a fixed number of times.

510
00:26:55,150 --> 00:26:57,910
I can say do this five times
or 10 times or 20 times.

511
00:26:57,910 --> 00:27:01,987
And then, finally, the repeat until
block that combines some of the ideas

512
00:27:01,987 --> 00:27:04,570
that we've been taking a look
at today, looking at the ability

513
00:27:04,570 --> 00:27:08,470
to repeat until a particular
question is answered true.

514
00:27:08,470 --> 00:27:10,480
And using those loops,
we have the ability

515
00:27:10,480 --> 00:27:12,730
to make our programs
even more interesting.

516
00:27:12,730 --> 00:27:15,010
Rather than have to click
a button multiple times

517
00:27:15,010 --> 00:27:17,080
and repeat ourselves
with code multiple times,

518
00:27:17,080 --> 00:27:21,537
we can let the code repeat itself
just via the use of these loops.

519
00:27:21,537 --> 00:27:24,370
That's it for an introduction to
Programming with Scratch for today.

520
00:27:24,370 --> 00:27:26,995
We have a couple more opportunities
to take a look at what else

521
00:27:26,995 --> 00:27:28,150
we can do with Scratch.

522
00:27:28,150 --> 00:27:30,450
So we'll see you next time.

523
00:27:30,450 --> 00:27:31,239