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Okay, so that's sentences, now paragraphs,
let's talk about paragraphs.

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Paragraphs are the combination of
sentences to make

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sort of a thought together. Multiple
sentences, multiple lines.

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So, the interactive Python that I
just showed you

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is fine for running one, two, or five or
six commands.

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But ultimately, we're going to write much
longer bits of Python.

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And so we write what's called a Python
script, or a Python program.

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And we put these in a file.

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And we, and if you went through the

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prerequisite, you will see have seen me
edit in a text

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editor, saved a file, and then run from
the Python file.

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Okay?

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And so we call these files put .py on the
end of them, P-Y on the end of them.

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And we're giving Python a script to
execute.

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[COUGH] So, interactive, you're typing
directly into Python

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and it's doing it right as you're talking.

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You're still doing it in order, and the
order does matter.

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In a script you type it all into a file
once and say Python, do it all.

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[SOUND] Now when you write one of

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these things there are patterns for
combining these.

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There are things that we do to these
lines that sort of treat them differently.

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Is like a recipe, a set of instructions.

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Start at the beginning.

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[SOUND] But it's a little more complex
than that.

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Some steps are just sequential.

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Some steps might be skipped.

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Some steps we do multiple times.

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And other times we have kind of like a
set of steps we do over and over again.

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So here's some pictures.

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And here's the four lines of Python, a
little simple paragraph.

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And it's got a sentence that says,
x equals 2.

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Print x.

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x equals x plus 2, which says, go grab the
old value of x, add 2 to it.

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Stick it back in x.

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And print x.

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The output of this program is 2,
then 4.

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because x was 2, we printed it, and we
added 2

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to it, and then we printed it again so it
was 4.

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Now, these flowcharts, don't worry, I'm
not going to make you draw these.

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I just draw these in case, cognitively, it
makes

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it easier for you to understand what's
going on.

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So, x = 1.

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is the first step.

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Sequentially, it just continues on.

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It runs the print.

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x equals x plus 1.

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Runs the print.

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So this is just straight through.

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It'll make more sense when we see a little
more convoluted things.

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So, this program just starts naturally.

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Python starts at the beginning and works
its way down through the end.

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That's sequential stuff.

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That's the normal order of business.

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Now, a conditional is a step that may or
may not get executed.

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If all we did was sequential steps,
programs would be kind of dull.

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Right, they would just be like blah, blah,
blah, blah, stop.

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So, there's things like oh what if you do
this or

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what if you do that and so we do things
like if.

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If you have more than 40 hours, I'm
going to pay

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you a different rate than if I have under
40 hours.

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Those kinds of things are if, the word if.

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So in Python, the way we express this is
we use the keyword if.

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So we say x equals 5, and then we say
if x is less than 10.

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This is a question that's being asked, is
x less than 10 or not, yes or no?

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If it is, we execute the indented bit.

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If it's not, we skip it.

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This case, since x is 5, we execute it.

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Then we come back here, and we're going to
do another one.

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If x is greater than 20, well, this turns
out to be false.

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So we skip that.

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So Bigger does not run.

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That line never runs.

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So, we, the output is Smaller, Finis.

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Now, here, we can take a look at it, sort
of, in the picture diagram.

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We run x equals 5.

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We ask a question, this doesn't hurt x to
ask the question, is x less than 10?

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The answer is yes so we kind of drive down
this little path.

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We print Smaller and then we rejoin the
freeway.

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Is x less than 20? No, so we skip and we
continue.

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So, this never gets executed.

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So you can think of it either way.

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You can think of either sort of like

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Gestalt, say, if this true, do what's
indented.

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Or you can imagine sort of a little car

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driving down a highway making turn choices
as it goes.

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They're equivalent. Over time it's probably
you'll just

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start seeing this and start thinking this
way.

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But sometimes it helps to think about it
this way, for a little while.

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Okay.

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Now, the next thing I want to show you is

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repeated steps, steps that happen over and
over and over again.

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Okay?

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And that again, when I said oh, computers
are good at handling a billion words.

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Well that means it has to have a loop or a
repeated section, one for each word.

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It's going to do something for each word.

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And so,

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so here we go.

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And in Python, let's pick a different
festive color.

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Let's pick purple as a festive color.

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So here's our program, starts at the
beginning, sets the variable n to 5.

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And then a key word, reserved word while,
while

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n greater than 0, again this is asking
a question.

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This is asking a question.

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Is n greater than 0?

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That's a question.

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If yes we're going to do this, if no we're
going to do that.

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Over here if it's true we're going to
execute the

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indented part, and then come back and do it
again.

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If it's false we're going to skip down.

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So it's kind of like an if, except it

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keeps doing it over and over and over
again.

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So, it comes in, sets n to 5.

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Is n greater than 0?

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Yeah, sure.

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So we print n, out comes 5.

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Then it says, and n equals n minus 1, so n
becomes 4.

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We can change colors.

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Then it goes back up.

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Goes back up and asks the question again.

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n is 4.

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It's still greater than 0, so it comes
through.

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Prints out 4, subtracts 1.

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So n is now 3.

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Goes back up.

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Is n 0?

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Is n greater than 0?

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Yes, it is.

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Print out 3.

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Subtract 1.

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Now it's 2.

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So out come 3 and 2.

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Then it goes back up, still
greater than 0.

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Yes, it is.

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Print out 2.

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Or, wait.

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Now it's 1.

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[COUGH] Now we subtract 1, it becomes 0.

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Is it greater than 0?

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No, and we finally leave.

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And we finally drop down.

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And so the last thing that comes out is
the printed Blastoff.

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So this is a loop, the notion that we're

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going to run this little bit of code
five times.

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[COUGH]

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Excuse me.

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We're going to run this little bit
of code five times,

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And loops have these things we call
iteration variables.

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And that is this n.

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It's a variable that specifically is
changing

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each time it goes through the loop.

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And that way, we can sort of control the
loop.

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We can decide when it starts and when it
stops.

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We can tell if we're at the beginning or
at

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the end, or the first one or the last one.

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We'll do a lot of stuff with loops.

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This is an iteration variable, because

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we iterate repeatedly iterate through the
loop.

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Okay?

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Any questions?

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We can't do questions.

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Okay, so now, if we go back to the little

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story that I, you've got several chapters
to understand, don't worry.

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You actually got like through chapter
nine, so

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don't try to understand this program right
now.

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I'm just trying to give you a sense of

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what the picture is going to be, right?

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So, so here are some sequential
statements, because they aren't indented.

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Those five lines are sequential.

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They just go one after the other.

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Then we have for, and it's indented.

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This is a loop.

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This is going to run a bunch of times.

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Then we're done with that.

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We do some more sequential stuff.

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Now, we have a for loop, and that's
going to run a bunch of times.

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And then we have an if, which may or may
not run.

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So these little, this little block of

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code is conditionally executed based on
something.

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And here is the question that we're
asking.

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So that's the question.

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And then at the end, we do a
print.

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Now, again, don't try to make too much
sense of this.

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I'm just trying to show you sequential,
repeated, repeated, conditional.

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Okay?

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Just, those concepts show up in every,
pretty much every program that we build.

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Okay.