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[AUDIO LOGO]

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SPEAKER: So in a moment, we're going
to write a very simple program.

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And while we do, we're going to
think about what a program actually

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is at the end of the day.

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So we've seen so far that we
have this language called Python.

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And we can use Python to write programs.

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And Python itself has all
these tools at our disposal

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to actually write those programs with.

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So among them are things
like conditionals, variables,

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loops, and in this case,
importantly, functions,

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where a function is some piece
of code that takes an input

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and produces some output.

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You can think of it like a
verb that you can actually

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run inside of your own program.

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So here, I'll go ahead and
make a program called hello.py

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by typing code hello dot py.

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And I'll hit Enter.

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And I'll get this blank Python file.

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I know because it ends with .py here.

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And I want to call, or use,
my very first function.

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And one function built into Python
is called the print function,

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where the print function
takes some text as input

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and produces as output this
text in my terminal down below.

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So I'll type print here.

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And I'll follow it with
parentheses, just like this.

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And I'll type "Hello, world!"

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like that.

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So now I've used, or called,
my very first function, print.

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And maybe afterwards,
I'll also type print.

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Maybe this is CS50P, like this.

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So now I have a very simple program.

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And I've, in this case,
used two functions.

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Now, if you're new, you might be asking,
how do I know that these are functions?

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Well, it just so happens
that functions have

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these identifiers you can use to
determine when you're actually

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looking at a function.

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Now, one of these identifiers
are these parentheses here.

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Notice how I called the
name of the function, print.

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And I followed it with
parentheses, open and closed here.

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So when you use, or
call, functions, you're

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often going to type the name
of the function followed

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by parentheses, and then some input.

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Some functions might
not even take any input.

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You can call them a bit like this--
print, open and closed parentheses.

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And that would mean
this function doesn't

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take any input or, in this case, I
haven't given it any input on line 3.

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But I have, of course, on lines 1 and 2.

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So to give you an idea of what these
functions will do, let me go ahead

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and run the program.

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I'll type python hello.py down below
to run this program top to bottom

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so that as-- first, we'll see the
print function will run on line 1.

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And then on line 2, the second
print function will run.

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So I'll hit Enter here.

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And I should see "Hello, world!

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This is CS50P" from top to bottom.

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So we've used these functions.

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But as a programmer, you might also
want to create your own functions.

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And in particular, you might
want to write programs that

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are composed of individual functions.

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Now, to define your
own function-- that is,

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to tell us what code should
run when I call or use

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the function-- you can use
the keyword called def,

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D-E-F, where def stands for "define."

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So maybe because this
is the main function--

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I'm about to write the main
function in my program--

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I could just call it
main, def main, like this.

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And I want to include an open
parentheses and a closed parentheses,

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followed by a colon.

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And that then says I have
a new function that I've

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defined called main that, in this
case, won't take any particular inputs.

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But now if I want to
tell Python or my program

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in particular what code should
be run when I use this function,

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I need to indent those lines of code
and put it inside this definition here.

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So why don't I take these
two, these two prints,

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and put it inside of my
function called main?

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I'll indent both of them and
move this back up to the top.

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And now I have defined,
again, a function

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called main that doesn't
take any particular input.

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But it does run these two functions--
first the print on line 2,

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and then the print on line 3.

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So now that you've seen
how to define a function,

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you could think about how or who
defined the print function for us

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because right now, we're
saying print "Hello, world!"

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But what does it mean to print?

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What goes into printing
something to the terminal?

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Well, it just so happens
that the Python developers

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defined what happens
in the print function

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elsewhere in the Python
library, which you'll learn

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more about as we go through the course.

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But for now, you can just know that
you can use this function, print.

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But somewhere else, somebody before
you has defined it and allowed

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you to use it in this context, just
like I've now defined a function

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called main here.

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So let me go and run this program now.

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I'll say python hello.py.

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Hit Enter.

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I don't see anything,
which is maybe surprising

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if you're new because here, I said I'm
going to define a function called main.

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And within that function,
I want to do the following.

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I want to print "Hello, world!"
and then print, "This is CS50P."

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But in Python, defining a function
is different from running it,

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or this technical term
called calling it.

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So I've defined my function called main.

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But I haven't called main.

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And by convention, I can do that using
two new lines underneath my definition

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here and then calling main.

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So I'm going to now run whatever I've
defined as part of my main function

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here.

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I'll type python hello.py
again, hit Enter,

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and now I should see "Hello, world!

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This is CS50P."

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Because this program
is read top to bottom,

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it will first define a
function called main.

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It will then say what functions compose
that function, which functions should

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be run when I call this function.

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And then down below, I finally
actually call it on line 6.

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And so now in this case, you've seen
that my entire program is really

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a function.

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As you go off and build
new and better programs,

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you'll see that a lot
of what you're doing

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is just writing things that
take inputs and produce outputs.

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And those things are called functions.

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