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PROFESSOR: This is
Dr. MATLAB, lecture 3.

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If you're using MATLAB on the
command line for enough time,

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you realize that you really
want to save a bunch of commands

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in a file and then execute
all those commands in one go.

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This helps you so that
if you have typos,

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you can correct the typos
without introducing new ones.

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It also allows you to
create large pieces of code

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that call each other and really
have complicated projects.

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To see how this works,
we need an example.

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What we'll do is generate
the Fibonacci sequence.

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So in the Fibonacci sequence,
the first two terms are 1.

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And then each term is the sum
of the previous two, like that.

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Notice that this will
create a new term

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in f that will be the
sum of the very last one

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plus the second-to-last one.

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Here it created 2,
the sum of 1 plus 1.

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If we run that
command again, it will

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generate 3, and then 5, and
then 8, and then 13, et cetera.

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So now I want to take these
three important lines--

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this one, this
one, and this one--

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and create a file that
will do all of this

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and put this line
inside a loop so that it

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does that, say, 20 times.

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To do that, I open
the History browser.

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I want it to be docked.

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And it's docked.

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And here I find my commands
that I issued here.

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I'm going to select all of
them, the three commands,

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right click, and copy.

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So now I've copied
these three commands.

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Over here, I can
open a new script.

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The first thing I want to
do is get rid of this bar

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because it uses up a
lot of important space.

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So now I have a bit
more space, and I

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can close this command
history because I

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have copied those commands.

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And now I'm going to paste.

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After I've pasted
this, of course,

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I need to clean up a bit.

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So I'm going to add semi-colons
at the end of these commands

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because I don't need
to see the output.

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And I'm going to
add a for loop that

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will calculate the first
20 to the next 20 terms.

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So I'm going to
have 22 in total--

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two from here, and then
another 20 from here.

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At the end, add a
Tab character just

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for alignment to
make it easy to read.

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And now at the end, I will
just display f like that.

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Because everything
else is suppressed.

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I need to save this.

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I need to select a
place to save it.

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The default is in
Documents, Matlab.

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I want to save it
in a different place

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to demonstrate what the path is.

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So to do this, I
open a new folder.

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I will call it
Scripts, for example.

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So now this script
is under Matlab.

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And then in Scripts, I
will call this file Fibbo.

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It already is an M file,
which is the extension

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that you need to have.

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Fibbo stands for Fibonacci.

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And I save it.

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And now I go to my command
line and I try to run it,

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and it doesn't run.

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And the reason it
doesn't run is that Fibbo

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is sitting inside a
subdirectory of Matlab.

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And my current directory,
found by typing CD, is Matlab.

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So I'm sitting in Matlab.

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Fibbo is sitting in Scripts,
which is inside Matlab.

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We can see this by looking
at the current folder.

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So here's the current folder.

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Under Scripts, there's
Fibbo, but we are not there.

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We're up here in Matlab.

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So there's two ways
we can solve this.

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We can either change
directory to Scripts--

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for example, by
double clicking here.

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Now our current
folder is Scripts.

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Now if we go here
and call if Fibbo,

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it works, and it
calculates the 22 terms

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of the Fibonacci sequence.

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We could have also done
this from the command line.

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Here I'm going to
go back to MATLAB.

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And I can say change directory
scripts, Tab completion.

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And now I'm in Scripts,
and I can call Fibbo again.

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Another thing I could do--

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going back to MATLAB--

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is to add scripts to the path.

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What is the path?

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Let's see.

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So if I type path, I get
a list of directories,

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a long list of
directories that Matlab

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uses in order to look for
the commands that I issue.

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All the commands
that you issue will

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have to be in one of these
directories for MATLAB

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to look at them, or in
the current directory.

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So you see that the Matlab
directory, the Documents Matlab

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directory, is
always in the path.

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That's a default.
But you can add

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to the path whatever you want.

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So for example, I can
add add path scripts.

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Now that I've done that,
let's look at the path again.

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Scripts is in there.

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And if I try to run Fibbo, then
it works even though I'm here.

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The way I've written it,
it will look in Scripts

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as a relative directory and
not as an absolute directory.

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If I want an
absolute directory, I

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would have had to say add path
slash Users slash Documents

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slash Matlab slash Scripts.

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This is a different path
because this is the exact one.

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Let's look at the path now.

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Here we have the absolute
and the relative scripts.

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At any case, if you want to see
whether Matlab can see a file,

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you can type which Fibbo,
and then MATLAB tells you,

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if it sees it, which file
it actually is going to.

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So if you have the same file
name in different directories,

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both of which are in
your path, the first one

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that Matlab encounters will
be the one that MATLAB calls.

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Let's do one more thing.

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So let's see what happens when
we actually hide something.

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This can be
potentially dangerous.

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Let's open a new file.

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And we'll say-- it
doesn't do much.

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Let's say it says x equals 1.

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And we're going
to save this file,

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but we're going to be a
bit insidious about it,

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and we're going to call it Sin.

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And we're saving it in
the Matlab directory.

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So we're saving a file called
Sin in the Matlab directory.

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And now if I do sin of 3,
I get a very strange error.

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Notice that it's a function.

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But if I would have
called it sine,

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I would have gotten
the 1, because that

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would have been the script.

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So you have to be careful
with things like this.

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And I would recommend
not having such a file

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in your current folder.

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You can also execute a file from
the graphical user interface

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by clicking this button.

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If that file is not in the path
and not in the current folder,

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Matlab will ask you whether you
want to change folders or add

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it to the path.

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And you can answer whatever
makes sense to you.

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For a function, you need
to add the keyword function

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in the beginning of your file.

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Function f to c.

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So let's write a function that
accepts Fahrenheit and converts

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it to Celsius.

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So return value, name
of the function, input.

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And now we want to
say something like b

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equals n minus 32 times 5/9.

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Now v becomes this number.

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And that is the conversion
between Fahrenheit and Celsius.

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So this value v will be returned
to whoever calls this function.

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We need to save it.

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MATLAB already suggests f to c.

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And we can put it inside
Scripts next to Fibbo.

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Now we can write it
as a function f to c.

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We need to give it an input.

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If we don't give it an
input, we get an error.

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So if we type 32, we get 0.

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If we type 101, we get
this as a function.

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You can either end your
function in the file or not.

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This mostly affects the behavior
when you have sub-functions.

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And will talk about
this in the future.