[MUSIC] Hi, welcome to lesson three, Functions. So far, what we've learned is how to
perform individual operations with MATLAB, and we've used MATLAB as
a glorified calculator. But now, it's time to start programming. You'll remember that we've called
a few of MATLAB's built-in functions. For example, to plot lines in a figure,
to calculate the square root, to calculate the sine, and
to do other things. And there's hundreds more
functions that we could call. But what if we need a function
that's not provided by MATLAB? Guess what? We can write our own functions, and we can use them just as if
they were built into MATLAB. Let's see how. The built-in function named rand creates random numbers between zero and
one. The numbers are uniformly distributed, and that means that any number in that
range is equally likely to be chosen. Here's an example of how we might call it. [CLICKING] It creates a three by four matrix
of random numbers between zero and one. You can see the first argument
gave us the three, and the second one, the four: the height and
the width of the matrix that's returned. And you can see these numbers
are all between zero and one, but what if we needed random numbers
between one and ten instead? That's easy enough. [CLICKING] And here's some numbers between one and
ten. Multiplng by 9,
stretched the range to go from 0 to 9, and adding 1 shifts the range
to go from 1 to 10. Let's say
we need this functionality repeatedly. We could type in this expression
every time we need it, but that would be annoying. And more importantly,
it would be error prone. We might mistype something one or
more of these times, and result would be wrong and
potentially we might not even notice it. So let's create our own function
that does what we want it to do. Then we can just call
that function repeatedly. And let's call it myRand. We create a function by using the Editor. Let's see what that is. We can get it with the command, edit. e-d-i-t, return, and
there's the Editor window right there. The Editor is the place that you type
in code that you can save in a file and the code we're going to type into
the Editor will be a function. We type this: function myRand, return, a equals 1 plus rand(3,4) times 9, return. And I'm going to put
an optional “end” in here. Notice that function and
end both turn blue. These are the key words. We'll say,
something about that a little later. The word “myRand” is of course
the name of our function. So now what do we do? Well, we want to be able
to run this function, and in order to run the function,
we have to save it into a file. MATLAB reminds us that we haven't yet
saved it in a file with this little word untitled and
this little asterisk right here. Okay.
To save our function into its file, we click this little
diskette right over here. Up pops our operating
system’s file-save window, and it's suggesting that
we name this myRand.m. That's a good idea. This is no time to be imaginative. We're going to name it just that. We could pick another name, but
it's very bad practice to do that because it's very confusing. First off, we should always use the same
name for the file, not counting the dot-m here that we use for the function name,
which is myRand in the case. And second, the dot-m is required. All MATLAB must be stored in
files with the extension, dot-m. And as a result,
they are all called dot-m files. Okay, let's save this file. We hit Save. And notice that the file is saved
right here in the current folder. That's exactly where we want it to be because that's where MATLAB will look for
it when we tell it to run it. Okay, we've opened the Editor; we've typed in a function;
we've saved it into a file. We can now run it by typing its
name in the command window, just like we would for
a built-in function. Let's do it. [MUSIC] Voila. There's our three by four array of random
numbers selected from a range of one to ten. [MUSIC] [APPLAUSE]
