How to save and execute a file.
First, if you will be using matlab on the same server you want to
log in to that machine and create two directories in your home
directory: one called "matlab" and the other called "Mat1062".
If you're in the math department, you'll be working on the machine
If you'll be using matlab on random machines in a computer lab then
you want to have a flash stick or some other memory medium. On that
stick you'll make two directories: "matlab" and "Mat1062".
First save the files junk.m,
in the Mat1062 directory.
Open matlab and type the command "magic(3)". This will produce a
3x3 magic square.
The file "junk.m" is a script file that solves a linear algebra
problem, Ax=b, in two different ways. Execute this file by typing
"junk" at the matlab prompt.
When you start matlab, it will be in the wrong directory for your
purposes. You saw the error message "Undefined function or variable
`junk'." when you typed "junk". If this happens, type "pwd". This
will show you what directory you're presently in. Move to the correct
directory with the command "cd d:" (to get to the d drive, if that's
the name of the flash stick drive) followed by "cd Mat1062". To see
what's around, type "ls" (or "dir" if on a PC). If you see junk.m,
counter.m, and magic.m then you've arrived.
Now try executing "junk". It should work. Now type the command
"magic(3)". Note that rather than getting a 3x3 magic square you just
got the number 12.
The file "counter.m" is a file that creates a vector x, which is those
integers from a to b. Open matlab and execute this file by typing
"[x] = counter(3,50)" at the matlab prompt. (Or whatever numbers you
choose for a and b.)
Matlab looks for files that end in ".m". When you create a file for
matlab, it must end with .m To execute the file, you type the part of
the file name that comes before the .m Your file names can have pretty
much anything in them, but only one period. (You can't have a
file called "test.this.m".)
You have to be careful in the naming of .m files since there are some
names already used by matlab. For example, "magic.m" is a built-in
file that creates magic squares. If you create your own file called
magic.m then what happens when you type "magic(3)"? When you type a
command, the first thing matlab does is looks in the current
directory. So if there's a file "magic.m" in the directory Mat1062
and if you're in that directory (as shown by "pwd") then it will
execute that file. If there is no such file then the second place it
will look is in the matlab directory in your home directory. If there
is no such file then the third place it will look is in its reserves
of files. And so when you started matlab in the first place and typed
"magic(3)" it got to the third step and executed its own version of
magic.m . Whereas once you were in the Mat1062 directory it executed
your own version of magic.m
Why would you ever want different *.m files that all have the same
name? In practice if you're doing experiments and you have a lot of
data, you'll have directories and subdirectories and subsubdirectories
with different experiments in each subsubdirectory. And in each of those
subdirectories you might have a file called "make_plot.m" which will
load data and make a plot. (This is an excellent habit to get into, btw,
because it's not uncommon for referees to deliver reports that ask for
minor modifications of plots. If you made that plot by hand six months
ago, you may have no idea how you did it. But if you saved all the commands
in "make_plot.m" then you're happy as a clam.)
Note: if you're on a PC or a mac then you may have to inform matlab
about the matlab directory in your home directory. This means that you
have to add that directory to matlab's "path". (The path is the list of
places that matlab looks.) To do this, on a mac, in matlab go to "file" and
then choose "set path..." and then add the folder matlab in your home
directory and click "move up" so that it's the first place it looks after
failing to find something in the current directory.
In general, if you're thinking of creating a file with a particular
name and you want to test whether or not matlab already has a command
with that name, to test if your filename is a good one, say
"my_dog_fluffy.m", type "help my_dog_fluffy" at the matlab prompt. If
matlab has such a file built in, it will tell you about it. (If you
type "help magic" it would tell you all about magic and how to use it.
It will give you the names of other commands you might be interested
in.) Note: matlab has the annoying habit of doing things like producing
the following when you use the help command:
>> help magic
MAGIC Magic square.
MAGIC(N) is an N-by-N matrix constructed from the integers
Reading this, you'd think that to execute the command you should
type "MAGIC(3)", wouldn't you? Noooooooo... it's all in lowercase.
For some reason, matlab's built-in functions are all lowercase but
then they're referred to in uppercase when you ask after them.
The files "junk.m" and "counter.m" are
of different types:
junk.m is a script file, which is a file that contains a
sequence of matlab commands. If you're going to have to type the same
10 commands in a row, but with minor modifications each time, then
it's fastest to write a script file, like junk.m, and then use an
editor to type those minor modifications in and then rerun junk.m,
rather than typing the 10 commands over and over again.
counter.m is a function file. It takes two inputs,
the integers a and b, and gives you back a vector, x. Now that
you have x, you can do all sorts of things with it.