Harvey Mudd College
Computer Science 153
Assignment 1

Solutions by Hormel Foods

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Section 1: Matlab!

Matlab is a tool for modeling, prototyping, and investigating computational and mathematical systems. It is an interpreted computer language with lots of support for user interfaces, graphics, and mathematics built-in. What's more, it's extensible and (almost completely) platform-independent, so that you can easily incorprorate others' systems and experiment with them. If you already know matlab, skip to the next section. There are two large matlab documents in /cs/cs153/matlab/docs: using_ml.pdf is a complete introduction to all aspects of matlab (it's a big file!) and buildgui.pdf details the construction of graphical user interfaces in matlab. In addition, matlab has extensive documentation within the program itself. Just type "help" at the prompt.

You may run matlab wherever you wish. It exists on wilkes (not on turing) as well as on the lab machines in B105, or you can install it on your home computer (see http://www.hmc.edu/cis/servers/suppsoft/licensing.shtml#matlab for details). The files we will use are in /cs/cs153 on turing/wilkes, so you may need to copy files to your computer if you choose to run matlab there (if you can't directly access the cs filesystem).

One-minute intro/refresher

The following is the "one-minute" introduction to matlab.

To start matlab (on wilkes, at least), type 

% matlab
Note that you must have /usr/local/matlab_classroom/bin/matlab in your path. You will see a window pop up briefly and then matlab prompt will appear in your terminal window: 
>>
(Because matlab uses your terminal window for its prompt, it will complain if you try to run it in the background.)

Just to be sure you know matlab is working, see if it can answer important questions like 

>> 40+2
If it can handle this, handling images should be no problem. You might want to try 
>> 40+2;
The semicolon at the end of a statement is not a terminator, but suppresses matlab's printing of the evaluation of the expression. Typing 
>> quit
will get you out of matlab.



Variables

Variables work as you may expect them to in matlab. 

>> x = 42;
followed by 
>> x * 0
will convince you that the system has indeed remembered x's value. The variable ans always holds the result of the last computation. Matlab variables are case-sensitive. If you want to clear the value of a variable you can use 
>> clear x
The command 
>> clear
clears all variables. If you want to check up on your variables, try the commands 
>> who
>> whos



Vectors and Matrices

A big reason for using matlab is that it has considerable support for vector and matrix manipulation. For example, you can define a vector and a matrix as follows: 

>> x = [ 1 2 3 ]
This declares x to be a 1 by 3 row vector. 
>> A = [ 1 0 1 ; x ]
This declares A to be a 2 by 3 matrix. Note that the semicolon here indicates that you want to start a new row. Also, matlab lets you embed matrices within one another, as long as their dimensions work out.

The transpose of x is 

>> x'
and multiplication is as you might guess 
>> b = A * x'
If your dimensions don't coincide, matlab will warn you. It will let you do something like 
>> x + 5
(componentwise), however.



Getting help

Matlab has a considerable built-in help system. To get a list of high-level topics, type 
>> help
To get more detailed information about one of the topics, try 
>> help general
which provides yet more topics. Finally 
>> help path
gives you a matlab "manual page" about the path command (where matlab will look for files). You may also want to set your MATLABPATH shell variable to add your direcctories to your search path. Notice that since lots and lots of things in matlab are global variables (after all, this is a prototyping system), you may need to be careful about what you name your own variables and files. And, speaking of files,

m files

It's usually easiest to store commands in files and run them from there. There are two basic types of matlab "m" files: scripts and functions. As an example script, type the following into a file. To do this, you'll need to use a text editor outside of matlab -- like vi, (on second thought, perhaps not one not like vi). 

  x = 5;
  y = 10;
  answer = y*x - y + y/x
Name this file test.m . Then if you type 
>> test
at the matlab prompt, you will find the answer you sought. If matlab complains about not being able to find the file, then you put it in a directory not in its search path (the current directory is in the search path by default). Either move it to matlab's current directory, change matlab's current directory, or set the path variable. You can get help on path, cd, or pwd.

The second kind of m file is a function definition. Here's an example: type 

  function   z = fun(x,y)

  % z = fun(x,y) computes x to the y power

  a = x^y;
  z = a;
into a file named fun.m and then try 
>> fun(3,5)
The % is matlab's one-line comment character. The comment in the above file is actually used by the help system. Type 
>> help fun
to remind yourself what it does in case you forget... .

Note: there was no formal code to write or problems to solve in this section of the assignment.



Next Section on Images in Matlab