MATH2070: LAB #1: Preliminaries



Introduction Exercise 1
Grading Exercise 2
Starting up Matlab Exercise 3
Using a USB drive  
Using a browser to download files  
Getting help  
Quitting  
Lab summaries  
More on Matlab  


Introduction

You will find instructions for each lab, including this one, on the web, starting from my home page: http://www.math.pitt.edu/~sussmanm. I do not supply copies on paper. Many students find it helpful to print out copies of the lab instructions before the lab session, although it is not necessary. During the lab session, it is convenient to use the online version because you can ``copy-and-paste'' instructions from the web page directly into Matlab. If you prefer, you will find a version of this lab in Adobe pdf format here.

This lab will occupy today's lab session only. This session will introduce the mechanics of using Matlab, particularly on the Linux computers here in the lab. There is some reading to be completed before the second session and you can do that here in the lab or at any other computer with web access. The second lab, two sessions long, will present exercises in Matlab use.

The discussion that follows assumes that you are basically familiar with using a Unix-like operating system such as Linux and browsing the Web. The next few sections will give a brief introduction to Matlab and explain how to use it and those aspects of the environment that will be important to doing the labs.


Grading

The labs roughly follow the material presented in lecture, but are independent of the homework and other exercises presented in lecture. Lab grades count as 30% of your course grade.

Attendance is not required, but help is most readily available during the lab sessions.

You are encouraged to work together with other students, but you must provide your own diary and summary files (explained further below).

Each lab will be given a grade of A+, A, B, C, or 0. These grades correspond with percentage grades of 100, 95, 85, 75, and 0. At the end of the semester, your grades will be averaged and then integrated with your grade in lecture. The grading criterion is:

Grade Value description
A+ 100 The work is correct, or there are only very minor errors.
A 95 All exercises were attempted and are substantially correct.
B 85 All exercises were attempted but there are some serious errors.
C 75 Substantial portions of some exercises were omitted.
Zero 0 Lab was not submitted.

Some of the labs include extra credit exercises. The percentage value of these extra credit exercises are stated with the exercises themselves. At the end of the semester, the extra credit percentages will be added to the grade percentages and the average computed from the sum, except that averages will not exceed 100. Half of the unused extra credit on one lab will be applied toward later labs. Unsubmitted labs will not be eligible for extra credit.

Each lab is due before 11:59 PM the day the subsequent lab begins. Labs submitted after the day the subsequent lab begins will have 1% deducted from the percentage grade for that lab. An additional percent will be deducted for each further week they are late. The final due date for labs 2 through 7 is the last day of classes for the semester, and the final due date for labs 8, 9 and 10 will be announced near the end of the semester. Labs that are not submitted before their final due date will be given a grade of zero, except under special circumstances.


Starting Up Matlab

Matlab is available on the computers in GSCC 126 and on the computers in the labs in Thackeray hall, such as the seventh floor computer lab opposite the lounge. Matlab is also installed in computers in other labs at Pitt, and you can purchase it for your personal computer from the university software center for a nominal fee.

In this section you will see how to start up of Matlab using the Java windowing interface. These instructions are the same for Unix, Mac and MS-Windows versions of Matlab. I will also give the command-line equivalents of many of the commands. These command-line equivalents are valid for all versions of Matlab as well as for the Matlab clone named Octave. Generally speaking, anything you can do using a menu can also be done with command-line equivalents. You would use the command-line equivalents when writing scripts and the menus when working interactively.

  1. Start up Matlab by (a) double-clicking the icon on the status bar at the bottom (this is one of the icons along the bottom of the screen in the default configuration, and looks like this: \includegraphics[scale=.06]{matlabShape.eps}), (b) choosing Matlab from the K (or Gnome or Start) menu at the lower left corner, (c) choosing ``Run Command'' from the K (or Gnome or Start) menu with the command matlab, or (d) typing the command matlab at a command prompt. The first window that opens is a blank white window named ``Shell-Matlab,'' and you should minimize it using the dash at its top right. If you kill this window instead of minimizing it, Matlab will not start. The second window is the ``splash'' that identifies Matlab, and it will go away by itself. Then the main Matlab window will open up on your screen. This window will look something like the following:

    \includegraphics[scale=0.75]{lab01fig01.eps}

    Sometimes, the window will open up containing a large number of warning messages about files missing. These messages are ``normal'' and can be safely ignored.

    The default window is divided into a larger pane on the right, and two panes, each with identification tabs at the bottoms, on the left. The right windowpane is a command windowpane and you will be typing Matlab commands in it. On the left side of this command windowpane somewhere (near the bottom in the figure) there will be a prompt of the form ». Your typed commands go next to this prompt.

    There are two choices for display on each of the two windowpanes on the left. I suggest that you choose ``Workspace'' on the top and ``Command History'' on the bottom in general, although we will have occasion to use ``Current Directory'' on the bottom.

  2. It is a good idea to organize your work into directories (or folders--they are the same thing). I suggest that you do your work in your AFS (Andrew File System) directory. This is the same directory used by most Unix and Linux computers at Pitt and in the Math Department. It is permanent, unlike the default directory on the computers in GSCC. (Files left in the default home directory usually disappear when you log out!) Use the Matlab ``Current Directory'' windowpane to navigate to your AFS directory. (Go first to the ``Desktop'' and then to ``AFS'' and then to ``private''.) Inside that directory, I suggest that you use a directory named math2070 for all the work in this lab, and subdirectories lab01, lab02, ..., lab10 for each of the labs. You will need to create these directories before you can use them, and the ``Current Directory'' windowpane has the appropriate buttons. After a directory has been created, you can also use the small box named ``Current Directory'' in the middle of the line of icons near the top of the Matlab window.

    Warning: You must switch to one of these directories or some other permanent directory each time you start Matlab. It is easy to forget to switch and if you do forget and log off from the computer, your work will be lost! In that case, your ``Command History'' will be retained and you can use it to help reconstruct your work.

    If you wish to use the command line to create and switch directories, you can switch to your permanent ``AFS/private'' directory with the ``change directory'' commands 

    cd Desktop
cd AFS
cd private
    You create a directory named math2070 with the command 
    mkdir math2070
    and you can make it the current directory with the command 
    cd math2070
    You can always find the name of your current directory with the command 
    pwd
    and you can get a listing of the files in the current directory with either the command 
    dir
    or the command 
    ls

    Before going on, create the directory math2070, with subdirectories lab01, lab02, ..., lab10.

  3. The ``Command History'' windowpane is convenient for recalling what commands you have used recently. In addition, Matlab provides the capability to keep a record of both the commands and their output. The command to do this is 
    diary diary.txt
    The name diary.txt is actually the name of the file that is created and you can use any name you like. It is a text file, and you should use .txt to name it. Before going on, type this command in the ``Command'' windowpane. Terminate the command by hitting the ``Enter'' key. In labs that take up several sessions, using the diary diary.txt command in the second session will append to the previous work and will not overwrite it.

    You should also type a comment line that will help you identify your work later. It should start with the comment character, a percent sign %, and include the lab number and date. This line will serve as an identifier when you look at the file.

    Note: The diary file contains a complete record of all the work you do in Matlab. You are to retain this complete record to send to me for grading. Please do not turn the diary file on and off or edit the diary file to hide your errors. There are times when you write an explanation that is not, in my opinion, correct and I will need the complete diary to establish what really did happen.

  4. You can double-click a command in the ``Command History'' windowpane and it will be executed again. You can also use the mouse to drag a command into the ``Command'' windowpane and then change it using the arrow keys.

  5. You can also recover previous commands by using the up-arrow key and then you can change them using the right and left arrow keys.

  6. As will be described below, you will be sending your files to me by email. Since there are sometimes a large number of these files, it is inconvenient to attach them one by one, so I suggest you create a ``zip'' file. Before you create the ``zip'' file, exit your diary with the command diary. If you do not, your diary will be incomplete in the ``zip'' file. You can create the ``zip'' file from the Matlab command line with the command 
    !zip labfiles.zip *.m *.jpg *.txt
    where ``labfiles.zip'' is the name of the file and can be chosen as you wish. (The exclamation point is necessary to tell Matlab that the command is a ``system'' command and not a Matlab command.) You then only need to attach this one file to your email. You should do this only once, when you have completed all your work and after you have closed your diary file with the same command (diary diary.txt) you used to open it, or after you have closed Matlab down and then opened it again.

  7. After you have completed a lab, exit from Matlab with the command quit or use the File$\rightarrow$Close Matlab menu choice.


Using a USB drive

The storage in your AFS area is quite limited, amounting only to eight megabytes or so. This is plenty for the work we are doing, but if you try to keep many old files or files from other courses there, you will run out of space. The computers in GSCC support using USB drives (``flash'' or ``thumb'' or ``jump'' drives or ``memory sticks'') to save your files. You can insert a USB drive into one of the USB slots and wait a few seconds. A new icon will show up on the desktop and you can double-click on it to see the contents of the drive. You can drag and drop files from other directories into your USB drive directory, thus making portable copies of your files.

You must not simply remove the USB drive from its slot when you are done! Instead, when you are ready, go to the icon on the Desktop representing your USB drive and right-click on it. Choose ``unmount'' or ``eject'' on the menu and give it a little time to get its act together. Then you may remove the drive. The reason for this rigmarole is that data do not get written immediately to the drive, but are written at the computer's leisure. Unmounting the drive forces the last data to be written. You may have noticed that MS-Windows offers the option to ``safely remove'' a USB drive, which accomplishes the same thing.


Using a browser to download files

Some of the labs require that you download files from the web in order to use them. The following exercise illustrates how to download files. The file you will download is a very simple script file.

Exercise 1: If you are not using the online version of this lab, please start it up by starting the browser and finding the online version of this page, beginning from my home page,
http://www.math.pitt.edu/~sussmanm, clicking on Math2070, and then on Lab #1.

Right-mouse click on the file demoscript.m to bring up a menu. Choose ``Save link as'' and a file save box will pop up. Navigate to the directory you made: math2070/lab01, and save the file with the name demoscript.m. You must use the .m extension to tell Matlab that the file contains Matlab commands. Return to the Matlab command window. The file should be visible to Matlab, a fact that you can confirm with the dir command or by its presence in the ``Current Directory'' windowpane.

Edit the file by typing the command 

edit demoscript.m
or by double-clicking on the file in the ``Current Directory'' windowpane, or using the ``File$\rightarrow$Open'' menu pick. An edit window will show up. Read through the file: the comments make it self-explanitory.

You can tell Matlab to execute (that is, cause the statements in the file to be executed) the file by typing its name, without the ``.m'', on the command line. (There is another method for executing a file that is not so appropriate for this course: you can choose ``Run'' from the Debug menu on the edit window.)

Do not be confused by the final few statements in the file. They refer to the following exercise.

The following two exercises illustrate the use of the debugging capability of Matlab. Most of the time you will be able to see what is wrong from the Matlab error messages, but sometimes the error is not obvious. In Exercise 2 below, you will see what you might do when you just cannot see why something is wrong.

Exercise 2:
  1. Turn on debugging with the commands 
    dbstop if error
dbstop if naninf
    or through the Breakpoints menu on the Edit window.
  2. Now, look at demoscript.m in the edit window. At the bottom there is a statement %bad=1/(x-1);. The percent is a comment character, so this statement is not executed. Make this statement active by deleting the percent character and save the changed file.
  3. Execute the script file by typing its name without the .m at the command line or by choosing ``Run'' from the Debug menu on the Edit window.
  4. The division by zero caused an exception and Matlab popped up a window with the offending source line highlighted. You should also note the changed prompt in the command window. Note: The window with debugging information in it is the same as the edit window. It may not automatically pop up and you may have to look for it on your desktop.
  5. In the edit window, place the mouse pointer on top of the x in the expression bad=1/(x-1) and leave it there motionless for a second or so (this is called ``hovering''). The value of x should be displayed. You can then see why the error occurred.
  6. Look at the prompt in the command window. It has changed to K». You can do any legal Matlab command at this changed prompt. In particular, you can type the name x to get Matlab to print out its value. This is an alternative way to see the value of a variable in a debug situation.
  7. You can exit from debugging mode either using the menu in the edit window or with the command 
    dbquit
    This action will return Matlab to its usual » prompt.
  8. You can turn off debugging feature with the command 
    dbclear all
    or from the debugging menu in the edit window.

Debugging (finding errors in) code you have written is the most time-consuming and least rewarding task in writing code. I am sure you think you will not be making errors, but everyone makes errors and they can be very difficult to find. You can often find your errors based on the line numbers included with Matlab's error messages, but sometimes the error remains a mystery. In that latter case, the Matlab debugger is the most powerful tool you have available.

Another important use of the debugger is for tracing through a piece of code to help you understand how it works. The benefit of the debugger used for tracing is that the effect of each statement is immediatly clear. Careful tracing is the quickest way to learn how code works.

The following exercise illustrates how you can use the debugger to trace execution. It uses the same demoscript.m file as before.

Exercise 3:
  1. Type the command clear at the command line. This will return Matlab to its state just after starting up. No variables will show in the ``Workspace'' windowpane.
  2. Click on the first executable line (x=1.3) of the demoscript.m file in the edit window. Go to the ``Breakpoints'' menu and ``set'' a breakpoint there. A breakpoint is a place where the debugger will always stop execution. When a breakpoint is set, a red dot will be placed next to the line number in the edit window. You can also click on the dash to the left of the line number to set a breakpoint at that line.
  3. Begin executing the file either by choosing ``Run'' from the debug menu in the edit window, by pressing the ``Run'' button, or by typing the name demoscript at the command line. The edit window will show that the script is poised to execute the first line of the file. (That line has not yet been executed.)
  4. Predict, in your mind, what will happen when the line is executed. In this case, the value of x will change to 1.3 and the result ``x=1.3'' will be printed because there is no semicolon at the end of the line.
  5. Choose ``Step'' from the Debug menu , press the ``Step'' button (hovering over a button brings up its description) or issue the command ``dbstep'' at the command line. This will cause the current line (line 8) to be executed. You will see the variable x appear in the Workspace windowpane and x=1.3000 appear in the command window.
  6. Predict what will happen on the next step. What will the value of the variable xsquared become?
  7. Predict what will happen on the next step. What will the value of the variable p become? (An estimate is good enough.)
  8. Continue using ``Step'' and watch the script execute, one line at a time.
  9. If the next line were to call a function, the button next to the ``Step'' button is the ``Step in'' button. The ``Step'' button goes on, using the value of the function while the ``Step in'' button jumps to the code inside the function and steps there.
  10. Pressing the ``Continue'' button causes Matlab to continue executing the script until another breakpoint is reached or until the end of the script is reached.
  11. Press the ``End Debugging'' button to exit from debug mode. Alternatively, choose Debug$\rightarrow$Exit Debug Mode from the menus or use the command dbquit.


Getting help

It is important to be able to get help when you need it. Matlab provides two help facilities from inside Matlab itself and a third on the web. The easiest way to get help is to use the ``Help'' menu at the top of the Matlab window. Command-line help is also available from the Matlab prompt by typing ``help command''. For example, 

help diary
You will get a short description of how to use the command. You will also get a list of related commands near the bottom of the help description, and you will often find other appropriate commands there. When you write your own Matlab files, you should always include some special comments in the beginning of the file. The comments up to the first executable statement or blank line will be printed out in response to the help command. For example, the command 
help demoscript
will give a quick help message from the first three lines of demoscript.m. You may notice that the first of these lines is included in the file listing in the ``Current Directory'' windowpane.

A second way to get help from the command prompt is the following. 

helpdesk
This command brings up a comprehensive help facility, the same one that the Help menu brings up. This help facility is very similar to the one on the web from the URL:
http://www.mathworks.com/access/helpdesk/help/techdoc/ We will be looking at the help facility on the web later in this lab.


Quitting

You exit Matlab by typing quit at the command line or by using the File$\rightarrow$Exit menu choice.


Lab summaries

You should complete a report of the results you obtained for each completed lab. This report need not be elaborate. The report consists of at least two files: the complete, original, unmodified, diary.txt file(s) plus a summary file. This summary file can be easily created as you do the lab by keeping a text file up in the editor and copying parts of the web page, your commands and output to the file as you work. Another way is to start from the diary.txt file and delete all your false starts and errors to produce a compact record. (If you work this way, do not forget to make a copy of your diary.txt file to send to me. Click here to see a sample summary file.

This summary file is very important. It is what I will read first and, if it is well-written and the work is done correctly, I will not need to read anything else. Never put incorrect Matlab statements into your summary because it will take me a lot of time to discover you really didn't mean them. I expect to see

Summarizing your work is important not only for my convenience in grading, but also to help fix in your mind the focus of each exercise.

Equally important, the summary file helps get you into the habit of keeping track of your numerical experiments in some formal manner. When you are doing research, you may be doing hundreds of numerical experiments, and you MUST get into the habit of documenting your work or you will not remember from month to month what each one did. The idea of the summary is that you can easily refresh your memory on exactly what you did to accomplish some particular task.

Here is what I want to see in the summary file:

  1. Those parts of the answers to each exercise that I ask for.
  2. Explanations of what you did, in full sentences. There should be enough information here to repeat the experiment. Matlab files will help in this documentation.
  3. I would like to see a few lines expressing your opinion of the point of each exercise.
  4. Easily identified answers to exercises, including numerical values. I do not want to look in your diary file to try to figure out what you did. Do not write, ``see summary file for details.''
  5. What was the result of the experiment? Not just the numbers, but what the experiment told you.

Here is what I do NOT want to see in the summary file:

  1. Matlab error messages (unless I ask for them).
  2. False starts, mistyped commands, etc.
  3. Incorrect results that are corrected later.
  4. Duplications of anything, unless I explicitly ask for them.
  5. Large numbers of printed values, for example, the contents of a vector of length 100. I will not read all these numbers and they end up being like spam.

If you want to know how much detail to include, think of the following scenario. You have completed this course and, a year from now, a friend who is taking the course is having trouble. Your friend comes to you and asks how you did a particular exercise. You have saved your work, so you go look at it. The first place you will look is in your summary to see what you did. If the summary file contains only ``Exercise 1.a: complete,'' you will then have to go re-read the original lab and look for your script files, etc. Instead, the summary should describe what you did so you can explain it in general to your friend without referring to other materials. If your friend needs more detail, you can look at the other files you wrote for the lab.

Do not write a summary of the work in today's lab. Instead, please read the following information about Matlab commands from either the PC here in the lab or from another computer on the web.


More on Matlab

The Mathworks, maker of Matlab, includes a short tutorial on using Matlab called Getting Started This tutorial is also available from the Matlab command prompt with the command helpdesk and also from the Help menu, and, if you have your own copy of the Matlab manuals, it comprises the ``Getting Started'' book.

If you feel you need more explanation of programming techniques, you may find the following reference useful.

Charles F. Van Loan and K.-Y. Daisy Fan,
``Insight Through Computing, A MATLAB Introduction to Computational Science and Engineering,'' SIAM, 2010, ISBN: 978-0-898716-91-7

The beginning of the ``Getting Started'' tutorial is the best presentation of the general capabilities of Matlab that I have come across. In order to have an overview of Matlab, read through the first sections of the tutorial. There is only the equivalent of about 100 pages of material here, mostly very easy to understand. The most important chapters are

Begin the ``Getting Started'' tutorial now, during this lab session. Read as much of it as you can now, and complete it at home or from any convenient computer connected to the web. The list of topics that follows includes topics that will be necessary for this semester's labs. The second lab addresses some of these topics, but you will be well-served if you have seen the introduction.

  1. What is a matrix in Matlab? What is a vector? How are they different from plain numbers?
  2. How to you refer to the elements of a matrix or vector in Matlab?
  3. How to you use a colon to represent a range of numbers or subscripts?
  4. What symbols are associated with the elementary arithmetic operations? (Addition, multiplication, exponentiation, etc.)
  5. What meanings do the special constants pi, i, j, inf (infinity), and NaN (not-a-number) have?
  6. How do you use the ellipsis ... to put a single statement on more than one line?
  7. What are script m-files? What are function m-files? What is the difference?
  8. What does format long mean and what other options might be useful?
  9. How are the arrow keys used to edit commands in the command window?
  10. What is the form of the plot command?
  11. What are the hold on and hold off commands used for?
  12. How do you indicate matrix sum, product, inverse, and transpose?
  13. What do the element-by-element matrix operations such as product mean and how are they denoted?
  14. What is an if ... else ... end statement used for? Can you give an example?
  15. How do you use the for statement to write a loop?
  16. How is the break statement used in a loop?
  17. What is the difference between the eval and feval statements and what are they used for?

Back to MATH2070 page.



Mike Sussman 2012-08-12