Lesson 3: Script that displays data

Write a simple script

A script, in any programming language, is a file containing a series of commands that are executed automatically in order as if you had typed them one by one. MATLAB  scripts end with the file extension ".m" In your MATLAB command screen there's a button that looks like a big yellow plus sign up in the toolbar labeled 'New.' If you click it, an Editor window will open up and your cursor will be in it, ready for you to type something.

You can also use the text editor of your choice to create a script. As long as the file you create has the right file extension and MATLAB knows where the file is located in your computer's file structure, you can run it. When I say "text editor of your choice" I do not mean a word processing program like Microsoft Word or Macintosh Pages. I mean a plain text editor like vi or emacs. If you don't know what I'm talking about, just use the built-in editor in MATLAB.

I wrote a short script to display a two-column table of data (below this paragraph). Copy and paste it into your editing window. Save the file. Name it sb.m. In order to run it, just type sb into the command line. Or you can hit the Run button in the MATLAB toolbar.

%display points scored by the first ten superbowl winners 
clearvars; 
yr=[1967 1968 1969 1970 1971 1972 1973 1974 1975 1976]; 
winnerPoints=[35 33 16 23 16 24 14 24 16 21]; 
tableYW(:,1)=yr'; 
tableYW(:,2)=winnerPoints'; 
disp('        YEAR       POINTS SCORED')
disp('                    BY WINNER')
disp(' ')
disp(tableYW) 

Hopefully when you ran it, the output looked like:

>> sb
        YEAR       POINTS SCORED
                    BY WINNER
 
        1967          35
        1968          33
        1969          16
        1970          23
        1971          16
        1972          24
        1973          14
        1974          24
        1975          16
        1976          21

New Stuff in this Script

Comments

The first line of the script begins with %. That's a comment. MATLAB ignores it but you shouldn't. Comments are some of the most important parts of a program, no matter what language you use. I rank comments right up there with help and lookfor in the MATLAB Pantheon of What's Useful. Comments are handy for saying in plain language what your code does, reminding yourself where you got the data, temporarily not running a certain problematic line of code while debugging, and more. Single line comments are preceded by %. If you want to write more than one line you can either put a % at the beginning of each line or enclose the whole block inside a %{ %} pair.

Clearing and displaying variables

The next line of my script says clearvars which tells MATLAB to clear all the variables from the workspace. I'm in the habit of putting clearvars at the top of my scripts so I don't have to worry about introducing some unexpected error by using a variable name that is already there. The next lines create row vectors to house the data. Then I turn the row vectors into column vectors and make an array out of them. I use disp to tell MATLAB to show me the data on the screen. Using disp is about the same as leaving off the ; at the end of the line except MATLAB doesn't echo back the variable name. You can also use disp as I did to display the actual text of what you typed by putting the text inside single quotes ' '.

Efficiency v. Clarity

In my script above I did a couple of things that expert programmers hate. First of all, I typed in my data by hand. This is not good practice because it takes time and is a really obvious way to introduce clerical errors. A better approach would be to find the data somewhere, download a file of it and read that file in. We'll learn how to do that soon, so no worries there. The second thing is that I created two vectors, then a created a whole 'nother array out of my two vectors, so essentially I'm duplicating effort there. I could have written the program like this instead and eliminated two lines of code:

%display points scored by the first ten superbowl winners 
clearvars;  
tableYW(:,1)=[1967 1968 1969 1970 1971 1972 1973 1974 1975 1976]'; 
tableYW(:,2)=[35 33 16 23 16 24 14 24 16 21]'; 
disp('        YEAR       POINTS SCORED')
disp('                    BY WINNER')
disp(' ')
disp(tableYW) 

Sure, this is better. From a computation time point of view, there's not much difference since the program is so short in the first place. The main thing for you to think about here is whether you, upon reading this program, would understand what I had done here. Can you picture in your head the all-in-one step creation of a column vector of data as part of a larger array? If you can't, then you might prefer my original version even though it is not as terse. Someone will yell at me for giving this advice but I'm going to say it anyway: A program that works is sometimes better than the most elegant program, especially when you are first learning.

Check yourself!