So far, everything we have learned how to do in MATLAB required typing in commands one by one on the command line. This is fine for testing things out, but, really, the whole point of computers is to make repetitive calculations quickly and accurately. As soon as you realize you want to run the same set of commands over again, it is time to write a script or a function so you can do it 100 times. In this lesson, we'll begin learning how to write functions and scripts. You want to be hard-working with the thinking, but lazy with the typing.

Learning Outcomes

By the end of this lesson, you should be able to:

• write a script to display a table of data.
• write a function with user inputs.
• write a script to make a plot.
• use MATLAB as a graphing calculator, also to display data.

Lesson Roadmap

To Read	content on this web site	here
To Do	Quiz	Canvas

Questions?

If you have questions, please feel free to post them to the Questions? discussion forum in Canvas. While you are there, feel free to post your own responses if you, too, are able to help a classmate.

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.

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: 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!

Click the + in MATLAB's editor window to open up a new blank file. Copy and paste this simple script into your editor. Save it, name it something, then run it by typing its name into the command window.

Explanation of the script

First I wrote a comment that says what the script does. Then I created a vector named x. Then I created two more vectors the same size as x that are functions of x. Next I used plot to plot both functions on the same axes. I set the axes limits, and then I made a legend to label both lines. legend is a new command. By default it correctly labels the plotting elements in the same order as you plot them in your script. Enclose any plain text in single quotes so MATLAB will render it as text. This works in the legend and also in axes labels and tick labels. Many special symbols such as Greek letters can be rendered by typing the word out with a backslash in front of it. The table below tells how to make some common special characters so they'll be rendered correctly on labels.

Creating special characters for plot labels in MATLAB

SYMBOL	AN EXAMPLE	HOW TO TYPE IT
lowercase Greek letter	α	'\alpha'
uppercase Greek letter	Γ	'\Gamma'
degree	40.8° N	'40.8\circ N'
superscript	x2, ∂O18	'x^2', '\deltaO^{18}'
subscript	H2SO4	'H_2SO_4'

Efficiency vs. Readability, take two

I could have saved a couple of lines in my script if I had just written: MATLAB can do calculations within plot. But, even though the second example is shorter, it's my personal preference to do calculations and plotting as separate parts of my scripts. (This is probably a holdover from growing up with programming languages like C and FORTRAN in which you would do all your calculations there, and then pipe the output to a separate plotting package.) Nevertheless, I think separating data manipulation from plotting is not such a bad habit, especially once you get to more complicated programs. When you are working through difficult concepts, it is easier to isolate and fix mistakes if the math and/or science analysis is not wrapped up inside the choice of how to display the output, in my mind.