Lesson 3: Script that makes a plot

Syntax introduced:

legend

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.

% this is a test script. it makes a plot
x=0:.1:2*pi;
y=sin(x);
z=sin(2*x);
plot(x,y,x,z);
axis([0 2*pi -1 1]);
legend('sin(\theta)','sin(2\theta)');

plot of code snippet above. Two sin functions on the same axes.

Figure generated by script above.

E. Richardson, created with MATLAB

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:

% this is a test script. it makes a plot
x=0:.1:2*pi;
plot(x,sin(x),x,sin(2*x));
axis([0 2*pi -1 1]);
legend('sin(\theta)','sin(2\theta)');

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.