Plotting Ellipse in Matlab

Question

In addition to my R programming course, I am also taking an intro to LaTeX course. We have a new assignment which is to generate a graph of a given function (in Matlab), save it and then recall it in LaTeX code as though we're making a publication. My Matlab is a little rusty, and I don't think that I quite got the code right. Furthermore it doesn't look anything like the example we were given for reference. The function that we were asked to plot is the following: 1 = x^(2)/9 - y^(2)/4

What my image is supposed to look like:

What my Matlab code generated:

function graph_1

clc; clear all; 

a = 9;
b = 4;
x0 = 0;
y0 = 0;
t = -pi:0.1:pi;
x = 1 - x0 + a*cos(t);
y = 1 - y0 + a*sin(t);

figure(1); clf; 
    
plot(x,y,'b','linewidth',1)
set(gca, 'fontsize', 12)
xlabel('X')
ylabel('Y')
grid on

end

Once I get the code to run in Matlab, I get a weird looking ellipse and it doesn't look "pretty" like the one my professors gave us as reference which I have attached. The full assignment is asking us to generate a plot with our programming language of choice, save it, and then have the LaTeX code actually recall the file and then insert into a pdf. How would I export this figure to somehwere else in my computer?

(1) Your ellipse parameters are wrong, hence the size doesn't match; you also don't use b in your computation. (2) Your t doesn't end with the same angle it starts with, hence the gap; use linspace instead. (3) Add axis equal after plotting so that the aspect ratio is correct. (4) Use print to export as PDF (check the docs). (5) Don't ever use clear all, but especially not inside a function! clear by itself clears all variables, but at the start of your function you don't have any variables to clear. — Cris Luengo
@CrisLuengo, I just noticed that I never used b in my code. Could you please explain what you meant by me using the wrong parameters for my ellipse? — Mehmet
@CrisLuengo, also what would I use for my arguments in the linspace function? — Mehmet
You can type help linspace to get documentation, or doc linspace to get more elaborate documentation. Or just look online: mathworks.com/help/matlab/ref/linspace.html — Cris Luengo

Mehmet Mehmet · Accepted Answer · 2020-11-05T20:07:43

Originally, I thought I needed to plot an ellipse, but found that I actually needed to plot a hyperbola. At any rate, I figured out how to plot both functions, because their format is nearly identical, and have included the code for the ellipse. In my experience, the best way to plot these functions was to use the fimplicit function in Matlab. To fully understand how it works, I strongly suggest typing help fimplicit into the command window. From there, you can see the documentation and examples of how to plot functions like these.

This is the code I ran in Matlab to make things work correctly.

function graph_1

clc; clear; 

f1 = @(x,y) x.^(2)/9 + y.^(2)/4 - 1;

fimplicit(f1, 'b');
grid on;
axis equal;

print(figure(1), '-dpng') %% Saves a picture of the image in png format. 

end

And then this is the ellipse I generated:

Plotting Ellipse in Matlab

1 Answers