Matlab Second Derivative Ploting Problem

[Zaragoza_S]

Question: How can I plot the derivative of a function?

Following code for problem 5.9 in MatLab by Gilat 4th Ed:

t=[0:1:20];
theta=pi*(1-exp(-0.2*t));
r=20+30*(1-exp(-0.1*t));
v=diff(theta)
table=[t' theta' r']
polar(theta,r)
polar(theta,v)

Error message:

? Error using ==> polar at 61
THETA and RHO must be the same size.

Error in ==> Prob5_16 at 12
polar(theta,v)


I have looked around for what may be the larger issue and it seems that when you use the "diff" function it shortens one dimension of the array (https://www.physicsforums.com/showthread.php?t=55524)

Any help please? The link provides "an answer" but i don't understand what he means.

 

[NateD]

Answer: To plot the derivative of a function, you need to make sure that the arrays used for the theta and v values have the same size. The diff function shortens one dimension of the array, so you will need to add in a zero value to make them equal. You can do this by adding the following line of code before the polar command:v = [0, v];This will add a zero at the start of the v array, making it the same length as the theta array. After doing this, the polar command should work correctly.