How do you know that "p" has a dimension of 4 columns for the code given under "Body" herein below?

sim('focbls',[0 2]); 
[p,f]=spectrum(posin,posfb,1024,0,[],1000); 
figure(1) 
clf; 
subplot(211); 
loglog(f,abs(p(:,4))),grid;

Answer 1

[Sean de Wolski]

You don't know what it has! It does not necessarily have four dimensions either, just four columns.

You can find out its size by asking:

size(p)

and if p does not have four columns the above would error.

p = magic(3);
p(:,4)