polyfit vs csapi revolve - can't square the integral on csapi

Hi all,

I'm trying to compare the volumes of two revolves around the x axis, defined by the same set of (x,y) coordinates. One is a polyfit approximation, which works very well. I'm now trying to do the same with a csapi spline that goes through the actual points, but can't seem to get the integral right. The polyfit works very well;

p=polyfit(x,y,10);
Vp=pi*(polyint(conv(p,p)));

But I can't get the csapi version to work at all;

c=csapi(x,y);
Vs=pi*diff(fnval(fnint(c)^2,[0 143]));

It just spits out errors all the time. Area integrals for both work just fine;

Ap=polyint(p);
As=fnint(c);

The error I get is

> In polyfit (line 84)
In Vase (line 8) 
Operator '.^' is not supported for operands of type 'struct'.

Error in  ^  (line 22)
    Z = X.^Y;

Error in Vase (line 15)
Vs=pi*diff(fnval(fnint(c)^2,[0 143]));    %Volume of Spline revolve

What am I doing wrong? How do I solve this?

Best of thanks inadvance,

Kai.

Answer 1

[Torsten]

Take a look here on how to to generate a spline function squared and integrate it:

https://de.mathworks.com/matlabcentral/answers/256891-integrate-a-spline-function-squared

[Kai]

Thank you very much - I guess I should have found that on my search but didn't,

I'll try to make this work with the csapi spline...

Thanks again,

Kai.