Definite integral over discrete data

Hi, I have a set of discrete data "A" over frequency "f". I want to integrate "A" over "f" from "f1" to "f2". I don't want to integrate over all frequency range. Using trapz(f,A) it gives me the integral over all frequencies... How can I do that? -Thank you

Answer 1

[Star Strider]

Without your data I can only outline the approach:

ix1 = find(f <= f1, 1, 'last');
ix2 = find(f >= f2, 1, 'first');
Alim = A(ix1:ix2);
flim = f(ix1:ix2);
int_f1_f1 = trapz(flim, Alim)

This finds the last index of ‘f’ less than or equal to ‘f1’ and the first index greater than or equal to ‘f2’. It then uses those indices to create ‘flim’ and ‘Alim’.

This is UNTESTED CODE but should work.

[Zoro]

It worked... Thanks!

[Star Strider]

My pleasure!

Answer 2

[Adam]

trapz( f( startIdx:endIdx ), A( startIdx:endIdx ) )

where startIdx and endIdx are the indices of the frequencies f1 and f2 which you can find by e.g.

startIdx = find( f >= f1, 1 );
endIdx = find( f >= f2, 1 );

if you don't already have them. I use >= there because exact equality tests are not stable with doubles, but you can change the test there to whatever you prefer.

[Adam]

How did my answer end up being marked as spam?!

[Star Strider]

You just lucked out!

This happens to us all from time to time, and requires someone else to rescue the post. There have been repeated requests that for those above a certain number of reputation points a post would never be flagged by the spam trap, but we’ve been unsuccessful in getting it implemented.

I’ll rescue it.

[Adam]

Thanks! I've never come across it before.

[Star Strider]

My pleasure! You will if you post often enough, although the spam trap has gotten progressively more sophisticated since the spam inundations in the Spring of 2014.

There should be a link on the spam notice you see that tells you to contact MathWorks if you believe the spam trap flagged it erroneously. I always click on it and include the URL of the post in my message, although I suspect several people with privileges to do so see the false positive and remove it, as I did with yours.