What is the difference between ifft(f) and ifft(f,n)?

Hi. I have one vector f with some values of velocity in function of the frequency. I have to plot this velocities in function of the time. I don't know if I have to use ifft(f) or ifft(f,n). In this last case, what is n?

Thanks.

[Daniel Shub]

I am not a huge fan of RTFM answers, but in this case ... what does help ifft or doc ifft say about what n is?

Answer 1

[John Petersen]

if N =length(f), there is no difference between ifft(f) and ifft(f,N)

[Marcia Azeredo]

But, if N is not equal length(f)? I think that N is the number of points that i'd like, or, in this case, N is length of the vector that contain the values of the times that i'd like to plot, but i'm not sure.

[John Petersen]

You can zero pad f so that it has the N samples. e.g. Say f is a 256 element column array and N = 400.

f1 = [f; zeros(400-256,1)];
iF = ifft(f);
iF1 = ifft(f1,N);
figure;plot(iF1);
hold on; plot(iF,'r')

Now you can see the difference, just zero padded at the end. Is this what you wanted?

Does the time vector correspond to the sampling points of the signal f?