diff gives back one less valor

So I have this set of data, it's humidity vs time when drying food. I need the speed of drying so what do I do? I use cftool to find best fit of data then I tag the fit and export to workspace.

I have:

time=[1;2;3;4;5;6] x=[0.5;0.4;0.3;0.25;0.22;0.18]

I use cftool to fit it and export to worskpace

Then i use;

Rc=diff(finame(time))

Rc stands for speed in water removal (drying) and it gives back a vector with only 5 numbers, when it should give me a vector with 6 elements, each one the result of the derivate for each time...yet, as i said it doesn't... why is this?

Thanks a lot for the answers.

Answer 1

[Sean de Wolski]

From the doc for diff

If X is a vector of length m, then Y = diff(X) returns a vector of length m-1. The elements of Y are the differences between adjacent elements of X.
Y = [X(2)-X(1) X(3)-X(2) ... X(m)-X(m-1)]

Thus it does the backward differences requiring one element shorter. IF you want the vector to be the same length, pad it with a 0.

There's also this file that might interest you:

http://www.mathworks.com/matlabcentral/fileexchange/28920-derivative

[Thomas]

do not pad with a 0, this would alter your result. Rather pad 0.6 at the front or 0.14 at the end.

[Juan Carlos]

Thanks! :)

[Sean de Wolski]

@Thomas, padding the results with a zero would not alter the results, just shift them.

Answer 2

[Thomas]

It cannot give 6 elements since it calculates differences between successive elements and for 6 input values there are only differences e.g. assuming time=1:6 you have

INPUT : x=[ a b c d e f] (count 6) OUTPUT: differences used are b-a, c-b, d-c, e-d, f-e (count 5)

If you really need 6 values you could extrapolate your data boundaries and use a central differencing scheme.

[Juan Carlos]

Yes! I thought i was something like that, i'll extrapolate. THANKS!