trying to calculate the central diff approximation

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i tried multiple ways but none of them worked. kind of stuck and not sure what to do. what i'm trying to do is find current value that is equal to one ahead minus one below all divided by .2
code is:
Phase_1 = DATA(DATA(:,4)==1,:);
Vroll_avg1_1 = movsum((1/5).*Phase_1(:,3),[2 2]);
Vdi_1 = movsum(1/5.*Vroll_avg1_1,[2 2]);
---> first try: Adi_1 = [Vdi_1(2:Vdi_1+1,:)-Vdi_1(1:Vdi_1-1,:)/(2*T)];
---> second try: Adi_1 = ((Vdi_1(2:end+1) - Vdi_1(1:end-1))/(2*T)); % error says index exceeds array bounds. i understand why but how((2:end+1)) would i get it to work?
---> third try: %K = length(Vdi_1);
%Q = length(Vti_1);
%for M = 2:(length(Vdi_1)-1)
% Adi_1 = ((Vdi_1 - Vdi_1)./(2.*T));
%end
%Adi_1 = (Vdi_1(2) - Vdi_1(1))./(2.*T);
%Adi_1(length(Vdi_1)) = (Vdi_1(K) - Vdi_1(K))./(2.*T);
really need help, please try to help me figure this out.

Accepted Answer

Jim Riggs
Jim Riggs on 10 Feb 2020
Edited: Jim Riggs on 10 Feb 2020
There is a Matlab function "diff" which will do this.
Otherwise, your subscripts must all match (2:end-1)
Adi_1 = diff(Vdi_1);
or
for i=2:numel(Vdi_1)-1
Adi_1(i) = (Vdi_1(i+1) - Vdi_1(i-1))/2/T;
end
  11 Comments
isamh
isamh on 10 Feb 2020
Edited: isamh on 10 Feb 2020
im so sorry jim, just got it to work. forsome reason i had one of the numel as nume1 with the #1. got it to work, thanks!

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