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.
采纳的回答
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 个评论
isamh
2020-2-10
Jim Riggs
2020-2-10
I am not sure what you are asking.
For a vector, it doesn't make any difference if it is a row vector or a column vector.
If there is more than one dimension, you will need to describe what it is - what are the dimensions, and what they represent, so I can understand what the differential is that you want.
First, don't post comments as an answer.
Simply preallocate Adi_1 to start with;
Adi_1 = nan(1,numel(Vdi_1);
Now the dimensions will match.
(Note that, when calculating the central difference, Adi_1(1) and Adi_1(end) are not defined)
Jim Riggs
2020-2-10
It looks to me that the only way that this could fail is if T is not a scalar.
T should be a scalar value. How is T defined?
in the second section you have defined
Ati_1 = nan(1,numel(Vdi_1));
This should be
Ati_1 = nan(1,numel(Vti_1));
isamh
2020-2-10
Jim Riggs
2020-2-10
This does not make sense.
Type the following command and post the result:
whos Vdi_1 Vti_1 T Adi_1 Ati_1
Jim Riggs
2020-2-10
Very good.
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