trying to calculate the central diff approximation

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.

Answer 1

[Jim Riggs]

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

[isamh]

thanks, the answer is in 1 row instead of 1 column, how would i fix that?

[Jim Riggs]

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.

[Jim Riggs]

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)

[isamh]

ok, so i tried that but got an error saying "matrix dimensions must agree" i have two variables Adi_1 and Ati_1 which is the same but use different values. tried to use the samething on Ati_1 but the dimensions arent the same. also, randomly get a error message saying nume1 is a undefined variable or function.

code is:

tic

Adi_1 = nan(1,numel(Vdi_1);

for i=2:numel(Vdi_1)-1

Adi_1(i) = (Vdi_1(i+1) - Vdi_1(i-1))/2/T;

end

toc

tic

Ati_1 = nan(1,numel(Vti_1);

for j=2:numel(Vti_1)-1

Ati_1(j) = (Vti_1(j+1) - Vti_1(j-1))/2/T;

end

toc

[Jim Riggs]

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?

[isamh]

T = .1;

but Vdi_1, Vti_1 are matrix columns.

[Jim Riggs]

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]

yeah, that was on accident. it was correct when i ran it but accidentally typed that wrong. still got the same message though.

[Jim Riggs]

This does not make sense.

Type the following command and post the result:

whos Vdi_1 Vti_1 T Adi_1 Ati_1

[isamh]

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!

[Jim Riggs]

Very good.