I need to perform element by element subtractions within a matrix.

I have a 21x2 matrix for a given row say row 12, I need to calculate the errors by using the following numerical schemes; backwards - ((y(12)-y(11)/x(12)-x(11)), forwards - (y(13)-y(12))/(x(13)-x(12), and central (y(13)-y(11))/(x(12)-x(11)). My program needs to use matrix based calculations to solve for each element of the matrix. The following code works for the forward but I cant get the backwards or central working.

yn1 =y(2:length(y));
yn = y(1:length(y)-1);
xn1 =x(2:length(x));
xn = x(1:length(x)-1);

Thanks for the help

[Sean Flanagan]

[Sean Flanagan]

This is the series of equations I need to satisfy. The matlab script that I have posted solves the forward equation. But I am stuck on the backwards and central equation.

Answer 1

[Image Analyst]

Not sure what you mean by the 11, 12, and 13, but why not use the built in diff, conv, and, if you have to flip?

Give an short actual numerical example (inputs and outputs) if you still need help.

[Sean Flanagan]

The 11, 12 and 13 are the different rows required to calculate the values for row 12.

[Image Analyst]

Yes, it looks like diff() will do it. Have you tried it yet? If not, why not -- I already suggested it hours ago?

[Sean Flanagan]

Haha sorry Image Analyst,

School holidays and sick kids, I have only just gotten back on Matlab. I cant get diff() to work for the equations, diff(X), for a matrix X, is the matrix of row differences, [X(2:n,:) - X(1:n-1,:)]. This doesnt produce the answers I require.

[Steven Lord]

It's one of the building blocks you need.

x = (1:5).^2

x = 1×5

1 4 9 16 25

y = diff(x)

y = 1×4

3 5 7 9

check = [y(3), x(4)-x(3)] % should match

check = 1×2

7 7

Read carefully through the diff documentation page, paying specific attention to the "Approximate Derivatives with diff" example and the description of the dim input argument.

[Sean Flanagan]

Thanks for the help mate. Its really apreciated.