matrix

I need to calculate the difference between values in a matrix without using the diff command. Is this done with a loop? I've tried:

for i=1:size(rho,2);
dz(:,i)=rho(:,i+1)-rho(:,i);
end

But obviously this wont work as the loop tries to continue beyond the the size of the matrix, how can this be done?

cheers

Answer 1

[Andrei Bobrov]

please read doc diff

e.g.

rho = randi(124,6)
dz = diff(rho,1,2) % in your case

[ricco]

diff works fine in obtaining what I need its just that the differences calculated are then used in an equation which includes the original values and the difference between the values. Where as the matrices are now of different dimensions they cannot be multiplied together. Is there a way of getting the matrices the same size? or is this just a case of adding an empty matrix in the begining to make up for it?

thanks

[Walter Roberson]

We cannot really advise you how to proceed without knowing more about the mathematics involved.

Specifying that you want differences between adjacent elements is inherently going to get you a smaller matrix, unless you define the difference carefully. For example, you can get an equal sized matrix if you define the difference as:

- absolute value of the forward difference for the left edge

- absolute value of the backward difference for the right edge

- maximum (or minimum) of the absolute value of the forward and backward difference for any interior point

But of course whether that makes any sense for what you are doing, we do not know.

If you tack on an extra row than you end up with a distortion along that edge. That could be important. Or not. We don't know.