looping through columns in the same nxm matrix

hey can anybody tell me how to loop through the columns of the same matrix? I got stuck at

    % code
    for i=1:length(diff12)-1
    Phint(i) = diff12(i+1)-diff12(i);
  end

but it only does it for the first column. How do i get it to do it for all the columns in diff12?

[Kye Taylor]

What is output from the command

whos diff12

[kwadwo]

whos diff12

Name          Size             Bytes  Class     Attributes

diff12      912x35            255360  double

but the code is supposed to take the difference as: (row i+1 - row i) continuously from the same j column. then repeat this arimethics for the next j+1 column.

I thought that would be the case with the next code

if true
   for i=1:length(diff12)-1
  for h=1:35
      diff12(h)
  Phint(i,h) = diff12(i+1,h)-diff12(i,h);
  end

end end

But it returns a 912 by 912 matrix, while its supposed to be a 911 by 35 matrix