MATLAB: How to subtract elements in a matrix from each other

matrix subtraction

Hello there!
I want to find out how I can subtract the elements of a matrix from one another in a particular order. Lets say that I have the following matrix:
 matrix_1 = [30 39 42; 10 15 22; 1 5 7]
This matrix represents the running total counts of some function. To get the counts for each individual cell, I want to subtract the element in a cell from the one before it. In this case, the solution will do something like this:
 matrix_2 = [30-22 39-30 42-39; 10-7 15-10 22-15; 1 5-1 7-5]
in order to produce:
 matrix_3 = [8 9 12; 3 5 7; 1 4 2]
Does anyone know of a way I can automize this process so I don't need to calculate this by hand?
Thank you in advanced!
Sincerely,
Colin

Best Answer

With some juggling of matrices this can be done with just one line of code:
>> A = [30 39 42; 10 15 22; 1 5 7]
A =
   30   39   42
   10   15   22
    1    5    7
>> flipud(reshape([A(end,1),diff(reshape(flipud(A).',1,[]))],size(A,2),[]).')
ans =
   8   9   3
   3   5   7
   1   4   2
The required order of data is very unusual: the flipud, reshape, and transpose operations are just to get the data into a simpler order (i.e. the order of linear indexing), which then makes calculating the difference easy with just one diff call.
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