Row subtraction in matrix

I am trying to subtract each row in a matrix from the row after it. The function diff does x_{i} - x_{i+1}, I need to do x_{i}-x_{i-1}. I cannot find anything that will tell me how to do this. I am sure it is something simple that I am just overlooking.

[James Tursa]

No. diff() does row{i}-row{i-1} just like you apparently want.

[Stephen23]

The diff help states that for a matrix it returns

[X(2,:)-X(1,:); X(3,:)-X(2,:); ... X(p,:)-X(p-1,:)]

which seems to match what you asked for.

[Ian Loveless]

Then I mistyped. Is there a way I can do row{i}-row{i+1}?

Answer 1

[James Tursa]

Just negate the usual diff() result. E.g.,

x = your matrix
result = -diff(x);

[Ian Loveless]

That just gives me the negative of diff(), it does not subtract in the opposite direction.

[James Tursa]

Really? Have you tried it?

[Ian Loveless]

Yes I did. It is the same as my diff() matrix, except each value has "-" in front of it.

[James Tursa]

So, there appears to be a disconnect of some sort with your description and what you want. Consider this:

>> x = randi(10,3,4)
x =
     8     1     7     4
    10     9     8     7
     7    10     8     2
>> -diff(x)
ans =
    -2    -8    -1    -3
     3    -1     0     5

It certainly looks to me like the first row of the result is x_row1-x_row2 etc. Similarly for the second row of the result. So for the x shown above, what is the result that you want exactly if it isn't what I have shown?

[Stephen23]

"It is the same as my diff() matrix, except each value has "-" in front of it."

That would be exactly correct then. The only difference between these

row(i)-row(i+1)
row(i)-row(i-1)

will be the sign. So if all were positive in the first case, then in the second case all would be negative.

[Ian Loveless]

You are all correct, I apologize for my ignorance. I was looking at spacings instead of differences. Thank you for your help and patience.