Difference

I have vector A =

Columns 1 through 5

         0         0         0         0         0
         0         0         0         0         0
         0         0         0         0         0

Columns 6 through 10

         0         0         0         0         0
         0         0         0         0         0
         0         0         0         0         0

Columns 11 through 15

    1.4602    1.4606    1.4609    1.4612    1.4615
         0         0         0         0         0
         0         0         0         0         0

Columns 16 through 20

    1.4619    1.4622    1.4625    1.4629    1.4632
         0         0         0         0         0
         0         0         0         0         0

Column 21

    1.4635
         0
         0

Why does

diff(A) =

ans =

Columns 1 through 5

         0         0         0         0         0
         0         0         0         0         0

Columns 6 through 10

         0         0         0         0         0
         0         0         0         0         0

Columns 11 through 15

   -1.4602   -1.4606   -1.4609   -1.4612   -1.4615
         0         0         0         0         0

Columns 16 through 20

   -1.4619   -1.4622   -1.4625   -1.4629   -1.4632
         0         0         0         0         0

Column 21

   -1.4635
         0

diff(A(1,:)) =

ans =

Columns 1 through 5

         0         0         0         0         0

Columns 6 through 10

         0         0         0         0    1.4602

Columns 11 through 15

    0.0003    0.0003    0.0003    0.0003    0.0003

Columns 16 through 20

    0.0003    0.0003    0.0003    0.0003    0.0003

give different result?

[Andrei Bobrov]

doc diff

[Lizan]

I want to take the difference row-wise. Which one is correct?

[Andrei Bobrov]

diff(A,[],1)) % difference row-wise

[Daniel Shub]

of course it did because you are not reading the documentation of diff. The [] is important. The first argument to diff is the matrix, the second is the order of the difference. You want this to be 1, which is the default so [] works or you could use 1. The third argument is the dimension over which to take the difference. Since you want the difference across rows, you want 2. Grab a pencil and paper and see which one you want.

Answer 1

[Daniel Shub]

If I remember correctly, diff works across the first non-singleton dimension. So diff(A) works across columns and diff(A(1, :)) works across rows.

x = [1,2,3; 4,5,6];
diff(x, [], 1)
ans =
   3     3     3
diff(x, [], 2)
ans =
   1     1
   1     1

Answer 2

[Lizan]

I want the difference per row, row one represents one solution and the second row represents another type of solution.

Which of the above will give me the correct solution for my case?