MATLAB: Difference

diff()MATLABvectors

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?

Best Answer

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
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