difference of datetimes using the "diff" function

I would like to find the temporal difference between the elements in the first (left) column and the elements in the second (right) column. I have tried the diff function, but it gives me an unexpected result. How can I still use the diff function correctly ?

% input
a = datetime({'2022-05-17 05:57:11.568'  '2022-05-17 06:01:23.552'
   '2022-05-17 06:01:23.552'   '2022-05-17 06:02:32.760'
   '2022-05-17 06:02:32.760'   '2022-05-17 06:03:57.660'
   '2022-05-17 06:03:57.660'   '2022-05-17 06:19:28.740'
   '2022-05-17 06:19:28.740'   '2022-05-17 06:20:50.100'
   '2022-05-17 06:20:50.100'   '2022-05-17 06:21:49.408'
   '2022-05-17 06:21:49.408'   '2022-05-17 06:23:15.980'
   '2022-05-17 06:23:15.980'   '2022-05-17 06:24:22.200'})

a = 8×2 datetime array
   17-May-2022 05:57:11   17-May-2022 06:01:23
   17-May-2022 06:01:23   17-May-2022 06:02:32
   17-May-2022 06:02:32   17-May-2022 06:03:57
   17-May-2022 06:03:57   17-May-2022 06:19:28
   17-May-2022 06:19:28   17-May-2022 06:20:50
   17-May-2022 06:20:50   17-May-2022 06:21:49
   17-May-2022 06:21:49   17-May-2022 06:23:15
   17-May-2022 06:23:15   17-May-2022 06:24:22

% If I use "diff(a)", why do I get two columns of "differences", instead of only one ?
diff(a)

ans = 7×2 duration array
   00:04:11   00:01:09
   00:01:09   00:01:24
   00:01:24   00:15:31
   00:15:31   00:01:21
   00:01:21   00:00:59
   00:00:59   00:01:26
   00:01:26   00:01:06

% the following command looks like to give the correct result (but I would like to use "diff" if possible)
abs(a(:,1)-a(:,2))

ans = 8×1 duration array
   00:04:11
   00:01:09
   00:01:24
   00:15:31
   00:01:21
   00:00:59
   00:01:26
   00:01:06

[Stephen23]

"why do I get two columns of "differences", instead of only one ?"

The DIFF documentation explains this. The documentation states "diff(X) calculates differences between adjacent elements of X along the first array dimension whose size does not equal 1..." and it then procedes to give a detailed explanation of the returned sizes for vectors and matrices: "If X is a nonempty, nonvector p-by-m matrix, then Y = diff(X) returns a matrix of size (p-1)-by-m, whose elements are the differences between the rows of X"

So because you did not specify the dimension argument and the first dimension of your matrix is non-scalar then DIFF operates along the first dimension. So far everything you show is exactly as documented and expected.

[Sim]

ah ok, thanks a lot for your explanation! :-)

Answer 1

[Sim]

ah.... probably this one ?

>> diff(a,1,2)

ans = 

  8×1 duration array

   00:04:11
   00:01:09
   00:01:24
   00:15:31
   00:01:21
   00:00:59
   00:01:26
   00:01:06