vector subtraction

I have a set of data stored in (kk): kk =

Columns 1 through 8

         0   -0.1128   -0.4684   -1.1005   -2.0583   -3.4158   -5.2942   -7.9141

Columns 9 through 16

-11.7597  -18.3555  -56.2068  -19.9206  -15.3116  -13.5578  -13.3223  -14.342

then i used the (diff)command to perform the kk(1)-kk(2) operation. i need to do this until the end: [kk(2)-kk(3)],[kk(3)-kk(4)]..till kk(16).

this is what i get when used the (diff) command:

q =

Columns 1 through 8

   -0.1128   -0.3556   -0.6322   -0.9578   -1.3576   -1.8784   -2.6199   -3.8456

Columns 9 through 16

   -6.5959  -37.8513   36.2863    4.6090    1.7538    0.2355   -1.0200   -2.4722

for q(1) i suppose to get 0.1128, but im getting a negative value. how can i rectify this problem?

Answer 1

[Sean de Wolski]

Because diff does q(2)-q(1),q(n)-q(n-1).

doc diff

for more info. To correct it, just take the negative.

dqf = -diff(q);