Alternating equations every other row

I have A= [3 2 1, 3 8 2, 3 9 2, 9 5 1, 3 8 3, 7 4 2]

For the first row I would like to subtraction Column 2 from Column 1, then for row I want to subtract Column 1 from Column 2. I want these two equations to alternate when going through the rows. In the end I want something that looks like this:

B= [1, 5, -6, -4, -5, 3]

Thanks!

[Azzi Abdelmalek]

There is one row in A, maybe you mean

A=[3 2 1; 3 8 2; 3 9 2; 9 5 1; 3 8 3; 7 4 2]

You should explain how you get B. what is row I?

Answer 1

[Andrei Bobrov]

B = diff(A(:,[1,2]),1,2).*(-1).^(1:size(A,1))'

Answer 2

[Azzi Abdelmalek]

A= [3 2 1;3 8 2; 3 9 2;9 5 1; 3 8 3; 7 4 2]
sg=ones(size(A,1),1)
sg(2:2:end)=-1
(A(:,1)-A(:,2)).*sg