Lets say I have a dataframe like this

A B

0 a b

1 c d

2 e f

3 g h

0,1,2,3 are times, a, c, e, g is one time series and b, d, f, h is another time series. I need to be able to add two columns to the orignal dataframe which is got by computing the differences of consecutive rows for certain columns.

So i need something like this

A B dA

0 a b (a-c)

1 c d (c-e)

2 e f (e-g)

3 g h Nan

I saw something called diff on the dataframe/series but that does it slightly differently as in first element will become Nan.