Refer to previous row

x <- read.table(text = 'ID  X1  X2

x$X3 <- c(0, diff(x$ID) <= 0)
x

Hi all,

I have a very basic doubt -- but still, I am a newby!

My question is about referring to the previous row: in a sample as the
following...

ID  X1  X2
1   A   12
2   A   6
3   A   10
1   B   17
2   B   19
1   C   22
1   D   13
2   D   19
3   D   21

... I would like to create a dummy variable equal to 1 whenever the
value of ID of the current row is lower or equal than the value of ID
of the previous row -- check the new vector X3 I'd like to obtain:

ID  X1  X2  X3
1   A   12  0
2   A   6    0
3   A   10  0
1   B   17  1
2   B   19  0
1   C   22  1
1   D   13  1
2   D   19  0
3   D   21  0

I have searched a lot without finding a decent and working solution. I
suppose it is just some basic matter of indexing language, something
like

X3<- as.numeric ( ID[n] <= ID[n-1])

but it is not so simple!


thanks!
Paolo