[R] Refer to previous row

[Berend Hasselman]

On 07-01-2013, at 14:33, Paolo Donatelli <donatellipaolo at gmail.com> wrote:

> 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])

Assuming your data are in a data.frame called dat this will work

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

Berend