[R] Refer to previous row

[Duncan Murdoch]

On 07/01/2013 8:33 AM, Paolo Donatelli 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])
>
> but it is not so simple!

Negative indexing lets you leave out an entry, so x[-1] leaves out the 
first entry, and x[-length(x)] leaves out the last one.  To talk about 
previous entries, you need to do something about the fact that the first 
row has no previous entry.  You gave X3[1] the value 0, suggesting that 
you want to implicitly have the "zeroth" row to have the smallest 
possible value.  So

prevID <- c( -Inf, ID[-length(ID)] )
X3 <- as.numeric( ID <= prevID )

should do what you want.

Duncan Murdoch