[R] Conditional operation on data frame, shift/roll of vector

[David Winsemius]

On Sep 26, 2009, at 11:46 AM, jiangrm wrote:

> Conditionally, when Ind of a certain row is 1, want to get sum or  
> delta of Val in that row and 1 row
> above.
>
> Val	Ind                 Val	Ind  Del
> 10	0                    10	0    NA
> 11	0                    11	0    NA
> 13	1      ------->      13	1    24 or 2
> 16	0                    16	0    NA
>
> A simple way I guess is to get shifted vector of Val (say, c(NA, 10,  
> 11, 13)), add to or minus from
> Val, then and logically AND with Ind.

?diff

 > df1<-data.frame(Val=c(10,11,13,16), Ind=c(0,0,1,0))
 > c(NA, diff(df1$Val))[df1$Ind==1]
[1] 2

>
> Which function provides the shift operation of the vector Val?

?"["   # with a suitable index vector
?lag   # for time series

>
> Also welcomed if any better way to do this. Thanks.



-- 

David Winsemius, MD
Heritage Laboratories
West Hartford, CT