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

[David Winsemius]

On Sep 26, 2009, at 12:52 PM, David Winsemius wrote:

>
> 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
>
I suppose I ought to answer the question more fully. One approach  
using indexing, is to use the logical vector produced by  df1$Ind==1  
on both sides of an assignment operation at once to determine which of  
the values of hte above set of differences get transfered:

 > df1$Del[df1$Ind==1] <- c(NA, diff(df1$Val))[df1$Ind==1]
 > df1
   Val Ind Del
1  10   0  NA
2  11   0  NA
3  13   1   2
4  16   0  NA

ifelse might also provide a solution. Something along the lines of:

df1$Del3 <- ifelse(df1$Ind ==1, c(NA, df1$Val[2:nrow(df1)]-df1$Val[1: 
(nrow(df1)-1)]), NA)

But that seems so Baroque that I think you will agree that the  
indexing method is preferable in this question.

>>
>> 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
>
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David Winsemius, MD
Heritage Laboratories
West Hartford, CT