[datatable-help] Efficiency help - create variable conditional on row-specific column values

[CCRyder]

Each row of the table has 15 columns with "ID" and 15 corresponding "ID
Indicator" columns.  These are alternating in order (i.e., ID1 / IDind1 /
ID2 / IDind2 /....).  The ID Indicator variable specifies whether the
corresponding ID is the "primary ID" for each row/observation (there is only
1 primary ID for each).  

My goal is to create a new variable that equals the Primary ID for each row.

I'm doing this now using for loops (see code below).  This works, but I am
certain there is a much more efficient way to accomplish the task.

/*RData$PrimaryID <- NA_character_
for(i in seq(9:37, by = 2)) {
     for(j in 1:nrow(RData)) {
          if(!(is.na(RData[j:i])) & (RData[j,i] == 'Y')) RData[j, 38] <-
NPI[j, (i-1)]
     }
}*/

/NOTE:  The ID and IDind variables are alternative in positions 9-37 on the
dataframe.  The 38th columns is the newly created "PrimaryID"./

I am thinking the better way to do this is with apply/sapply/lapply, or by
vectorizing the work somehow.  I haven't been able to get anything working,
though.  Any help, tips, or guidance would be very much appreciated!



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