Multiply many columns by a specific other column in R with data.table?

Question

I have a large data.table in R with several columns with dollar values. In a different column I have an inflation adjustment number. I am trying to figure out how to update

Answer 1

You could try

DT[, (inc_cols) := lapply(.SD, function(x) 
        x * DT[['deflator']] ), .SDcols = inc_cols]
head(DT1,2)
#   id year         inc1         inc2       inc3   deflator
#1:  1    3  0.614838304  0.009796974  0.3236051  0.7735552
#2:  2    2 -0.001583579 -0.082289606 -0.1365115 -0.6644330

Or if you need a loop

for(inc in inc_cols){
  nm1 <- as.symbol(inc)
  DT[,(inc):= eval(nm1)*deflator]
}

 head(DT,2)
 #  id year         inc1         inc2       inc3   deflator
 #1:  1    3  0.614838304  0.009796974  0.3236051  0.7735552
 #2:  2    2 -0.001583579 -0.082289606 -0.1365115 -0.6644330

Or a possible option using set which should be very fast as the overhead of [.data.table is avoided (suggested by @Arun)

indx <- grep('inc', colnames(DT))

for(j in indx){
 set(DT, i=NULL, j=j, value=DT[[j]]*DT[['deflator']])
}
head(DT,2)
#  id year         inc1         inc2       inc3   deflator
#1:  1    3  0.614838304  0.009796974  0.3236051  0.7735552
#2:  2    2 -0.001583579 -0.082289606 -0.1365115 -0.6644330

where

inc_cols <-  grep('^inc', colnames(DT), value=TRUE)

data

set.seed(24)
DT <- data.table(id=1:1000,year=round(runif(1000)*10), 
      inc1 = runif(1000), inc2 = runif(1000), inc3 = runif(1000),    
      deflator = rnorm(1000))