Using loop to create new variables

Question

My data frame is called Subs .

My variables are REV_4 , REV_5 , REV_6 etc

I want to create new variables to calculate percentage change of revenue.
Eg: d.rev.5 <- Subs$REV_5/Subs/$REV_4 -1

I would like to use a loop to create these new variables. I've tried this:

for(i in 5:10){
Subs$d.data.[i] <- Subs$REV_[i]/Subs$REV_[i-1] - 1 }

But it doesn't work . I suspect it's not recognizing the i as part of the variable name.

Is there any way to get around this? Thank you so much.

Answer 1

You can't reference columns like you're attempting ( Subs$REV_[i] ), you need to create a string to represent the column.

What I think you're trying to do is (in the absense of your data I've created my own)

set.seed(123)
Subs <- data.frame(rev_1 = rnorm(10, 0, 1),
                   rev_2 = rnorm(10, 0, 1),
                   rev_3 = rnorm(10, 0, 1),
                   rev_4 = rnorm(10, 0, 1))

for(i in 2:4){
  ## looping over columns 2-4
  col1 <- paste0("rev_", i)
  col2 <- paste0("rev_", i - 1)

  col_new <- paste0("d.rev.", i)

  Subs[, col_new] <- Subs[, col1] / Subs[, col2]

}

---

## A note on subsetting a data.frame
Subs$rev_1   ## works
i <- 1
Subs$rev_[i] ## doesn't work
Subs[, rev_[i]] ## doesn't work
Subs[, "rev_1"] ## works
Subs[, paste0("rev_", i)] ## works
## because
paste0("rev_", i)  ## creates the string:
[1] "rev_1"