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.

why do you want to use a loop? Subs$d.rev.5 <- Subs$REV_5 / (Subs$REV_4 - 1) should do. — SymbolixAU
– SymbolixAU, Commented Feb 28, 2016 at 6:57
Oh I guess I didn't explain myself properly. I have about 40 different revenue variables, i.e. REV_4, REV_5.......REV_39, REV_40 etc. and I was hoping to find a faster way to create a new variable to calculate their percentage change. — jtan8999
– jtan8999, Commented Feb 28, 2016 at 7:12
You should update your question with the output of dput(head(Subs)) so others can see the structure of the data you're working with, and can therefore give you a solution — SymbolixAU
– SymbolixAU, Commented Feb 28, 2016 at 7:13

SymbolixAU · Accepted Answer · 2016-02-28 07:27:53Z

2

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"

edited Feb 28, 2016 at 7:27

answered Feb 28, 2016 at 7:22

SymbolixAU

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3 Comments

user5947301 Over a year ago

why one should use Subs[, paste0("rev_", 2)] ? cannot you use Subs[, 2] or Subs[,i]

jtan8999 Over a year ago

That was exactly what I needed. Thank you so much!

SymbolixAU Over a year ago

@Learner you're right, you can, I just assumed they wanted to reference the column by name

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