R - dynamically create columns using existing column names in sequence - r

Question

I have a dataframe, df, with several columns in it. I would like to create a function to create new columns dynamically using existing column names. Part of it is using the last four characters of an existing column name. For example, I would like to create a variable names df$rev_2002 like so:
df$rev_2002 <- df$avg_2002 * df$quantity
The problem is I would like to be able to run the function every time a new column (say, df$avg_2003) is appended to the dataframe.
To this end, I used the following function to extract the last 4 characters of the df$avg_2002 variable:
substRight <- function (x,n) {
substr(x, nchar(x)-n+1, nchar(x))
}
I tried putting together another function to create the columns:
revved <- function(x, y, z){
z = x * y
names(z) <- paste('revenue', substRight(x,4), sep = "_")
return x
}
But when I try it on actual data I don't get new columns in my df. The desired result is a series of variables in my df such as:
df$rev_2002, df$rev_2003...df$rev_2020 or whatever is the largest value of the last four characters of the x variable (df$avg_2002 in example above).
Any help or advice would be truly appreciated. I'm really in the woods here.

Answer 1

dat <- data.frame(id = 1:2, quantity = 3:4, avg_2002 = 5:6, avg_2003 = 7:8, avg_2020 = 9:10)
func <- function(dat, overwrite = FALSE) {
nms <- grep("avg_[0-9]+$", names(dat), value = TRUE)
revnms <- gsub("avg_", "rev_", nms)
if (!overwrite) revnms <- setdiff(revnms, names(dat))
dat[,revnms] <- lapply(dat[,nms], `*`, dat$quantity)
dat
}
func(dat)
# id quantity avg_2002 avg_2003 avg_2020 rev_2002 rev_2003 rev_2020
# 1 1 3 5 7 9 15 21 27
# 2 2 4 6 8 10 24 32 40