Error with creating function in R - r

Question

I'm trying to create a function as I need to apply the same code multiple times to different columns in my data.
My data (df) looks like this:
WEEK1.x WEEK1.y WEEK2.x WEEK2.y WEEK3.x WEEK3.y
1 660.14 1 690.74 2 821.34 1
2 -482.89 99 -368.12 99 -368.12 99
3 284.48 3 399.90 1 375.32 1
4 -554.18 99 -300.28 99 -300.28 99
Then my function looks like:
extra<-function(first_var, second_var){
df$first_var=ifelse((df$first_var == 99),"99",
ifelse((df$first_var %in% c(1,2,3,4,5)),"1-5",NA))
output=as.data.frame(aggregate(second_var~first_var, data = df, mean))
return(output)
}
WEEK1<-extra("WEEK1.y", "WEEK1.x")
WEEK2<-extra("WEEK2.y", "WEEK2.y")
This then gives me the error:
Error in $<-.data.frame(*tmp*, first_var, value = logical(0)) :
replacement has 0 rows, data has 1416
When I press view traceback this is what it says:
stop(sprintf(ngettext(N, "replacement has %d row, data has %d",
"replacement has %d rows, data has %d"), N, nrows), domain = NA)
$<-.data.frame(*tmp*, first_var, value = logical(0))
$<-(*tmp*, first_var, value = logical(0))
extra("WEEK1.y", "WEEK1.x")
I'm not sure what the problem is?

Answer 1

Here is a working version of your function.
I have used a variant of the suggestion by #A.Suliman, but with [[.
extra <- function(first_var, second_var){
df[[first_var]] <- ifelse((df[[first_var]] == 99), "99",
ifelse((df[[first_var]] %in% c(1,2,3,4,5)), "1-5", NA))
fmla <- as.formula(paste(second_var, first_var, sep = "~"))
aggregate(fmla, data = df, mean, na.rm = TRUE)
}
WEEK1 <- extra("WEEK1.y", "WEEK1.x")
WEEK1
# WEEK1.y WEEK1.x
#1 1-5 472.310
#2 99 -518.535
WEEK2 <- extra("WEEK2.y", "WEEK2.x")
WEEK2
# WEEK2.y WEEK2.x
#1 1-5 545.32
#2 99 -334.20
Note that I would also suggest that you pass df as an argument to the function. It is generally considered bad practice to rely on objects existing elsewhere than in the function´s environment. In this case, df exists in .GlobalEnv and you are forcing R to leave th environment where it is needed to find it.
DATA.
df <- read.table(text = "
WEEK1.x WEEK1.y WEEK2.x WEEK2.y WEEK3.x WEEK3.y
1 660.14 1 690.74 2 821.34 1
2 -482.89 99 -368.12 99 -368.12 99
3 284.48 3 399.90 1 375.32 1
4 -554.18 99 -300.28 99 -300.28 99
", header = TRUE)