Passing column name and data frame to custom function in R - r

Question

I am trying to write a function in R that:
1) Receives a data frame and column name as parameters.
2) Performs an operation on the column in the data frame.
func <- function(col, df)
{
col = deparse(substitute(col))
print(paste("Levels: ", levels(df[[col]])))
}
func(Col1, DF)
func(Col2, DF)
mapply(func, colnames(DF)[1:2], DF)
Output
> func(Col1, DF)
[1] "Levels: GREEN" "Levels: YELLOW"
> func(Col2, DF)
[1] "Levels: 0.1" "Levels: 1"
> mapply(func, colnames(DF)[1:2], DF)
Error in `[[.default`(df, col) : subscript out of bounds

Answer 1

Two things :
in your function func, you apply deparse(substitute(col)) to an object col you expected is not a string. So it works with func(Col1, DF). But in your mapply() call, your argument colnames(...) is a string, so it create an error. Same error obtained with func('Col1', DF).
in a mapply() call, all arguments need to be a vector or a list. So you need to use list(df, df), or if you don't want to replicate, remove the argument df of your function func.
This is one alternative that should work:
func <- function(col, df)
{
print(paste("Levels: ", levels(df[,col])))
}
mapply(FUN = func, colnames(DF)[1:2], list(DF, DF))

Answer 2

Please have a look at the last comment of #demarsylvain - maybe a copy-paste error on your side, you should have done:
func <- function(col,df) {
print(paste("Levels: ", levels(df[,col])))
}
mapply(FUN = func, c('Species', 'Species'), list(iris, iris))
you did:
func <- function(col) {
print(paste("Levels: ", levels(df[,col])))
}
mapply(FUN = func, c('Species', 'Species'), list(iris, iris))
Please upvote and accept the solution of #demarsylvain, it works
EDIT to adress your comment:
To have a generic version for an arbitrary list of column names you can use this code, sorry for the loop :)
func <- function(col,df) {
print(paste("Levels: ", levels(df[,col])))
}
cnames = colnames(iris)
i <- 1
l = list()
while(i <= length(cnames)) {
l[[i]] <- iris
i <- i + 1
}
mapply(FUN = func, cnames, l)