Function to subset dataframe on optional arguments - r

Question

I have a dataframe as follows:
df1 <- data.frame(
Country = c("France", "England", "India", "America", "England"),
City = c("Paris", "London", "Mumbai", "Los Angeles", "London"),
Order_No = c("1", "2", "3", "4", "5"),
delivered = c("Yes", "no", "Yes", "No", "yes"),
stringsAsFactors = FALSE
)
and multiple other columns as well (around 50)
I want to write a function which is generic and can take in as many parameters as the user wants and return a subset of only those specific columns. So the user should technically be able to pass 1 column or 30 columns to get the result back from function
With what I was able to find online on optional arguments, I wrote this following code but I am running into issues. Can anyone help me out here?
SubsetFunction <- function(inputdf, ...)
{
params <- vector(...)
subset.df <- subset(inputdf, select = params)
return(subset.df)
}
This is the error I am getting -
error in vector(...) :
vector: cannot make a vector of mode 'Country'.

Answer 1

The use of vector(...) is making problems here. The ellipsis has to be converted into a list instead. Therefore, in order to finally obtain a vector out of the three-dot parameters, the seemingly awkward construction unlist(list(...)) should be used instead of vector(...):
SubsetFunction <- function(inputdf, ...){
params <- unlist(list(...))
subset.df <- subset(inputdf, select=params)
return(subset.df)
}
This allows to call the function SubsetFunction() with an arbitrary number of parameters:
> SubsetFunction(df1, "City")
# City
#1 Paris
#2 London
#3 Mumbai
#4 Los Angeles
#5 London
> SubsetFunction (df1, "City", "delivered")
# City delivered
#1 Paris Yes
#2 London no
#3 Mumbai Yes
#4 Los Angeles No
#5 London yes

Answer 2

We can use missing function here to check if arguments are present or not
select_cols <- function(df, cols) {
if(missing(cols))
df
else
df[cols]
}
select_cols(df1, c("Country", "City"))
# Country City
#1 France Paris
#2 England London
#3 India Mumbai
#4 America Los Angeles
#5 England London
select_cols(df1)
# Country City Order_No delivered
#1 France Paris 1 Yes
#2 England London 2 no
#3 India Mumbai 3 Yes
#4 America Los Angeles 4 No
#5 England London 5 yes