How to define optional arguments in a function - r

Question

I'm writing a function to get some descriptive stats from a data frame. The function takes three argument: data set, set of numerical variables, set of character variables. I managed to write the function to successfully obtain the required result when the both numerical and character variables are identified within the argument. However, when one of these argument is missing, I'd like the function to return a list with two components with the missing argument as NULL within its component.
Here's the code I've written. Please let me know if you have an answer.
table1 <- function(dat, numvar, charvar){
result_n <- numeric()
result_c <- data.frame()
#This is the original table function for numerical values
for (i in 1:length(numvar)) {
new_row <- c(round(mean(dat[[numvar[i]]],na.rm = T),2) ,
round(median(dat[[numvar[i]]],na.rm = T),2),
round(sd(dat[[numvar[i]]],na.rm = T),2),
length(dat[[numvar[i]]])-sum(is.na(dat[[numvar[i]]])),
sum(is.na(dat[[numvar[i]]])))
result_n <- rbind(result_n,new_row)
}
rownames(result_n) <- numvar
colnames(result_n) <- c("Mean", "Median", "SD", "N", "N_miss")
#Thisi is the new table for char values
for (i in 1:length(charvar)) {
tab.dat <- as.data.frame(table(dat[charvar[i]],useNA = "ifany" ))
a1 <- as.character(tab.dat$Var1)
a1[3] <- "NMiss"
one.table <- data.frame(
Varname = c(charvar[i], rep(" ", nrow(tab.dat)-1)),
group = a1,
count= tab.dat$Freq)
result_c <- rbind(result_c, one.table)
}
result_list <- list(numericStats = result_n, FactorStats =result_c)
return(result_list)
}

Answer 1

You can set the default value to a function:
table1 <- function(dat = NULL, numvar = NULL, charvar = NULL) {...
From there, the script can determine which is missing and go from there.

Answer 2

Here's the answer:
table1 <- function(dat, numvar=NULL, charvar=NULL){
result_n <- numeric()
result_c <- data.frame()
#This is the original table function for numerical values
#I borrowed builtin function (ifmissing) from the internet
if(!missing(numvar)) {for (i in 1:length(numvar)) {
new_row <- c(round(mean(dat[[numvar[i]]],na.rm = T),2) ,
round(median(dat[[numvar[i]]],na.rm = T),2),
round(sd(dat[[numvar[i]]],na.rm = T),2),
length(dat[[numvar[i]]])-sum(is.na(dat[[numvar[i]]])),
sum(is.na(dat[[numvar[i]]])))
result_n <- rbind(result_n,new_row)
}
rownames(result_n) <- numvar
colnames(result_n) <- c("Mean", "Median", "SD", "N", "N_miss")}
#Thisi is the new table for char values
#I borrowed builtin function (ifmissing) from the internet
if(!missing(charvar)) {for (i in 1:length(charvar)) {
tab.dat <- as.data.frame(table(dat[charvar[i]],useNA = "ifany" ))
a1 <- as.character(tab.dat$Var1)
a1[3] <- "NMiss"
one.table <- data.frame(
Varname = c(charvar[i], rep(" ", nrow(tab.dat)-1)),
group = a1,
count= tab.dat$Freq)
result_c <- rbind(result_c, one.table)
}}
result_list <- list(numericStats = result_n, FactorStats =result_c)
return(result_list)
}