I would like to submit a data frame to a function and use it to subset another data frame.

This is the basic data frame:

foo <- data.frame(var1= c('1', '1', '1', '2', '2', '3'), var2=c('A', 'A', 'B', 'B', 'C', 'C'))

I use the following function to find out the frequencies of var2 for specified values of var1.

foobar <- function(x, y, z){
  a <- subset(x, (x$var1 == y))
  b <- subset(a, (a$var2 == z))
  n=nrow(b)
  return(n)
}

Examples:

foobar(foo, 1, "A") # returns 2
foobar(foo, 1, "B") # returns 1
foobar(foo, 3, "C") # returns 1

This works. But now I want to submit a data frame of values to foobar. Instead of the above examples, I would like to submit df to foobar and get the same results as above (2, 1, 1)

df <- data.frame(var1=c('1','1','3'), var2=c("A", "B", "C"))

When I change foobar to accept two arguments like foobar(foo, df) and use y[, c(var1)] and y[, c(var2)] instead of the two parameters x and y it still doesn't work. Which way is there to do this?