Suppose I have a data frame with columns c1, ..., cn, and a function f that takes in the columns of this data frame as arguments. How can I apply f to each row of the data frame to get a new data frame?

For example,

x = data.frame(letter=c('a','b','c'), number=c(1,2,3))
# x is
# letter | number
#      a | 1
#      b | 2
#      c | 3

f = function(letter, number) { paste(letter, number, sep='') }

# desired output is
# a1
# b2
# c3

How do I do this? I'm guessing it's something along the lines of {s,l,t}apply(x, f), but I can't figure it out.

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