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Dear list, Here's a little problem I already solved with my own coding style, but I feel there is a more efficient and cleaner way to write it, but had no success finding the "clever" solution. I want to produce a factor from a subset of the combination of two vectors. I have the vectors a et b in a data-frame : > df <- expand.grid(a=c(0, 5, 10, 25, 50), b=c(0, 25, 50,