[R] How do I make proper use of the by() function?

Dear list, I have a function that uses values from two vectors and spits out one new vector based on all the values of the two original vectors, and with the same length as them. Ok, I would now like to be able to apply that function simply on two columns in a data frame, divided by the levels

Re: [R] How do I make proper use of the by() function?

I make proper use of the by() function? Dear list, I have a function that uses values from two vectors and spits out one new vector based on all the values of the two original vectors, and with the same length as them. Ok, I would now like to be able to apply that function simply on two columns