iteratively constructed dataframe in R

I'm relatively new to R, and was wondering the most efficient way to iteratively construct a dataframe (one row at a time, the number of iterations "n" and the length of each row "l" are known beforehand).

1. Create empty dataframe, add a row each iteration
2. Preallocate n x l dataframe, modify a row each iteration
3. Preallocate n x l matrix, modify a row each iteration, make dataframe from matrix
4. Something else