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

Pre-allocate!!!

And use a matrix if the data are all the same type. It will be much faster than a data.frame.

For example:

> n <- 1000      # Number of rows
> row <- 1:20*1  # one row
> 
> # Adding row, one-by-one
> Data <- data.frame()
> system.time(for(i in 1:n) Data <- rbind(Data,row))
   user  system elapsed 
   2.18    0.00    2.18 
> 
> # Pre-allocated data.frame
> Data <- as.data.frame(Data)
> system.time(for(i in 1:n) Data[i,] <- row)
   user  system elapsed 
   0.94    0.00    0.93
>
> # Pre-allocated matrix (fast!)
> Data <- as.matrix(Data)
> system.time({ for(i in 1:n) Data[i,] <- row; Data <- as.data.frame(Data) })
   user  system elapsed 
      0       0       0