R: loop through data frame extracting subset of data depending on date

Question

I have a large data frame that consists of data that looks something like this:

        date    w    x    y    z    region
1    2012 01    21   43   12    3   NO         


        

Answer 1

This is a perfect situation for the plyr package:

require(plyr)
ddply(my_df, .(date), my_function, extra_arg_1, extra_arg_2)

where my_function is the function you want to perform on the split data frames, and extra_args are any extra arguments that need to go to that function.

ddply (data frame -> data frame) is the form you want if you want your results in a data frame; dlply returns a list.