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

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

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 NORTH
2 2012 02 32 54 21 16 NORTH
3 2012 03 14 32 65 32 NORTH
4 2012 04 65 33 75 21 NORTH
: : : : : : :
: : : : : : :
12 2012 12 32 58 53 17 NORTH
13 2012 01 12 47 43 23 SOUTH
14 2012 02 87 43 21 76 SOUTH
: : : : : : :
25 2012 01 12 46 84 29 EAST
26 2012 02 85 29 90 12 EAST
: : : : : : :
: : : : : : :
I want to extract section of the data that have the same date value, for
example to do this just for 2012 01 I would just create a subset of data
data_1 <- subset(data, date == "2012 01")
and this gives me all the data for 2012 01 but I then go on to apply a
function to this data. I would like to be able to apply my function to all
possible subsets of my data, so ideally I would be looping through my
large data frame and extracting the data for 2012 01, 2012 02, 2012 03,
2012 04... and applying a function to each of these subsets of data
separately.
But I would like to be able to apply this to my data frame even if my data
frames length were to change, so it may not always go from 2012 01 - 2012
12, the range of dates may vary so that sometimes it may be used on data
from for example 2011 03 - 2013 01.