title: R: loop through data frame extracting subset of data depending on date
body: <p>I have a large data frame that consists of data that looks something like this:</p>
  
  <pre><code>        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
  :        :      :    :    :    :       :
  :        :      :    :    :    :       :
  </code></pre>
  
  <p>I want to extract section of the data that have the same <code>date</code> value, for example to do this just for <code>2012 01</code> I would just create a subset of data</p>
  
  <pre><code>data_1 &lt;- subset(data, date == "2012 01")
  </code></pre>
  
  <p>and this gives me all the data for <code>2012 01</code> 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 <code>2012 01, 2012 02, 2012 03, 2012 04...</code> and applying a function to each of these subsets of data separately. </p>
  
  <p>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 <code>2012 01 - 2012 12</code>, the range of dates may vary so that sometimes it may be used on data from for example <code>2011 03 - 2013 01</code>.</p>