title: How to extract data from a RasterBrick?
body: <p>I have a RasterBrick consisting of monthly rainfall data over 7 years, so it has 7 layers with 12 slots each:</p>
  
  <pre><code>rainfall &lt;- brick("Rainfall.tif")
      &gt; rainfall
      class       : RasterBrick
      dimensions  : 575, 497, 285775, 7  (nrow, ncol, ncell, nlayers)
      resolution  : 463.3127, 463.3127  (x, y)
      extent      : 3763026, 3993292, -402618.8, -136213.9  (xmin, xmax, ymin, ymax)
      coord. ref. : +proj=sinu +lon_0=0 +x_0=0 +y_0=0 +a=6371007.181 +b=6371007.181 +units=m +no_defs 
      data source : in memory
      names       : layer.1.1, layer.2.1, layer.1.2, layer.2.2,   layer.1,   layer.2,     layer 
      min values  :  239.6526,  499.8343,  521.0316,  617.2896,  596.0397,  663.6633,  298.0572 
      max values  :  691.9075, 1158.2064, 1184.9858, 1198.7121, 1241.8077, 1114.7598,  832.6042 
  </code></pre>
  
  <p>From this I would like to extract a value for rainfall at points distributed both spatially and temporally.  These points are in a data frame:</p>
  
  <pre><code>points &lt;- read.csv("Points.csv")
      &gt; head(points)
          ID      x          y      ncell  jday  FRP_max    FRI   year   month
         69211  3839949  -171684.6    17    59      NA  230.2500  2001     2
         69227  3808720  -238808.7    16    52      NA        NA  2001     2
         69237  3793373  -267563.1     1    52      NA        NA  2001     2
         69244  3986574  -292118.7     1    43      NA        NA  2001     2
         32937  3864736  -164296.8   106    77    94.8  249.1524  2001     3
         32938  3871463  -163123.4    31    82      NA  253.5081  2001     3
  </code></pre>
  
  <p>I can handle the spatial aspect by converting the data frame to a spatial data frame and using the extract function:</p>
  
  <pre><code>points.sp &lt;- points
  coordinates(points.sp) &lt;- ~ x + y
  rainfall.points &lt;- extract(rainfall, points.sp)
  </code></pre>
  
  <p>However, I can't work out how to make sure the rainfall values are being extracted from the correct raster layer from within the raster brick.  I've tried various ways of indexing using the "year" and "month" columns from my data frame but nothing has worked.  Any tips would be much appreciated!</p>
  
  <p>This is my first post so apologies if there's too much/not enough info.  Let me know if seeing more of my code would be useful.</p>