title: How to take the mean of last 10 values in a column before a missing value using R?
body: <p>I am new to R and having trouble figuring out to go about this.  I have data on tree growth rates from dead trees, organized by year.  So, my first column is year and the columns to the right are growth rates for individual trees, ending in the year each tree died.  After the tree died, the values are "NA" for the remaining years in the dataset.  I need to take the mean growth for the 10 years preceding each tree's death, but each tree died in a different year.  Does anyone have an idea for how to do this? Here is an example of what a dataset might look like:</p>
  
  <pre><code>Year    Tree1   Tree2   Tree3
  1989    53.00   84.58   102.52
  1990    63.68   133.16  146.07
  1991    90.37   103.10  233.58
  1992    149.24  127.61  245.69
  1993    96.20   54.78   417.96
  1994    230.64  60.92   125.31
  1995    150.81  60.98   100.43
  1996    124.25  42.73   75.43
  1997    173.42  67.20   50.34
  1998    119.60  73.40   32.43
  1999    179.97  61.24   NA
  2000    114.88  67.43   NA
  2001    82.23   55.23   NA
  2002    49.40   NA  NA
  2003    93.46   NA  NA
  2004    104.67  NA  NA
  2005    44.14   NA  NA
  2006    88.40   NA  NA
  </code></pre>
  
  <p>So, the averages I need to calculate are:</p>
  
  <pre><code>Tree1: mean(1997-2006) = 105.01
  Tree2: mean(1992-2001) = 67.15
  Tree3: mean(1989-1998) = 152.98
  </code></pre>
  
  <p>Since I need to do this for a large number of trees, it would be helpful to have a method of automating the calculation.  Thank you very much for any help!  Katie</p>