title: How to find the position of the first non-NA element in a column starting with the last element in an xts object
body: <pre><code>&gt; vols
                 vol_tr     vol_yz     vol_cl    vol_iv
  2021-01-22 0.06260922 0.09798388 0.09861034        NA
  2021-01-25 0.09783596 0.10595096 0.10121109 0.1362547
  2021-01-26 0.10485836 0.10672985 0.10117991 0.1388527
  2021-01-27 0.08284200 0.10742612 0.09586469 0.1509771
  2021-01-28 0.08452010 0.11046722 0.10247347 0.1229756
  2021-01-29 0.07045891 0.11292108 0.10991404        NA
  </code></pre>
  <p>I'd like to find the index of the first non-NA in column vols$vol_iv starting at the end of the column. I'm looking for index or position 5. To find the first non-NA from <em>beginning</em> I can do:</p>
  <pre><code>&gt; d &lt;- coredata(vols$vol_iv)
  &gt; pos = Position(function(d)!is.na(d), d)
  &gt; pos
  [1] 2
  </code></pre>
  <p><code>Position</code> is nice b/c it only evaluates until a match is found.</p>