title: Return rownumber in R for first value with a whole hour in it
body: <p>For an analysis I'm running I want to know the first row in my dataset where the column "time" is a whole hour. So for instance I have:</p>
  
  <pre><code>df &lt;- data.frame(x = c("14:56:00","14:57:00","14:58:00","14:59:00","15:00:00","15:01:00"))
  </code></pre>
  
  <p>I want to find the rownumber where x is a whole hour, which would be 15:00:00 in this case.</p>
  
  <p>Now even better would be if it only returns the first case where this happens, but I think I can figure a way to work around this. I though maybe <em>grepl</em> was a way, but I don't think that works very well with time functions?</p>