title: Select specific value out of a data frame
body: <p>I want to select a specific value from a data frame and can't figure out how. Here's the data frame:</p>
  
  <p><a href="https://i.stack.imgur.com/KBi6L.png" rel="nofollow noreferrer">picture of data frame</a></p>
  
  <p>I want to select a specific value from the column "X1", but I don't want to access it through the index (1 to 10), but by using the "time"-column. </p>
  
  <p>If I wanted to select the value X1 from row 8 (0.89) through the index, I could do this:</p>
  
  <pre><code>df[8,"X1"]
  </code></pre>
  
  <p>But I want to access the value through the Date &amp; Time. I didn't find a solution yet. The time-column is basically the column X and X.1 merged into one separate column with a POSIXct format (%Y-%m-%d %H:%M:%S). </p>
  
  <p>Something like this (but doesn't work like that):</p>
  
  <pre><code>df[df$time[2018-08-10 13:29:20], "X1"]
  </code></pre>
  
  <p>I would be thankful for any ideas. </p>