title: Reverse row order in a data frame
body: <p>Suppose I got the following data frame:</p>
  <pre><code>A   B
  1   2
  3   4
  5   6
  </code></pre>
  <p>And I want to reverse row order in all columns (turn it &quot;upside down&quot;):</p>
  <pre><code>A   B
  5   6
  3   4
  1   2
  </code></pre>
  <p>I used the following code</p>
  <pre><code>sort(df[ , 1:2], decreasing = TRUE)
  </code></pre>
  <p>However, it gives me the following error:</p>
  <pre><code>Error in `[.data.frame`(x, order(x, na.last = na.last, decreasing = decreasing)) : undefined columns selected
  </code></pre>
  <p>when I specify only one column it works but I need both the columns to be sorted at once.</p>