title: creating rows with different combinations of columns
body: <p>For the below example data, I wonder to know how to create four rows with different combinations by crossing a low value of salary with high and low values of rent, and then a high value of rent with high and low values of salary.</p>
  <p>Thanks,</p>
  <p>MMR</p>
  <pre><code>salary &lt;- c(21000, 23400, 26800,40000, 40000, 50000)
  rent &lt;- c(1100, 1340, 1680,2000, 1000, 1000)
  employ.data &lt;- data.frame(salary, rent)
  </code></pre>