title: In R, how do I subset a data.frame by values from another data.frame?
body: <p>I have two data frames.  The first, <code>df.1</code>, contains two columns of paired numerical identifiers, where each column includes ~100,000 rows.  The second  data frame, <code>df.2</code>, includes one column (<code>df.2$C</code>) of numerical identifiers. This data frame has around 200 rows.</p>
  
  <p>How can I find the paired subset of data of <code>df.1</code> that includes only the rows with values of the identifiers found in <code>df.2$C</code>? </p>
  
  <p>The final subset would include the paired data of <code>df.1</code> which corresponds to identifiers found in <code>df.2$C</code> that match the identifiers found in <code>df.1$A</code>, <code>df.1$B</code> or both.</p>