title: Creating a Data Frame based on day of week from a Data Frame containing several months of data
body: <p>I am just starting with R and hoping to get some help. I have a set of data in a file as follows:</p>
  
  <pre><code>20111001|37|9|9
  20111002|48|6|11
  ...
  20111031|42|6|8
  </code></pre>
  
  <p><br>
  I have loaded in the file using the following:</p>
  
  <pre><code>    myClasses &lt;- c( "character", "numeric", "numeric", "numeric" )
      df &lt;- read.table( "inputfile", FALSE, "|", colClasses=myClasses )
      names( df ) &lt;- c( "datestamp", "cnt1", "cnt2", "cnt3" )
      df$datestamp &lt;- as.Date( df$datestamp, "%Y%m%d" )
  </code></pre>
  
  <p><br>
  How can I create another data frame containing only the data from a certain weekday (i.e., a dataframe containing all the same information as <code>df</code> but only for <code>Fridays</code>)?</p>