title: How to convert a character column of data frame to Posixlt type
body: <p>One of the column of data.frame is "character"<br>
  Ex:   9:56:53 PM 3/31/2012</p>
  
  <pre><code>/&gt; class(columnName)  
  [1] "character"
  
  /&gt;ttable[,c(3)] &lt;- as.POSIXlt(ttable[,c(3)],tz="GMT", format="%H:%M:%S %p %m/%d/%Y")    
  
  /&gt;class(columnName)     
  [1] "numeric"    
  </code></pre>
  
  <p>I expect it to be posixlt but it turns out to be numeric. How to make it posixlt ? </p>
  
  <p>If i apply the same instruction on actual value of the columns it works as expected ? </p>
  
  <pre><code>/&gt; dd &lt;- as.POSIXlt("9:56:53 PM 3/31/2012",tz="GMT", format="%H:%M:%S %p %m/%d/%Y")  
  
  /&gt; class(dd)  
  [1] "POSIXlt" "POSIXt" 
  </code></pre>