title: Remove duplicates column combinations from a dataframe in R
body: <p>I want to remove duplicate combinations of sessionid, qf and qn from the following data  </p>
  
  <pre><code>               sessionid             qf        qn         city
  1  9cf571c8faa67cad2aa9ff41f3a26e38     cat   biddix          fresno
  2  e30f853d4e54604fd62858badb68113a   caleb     amos                
  3  2ad41134cc285bcc06892fd68a471cd7  daniel  folkers                
  4  2ad41134cc285bcc06892fd68a471cd7  daniel  folkers                
  5  63a5e839510a647c1ff3b8aed684c2a5 charles   pierce           flint
  6  691df47f2df12f14f000f9a17d1cc40e       j    franz prescott+valley
  7  691df47f2df12f14f000f9a17d1cc40e       j    franz prescott+valley
  8  b3a1476aa37ae4b799495256324a8d3d  carrie mascorro            brea
  9  bd9f1404b313415e7e7b8769376d2705    fred  morales       las+vegas
  10 b50a610292803dc302f24ae507ea853a  aurora      lee                
  11 fb74940e6feb0dc61a1b4d09fcbbcb37  andrew    price       yorkville 
  </code></pre>
  
  <p>I read in the data as a data.frame and call it mydata. Heree is the code I have so far, but I need to know how to first sort the data.frame correctly. Secondly remove the duplicate combinations of sessionid, qf, and qn. And lastly graph in a histogram characters in the column qf</p>
  
  <pre><code>sortDATA&lt;-function(name)
  {
  #sort the code by session Id, first name, then last name
  sort1.name &lt;- name[order("sessionid","qf","qn") , ]
  #create a vector of length of first names
  sname&lt;-nchar(sort1.name$qf)
  hist(sname)
  }
  </code></pre>
  
  <p>thanks!</p>