title: How to extract unique elements from a data.frame in R?
body: <p>Can someone suggest a way to extract unique elements from a data.frame?</p>
  
  <p>I have noticed <code>unique</code>, but since it works on either rows or columns, it doesn't do what I'm looking for. I'm after unique cells from a data.frame.</p>
  
  <p>e.g.,</p>
  
  <pre><code>df&lt;-data.frame(V1=c("Hello","fat","man"),V2=c("cat","fat","Hello"),V3=c("man","dog","black"))
  </code></pre>
  
  <p>Extracting unique elements should give me  <code>c("Hello","fat","man","cat","dog","black")</code></p>