title: merging two columns of data frame in R
body: <p>Consider this data frame in R which contains two columns named a and b:</p>
  
  <pre><code>a     b
  a1   b1  
  a2   b2  
  a3   b3  
  </code></pre>
  
  <p>I want to produce a list by merging columns a and b in this way:</p>
  
  <pre><code> a1  b1  a2  b2  a3  b3
  </code></pre>
  
  <p>How can i do this?</p>