title: Write a dataframe to csv file with value of NA as blank
body: <p>There is a <code>dataframe</code> named <code>cnbd</code>, for example:</p>
  
  <pre><code>cnbd = data.frame(1,2,3,NA,NA,5)
  </code></pre>
  
  <p>Thus the expression:</p>
  
  <pre><code>dim(cnbd)[1]
  </code></pre>
  
  <p>give 1.</p>
  
  <p>I want to write a dataframe like <code>cnbd</code> to a csv with: </p>
  
  <pre><code>write(file = filename, cnbd, append = TRUE)
  </code></pre>
  
  <p>The problem comes:</p>
  
  <ol>
  <li>The values of csv file show <code>cnbd</code> with 6 rows not 1 row as <code>1,2,3,NA,NA,5</code>.</li>
  <li>I need output <code>cnbd</code> show as <code>1,2,3,,,5</code> in csv file, no NAs.</li>
  </ol>