title: counting zeros in columns in data frame in R and express as percentage
body: <p>I want to count number of zeros in each column in a R data frame and express it as a percentage. This percentage should be added to last row of the original data frame? 
  example </p>
  
  <pre><code>x &lt;- c(0, 4, 6, 0, 10)
  y &lt;- c(3, 0, 9, 12, 15)
  z &lt;- c(3, 6, 9, 0, 15)
  
  data_a &lt;- cbind(x,y,z)
  </code></pre>
  
  <p>want to see the zeros in each column and express as percentage</p>
  
  <p>Thanks</p>