title: How to remove "Not Available" in a data frame
body: <p>I want to remove the "Not Available" in the following data frame, but when I change <code>Number</code> to numeric using the following code, the "Not Available" becomes <code>4</code>:</p>
  
  <pre><code>c1 &lt;- c("India", "America", "China", "Europe", "Japan")
  c2 &lt;- c(2.3, 3.5, "Not Available", 1.2, 1.2)
  data &lt;- data.frame(Name=c1, Number=c2)
  data$Number &lt;- as.numeric(data$Number)
  </code></pre>
  
  <p>The result is:</p>
  
  <pre><code>data
  
  ##      Name Number
  ## 1   India      2
  ## 2 America      3
  ## 3   China      4
  ## 4  Europe      1
  ## 5   Japan      1
  </code></pre>
  
  <p>How can I remove the "Not Available" rows in this data frame?</p>