title: Replace missing values (NA) with blank (empty string)
body: <p>I have a dataframe with an NA row:</p>
  
  <pre><code> df = data.frame(c("classA", NA, "classB"), t(data.frame(rep("A", 5), rep(NA, 5), rep("B", 5))))
   rownames(df) &lt;- c(1,2,3)
   colnames(df) &lt;- c("class", paste("Year", 1:5, sep = ""))
  
   &gt; df
     class Year1 Year2 Year3 Year4 Year5
  1 classA     A     A     A     A     A
  2   &lt;NA&gt;  &lt;NA&gt;  &lt;NA&gt;  &lt;NA&gt;  &lt;NA&gt;  &lt;NA&gt;
  3 classB     B     B     B     B     B
  </code></pre>
  
  <p>I introduced the empty row (NA row) on purpose because I wanted to have some space between classA row and classB row.</p>
  
  <p>Now, I would like to substitute the <code>&lt;NA&gt;</code> by blank, so that the second row looks like an empty row.</p>
  
  <p>I tried:</p>
  
  <pre><code> df[is.na(df)] &lt;- ""
  </code></pre>
  
  <p>and</p>
  
  <pre><code> df[df == "NA"] &lt;- ""
  </code></pre>
  
  <p>but it didn't work..</p>
  
  <p>Any ideas? Thanks!</p>