title: Merge Multiple Data Frames by Row Names
body: <p>I'm trying to merge multiple data frames by row names.</p>
  
  <p>I know how to do it with two:</p>
  
  <pre><code>x = data.frame(a = c(1,2,3), row.names = letters[1:3])
  y = data.frame(b = c(1,2,3), row.names = letters[1:3])
  merge(x,y, by = "row.names")
  </code></pre>
  
  <p>But when I try using the <code>reshape</code> package's <code>merge_all()</code> I'm getting an error.</p>
  
  <pre><code>z = data.frame(c = c(1,2,3), row.names = letters[1:3])
  l = list(x,y,z)
  merge_all(l, by = "row.names")
  
  Error in -ncol(df) : invalid argument to unary operator
  </code></pre>
  
  <p>What's the best way to do this?</p>