title: dplyr mutate rowSums calculations or custom functions
body: <p>I'm trying to mutate a new variable from sort of row calculation,
  say <code>rowSums</code> as below</p>
  
  <pre><code>iris %&gt;% 
    mutate_(sumVar = 
              iris %&gt;% 
              select(Sepal.Length:Petal.Width) %&gt;%
              rowSums)
  </code></pre>
  
  <p>the result is that "sumVar" is truncated to its first value(10.2): </p>
  
  <pre><code>Source: local data frame [150 x 6]
  Groups: &lt;by row&gt;
  
     Sepal.Length Sepal.Width Petal.Length Petal.Width Species sumVar
  1           5.1         3.5          1.4         0.2  setosa   10.2
  2           4.9         3.0          1.4         0.2  setosa   10.2
  3           4.7         3.2          1.3         0.2  setosa   10.2
  4           4.6         3.1          1.5         0.2  setosa   10.2
  5           5.0         3.6          1.4         0.2  setosa   10.2
  6           5.4         3.9          1.7         0.4  setosa   10.2
  ..
  Warning message:
  Truncating vector to length 1 
  </code></pre>
  
  <p>Should it be <code>rowwise</code> applied? Or what's the right verb to use in these kind of calculations.</p>
  
  <p><strong>Edit:</strong></p>
  
  <p>More specifically, is there any way to realize the inline custom function with <code>dplyr</code>?</p>
  
  <p>I'm wondering if it is possible do something like:</p>
  
  <pre><code>iris %&gt;% 
    mutate(sumVar = colsum_function(Sepal.Length:Petal.Width))
  </code></pre>