title: get sum of the product of two columns
body: <p>Can someone explain how my order of operations for the R fuctions below is wrong? I expected to multiply across each row then sum the result, but the answer I got was very different.</p>
  
  <pre><code>qqq&lt;-data.frame(c(1,2,3),c(4,5,6))
  library(dplyr)
  qqq%&gt;%sum(.[,1]*.[,2]) #returns: 53
  
  # answer I expected: 1*4+2*5+3*6 = 32
  </code></pre>