title: sum two columns in R
body: <p>I feel a bit embarrassed as I am trying to add two columns in R to get the product.</p>
  
  <p>I have tried</p>
  
  <pre><code>sum(col1,col2)
  </code></pre>
  
  <p>but this returns</p>
  
  <pre><code>Error in Summary.factor(c(49L, 48L, 47L, 46L, 46L, 45L, 45L, 44L, 43L,  : 
    sum not meaningful for factors
  </code></pre>
  
  <p>I thought this would very simple! both columns contain integers.</p>