title: error with rowSums usng column names
body: <p>I am trying to segment Census data from fairly deaggregated data (e.g. age variables in 5-yr groups), &amp; creating summary variables based on aggregation (e.g. all males 18+ per county). My solution is rowSums, e.g. <code>county$MalesOver18 &lt;- rowSums(county[,c(68:87)])</code>, where vars 68-87 sum to males 18+ -- works fine. However, with 500 variables it is not efficient to count out the order of my start/end columns. </p>
  
  <p>But when I use my preferred solution, column names for rowSums (e.g. <code>rowSums(county[,c(H76007:H76025)]</code>, where H vars = field names), I get one of 2 msg errors:</p>
  
  <p>run w/ col names in quotes:  <code>Error in "H76007":"H76025" : NA/NaN argument
  In addition: Warning messages:
  1: In</code>[.data.frame<code>(county, , c("H76007":"H76025")) :
    NAs introduced by coercion
  2: In</code>[.data.frame<code>(county, , c("H76007":"H76025")) :
    NAs introduced by coercion</code></p>
  
  <p>run w/ col names not in quotes: <code>Error in</code>[.data.frame<code>(county, , c(H76007:H76025)) : 
    object 'H76007' not found</code></p>
  
  <p>I have tried using the na.rm command &amp; setting my variables as numeric -- although they are already integers -- and all to no result.</p>
  
  <p>any guidance? thanks.</p>