title: How to use a vector of strings to call dataframe columns by its header
body: <p>In R, I want to use a subset of a dataframe 'RL', by selecting specific headers (eg. 'RL$age01' etc.. I generate the selected headers as a vector of strigs:</p>
  
  <pre><code>v = c('ID', sprintf("sex%02d", seq(1,15)), sprintf("age%02d", seq(1,15)))
  </code></pre>
  
  <p>and the dataframe index as:</p>
  
  <pre><code>c = sprintf('RL$%s', v)
  </code></pre>
  
  <p>how can I evaluate these strigns to call the dataframe columns by header and rearange them in a matrix, in the sense of x = cbind(RL$ID, RL$age01, ...) ?
  cbind(c) does not work neither using things like eval(), parse() or expression().</p>
  
  <p>Thanks for any help</p>
  
  <p>Rafael</p>