title: What 1-2 letter object names conflict with existing R objects?
body: <p>To make my code more readable, I like to avoid names of objects that already exist when creating new objects. Because of the package-based nature of R, and because functions are first-class objects, it can be easy to overwrite common functions that are not in base R (since a common package might use a short function name but without knowing what package to load there is no way to check for it).  Objects such as the built-in logicals T and F also cause trouble.</p>
  
  <p>Some examples that come to mind are:</p>
  
  <p><strong>One letter</strong></p>
  
  <ul>
  <li>c</li>
  <li>t </li>
  <li>T/F </li>
  <li>J</li>
  </ul>
  
  <p><strong>Two letters</strong></p>
  
  <ul>
  <li>df</li>
  </ul>
  
  <p>A better solution might be to avoid using short names altogether in favor of more descriptive ones, and I generally try to do that as a matter of habit.  Yet "df" for a function which manipulates a generic data.frame is plenty descriptive and a longer name adds little, so short names have their uses.  In addition, for SO questions where the larger context isn't necessarily known, coming up with descriptive names is well-nigh impossible.</p>
  
  <p><strong>What other one- and two-letter variable names conflict with existing R objects?</strong>  Which among those are sufficiently common that they should be avoided?  If they are not in <code>base</code>, please list the package as well.  The best answers will involve at least some code; please provide it if used.</p>
  
  <p>Note that I am not asking whether or not overwriting functions that already exist is advisable or not.  That question is addressed on SO already:</p>
  
  <p><a href="https://stackoverflow.com/q/6135868/602276">In R, what exactly is the problem with having variables with the same name as base R functions?</a></p>
  
  <p>For visualizations of some answers here, see this question on CV:</p>
  
  <p><a href="https://stats.stackexchange.com/questions/13999/visualizing-2-letter-combinations">https://stats.stackexchange.com/questions/13999/visualizing-2-letter-combinations</a></p>