title: alternative to "!is.null()" in R
body: <p>my R code ends up containing plethora of statements of the form:</p>
  
  <pre><code>if (!is.null(aVariable)) { 
       do whatever 
  }
  </code></pre>
  
  <p>But this kind of statement is hard to read because it contains two negations. I would prefer something like: </p>
  
  <pre><code> if (is.defined(aVariable)) { 
        do whatever 
   }
  </code></pre>
  
  <p>Does a <code>is.defined</code> type function that does the opposite of !is.null exist standard in R?</p>
  
  <p>cheers,
  yannick</p>