title: R gotcha: logical-and operator for combining conditions is & not &&
body: <p>Why doesn't <code>subset()</code> work with a logical and <code>&amp;&amp;</code> operator combining two conditions?</p>
  
  <pre><code>&gt; subset(tt, (customer_id==177 &amp;&amp; visit_date=="2010-08-26"))
  &lt;0 rows&gt; (or 0-length row.names)
  </code></pre>
  
  <p>but they each work individually:</p>
  
  <pre><code>&gt; subset(tt, customer_id==177)
  
  &gt; subset(tt, visit_date=="2010-08-26")
  </code></pre>
  
  <p>(Want to avoid using large temporary variables - my dataset is huge)</p>