title: Creating a new column to a data frame using a formula from another variable
body: <p>I want to create a new column to a data frame using a formula from another variable.<br/> 
  Example:<br/> 
  I have a data set "aa" is; </p>
  
  <pre><code>x    y 
  2    3 
  4    5 
  6    7 
  </code></pre>
  
  <p>My R code is; </p>
  
  <pre><code>&gt;bb &lt;- "x+y-2" 
  &gt;attach(aa) 
  &gt;aa$z&lt;- bb 
  &gt;detach(aa) 
  </code></pre>
  
  <p>the result is; </p>
  
  <pre><code>x  y  z 
  2  3  x+y-2 
  4  5  x+y-2 
  6  7  x+y-2 
  </code></pre>
  
  <p>but I want like this; </p>
  
  <pre><code>x  y  z 
  2  3  3 
  4  5  7 
  6  7  11 
  </code></pre>
  
  <p>Could you please help me.. </p>