title: remove rows that don't contain names in another data frame
body: <p>I have a data frame that looks like below, we'll call statesA (but with all 50 states). I have a separate data frame that has specific states in one column (37 states), we'll call statesB (no ozone column). How can I remove all information in the rows from statesA to contain only the states from statesB? I want to keep the ozone number though for the 37 states.  </p>
  
  <pre><code>`    state           ozone
       &lt;chr&gt;           &lt;dbl&gt;
  50  Alabama     0.04537190      
  1   Alaska      0.02505039      
  12  Arizona     0.05191860      
  23  Arkansas    0.03815530      
  34  California  0.03625556      
  45  Colorado    0.04462810      
  47  Connecticut 0.04976119      
  48  Delaware    0.04170330      
  49  Florida     0.04038279      
  2   Georgia     0.04970213`
  </code></pre>
  
  <p>I was referred to another question and tried to do a right outer join but only received 25 of the needed 37 rows.
  <code>statesC = merge(x = statesA, y = statesB, by = "state", all.y = TRUE)</code></p>
  
  <p>What am I doing wrong?</p>