title: Merge data frames of unequal number of rows
body: <p>I want to create a new data frame df3 from data frames df1 and df2. The dataframes contain zipcodes and states. One data frame has more zipcodes than the other. I want combine them and add NA for Zipcodes that do not have a value for the corresponding Zip code in the other file.</p>
  
  <pre><code>df3  
  ZIP STATE   Total2 Total1  
  36541   AL  6747   11190  
  36542   AL  8093   10647  
  36549   AL  9404   NA  
  36551   AL  7048   10656  
  36561   AL  5104   5044  
  .
  .
  .
  
  
  df1  
  ZIP STATE   Total1  
  36541   AL  11190  
  36542   AL  10647  
  36551   AL  10656  
  36561   AL  5044  
  32314   FL  10624  
  32320   FL  5705  
  32322   FL  11457  
  32324   FL  5926  
  32508   FL  8885  
  32511   FL  9428  
  32514   FL  6168  
  32526   FL  11998  
  32533   FL  11188  
  32534   FL  11188  
  
  
  df2  
  ZIP STATE   Total2  
  36541   AL  6747  
  36542   AL  8093  
  36549   AL  9404  
  36551   AL  7048  
  36561   AL  5104  
  32314   FL  8959  
  32320   FL  9237  
  32321   FL  11417  
  32322   FL  5467  
  32324   FL  6672  
  32508   FL  6085  
  32511   FL  8782  
  32514   FL  11297  
  32522   FL  6892  
  32526   FL  9497  
  32533   FL  9114  
  32534   FL  8594  
  </code></pre>