title: How do I merge 2 dataframes?
body: <p>I have one <code>dataframe1</code> with a column containing firstnames and another <code>dataframe2</code> containing firstnames and the corresponding genders.</p>
  
  <p>Now I want to add the gender-column of <code>dataframe2</code> into <code>dataframe1</code> but it doesn't seem to work with the following line:</p>
  
  <p><code>df2new &lt;- merge(df1,df2,by.x="firstname",by.y ="gender")</code></p>
  
  <p>How can I merge these two dataframes?</p>