title: add a column to dataFrame from another dataFrame
body: <p>I have a data frame allData that has 3 columns,and a data frame userData with 1 column.that every row of all data refers to a row of user data.</p>
  
  <p>now I want to add userData column to my allData data frame.but I cant.</p>
  
  <p>I tried some codes Like :</p>
  
  <pre><code>allData$user &lt;- userData$userId
  </code></pre>
  
  <p>or:</p>
  
  <pre><code>cbind(allData , userData)
  </code></pre>
  
  <p>but non of theme do not work correctly.they produce a data with lots of columns unexpectedly.</p>
  
  <p>how can I add user column to my allData data frame?</p>
  
  <p>thank you.</p>