title: how I can add a column with all 1 to my dataframe?
body: <p>I have a data frame and I want to add a new column with entries 1. how I can do that?</p>
  
  <p>for example</p>
  
  <pre><code>   col1.  col2
      1.     2. 
      4.     5. 
      33.     4. 
      5.     3. 
  </code></pre>
  
  <p>new column</p>
  
  <pre><code>   col1.  col2.  col3
      1.     2.     1
      4.     5.     1
      33.     4.    1
      5.     3.     1
  </code></pre>