title: pandas copy value from one column to another if condition is met
body: <p>I have a dataframe:</p>
  
  <pre><code>df = 
  col1  col2  col3 
  1      2     3
  1      4     6
  3      7     2
  </code></pre>
  
  <p>I want to edit <code>df</code>, such that when the value of col1 is smaller than 2 , take the value from <code>col3</code>.</p>
  
  <p>So I will get:</p>
  
  <pre><code>new_df = 
  col1  col2  col3 
  3      2     3
  6      4     6
  3      7     2
  </code></pre>
  
  <p>I tried to use <code>assign</code> and <code>df.loc</code> but it didn't work.</p>
  
  <p>What is the best way to do so?</p>