title: Replace values in one column without NA
body: <p>I've a dataframe where I want replace some values in one column.
  My column:</p>
  
  <pre><code>         col1
  0         0.0
  1 -89999991.0
  2         1.0
  3         2.0
  4         0.0
  5        11.0
  </code></pre>
  
  <p>I want to replace all values <code>&lt;0 by 0</code> in col1 by using <code>df.loc[df.col1 &lt;0, 'col2'] = 0</code>and write them into col2:</p>
  
  <pre><code>   col1
  0   0.0
  1   0.0
  2   1.0
  3   2.0
  4   0.0
  5  11.0
  </code></pre>
  
  <p>But the problem was that it replaced all  values by NAs:</p>
  
  <pre><code>  col2
  0   NA
  1  0.0
  2   NA
  3   NA
  4   NA
  5   NA
  </code></pre>
  
  <p>And I don't understand where is the problem.</p>