title: conditionally fill all subsequent values of dataframe column
body: <p>i want to "forward fill" the values of a new column in a DataFrame according to the first instance of a condition being satisfied. here is a basic example:</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  
  x1 = [1,2,4,-3,4,1]
  
  df1 = pd.DataFrame({'x':x1})
  </code></pre>
  
  <p>i'd like to add a new column to df1 - 'condition' - where the value will be 1 upon the occurrence of a negative number,else 0, <em>but</em> i'd like the remaining values of the column to be set to 1 once the negative number is found</p>
  
  <p>so, i would look for desired output as follows:</p>
  
  <pre><code>    condition   x
  0   0   1
  1   0   2
  2   0   4
  3   1   -3
  4   1   4
  5   1   1
  </code></pre>