title: Replacing zero values in dataframe using another dataframe
body: <p>I would like to replace some of the values in the foll. dataframe:</p>
  
  <p>dataframe_a</p>
  
  <pre><code>Y2000   Y2001   Y2002    Y2003    Y2004    Item    Item Code
  34        43      0      0          25     Test      Val
  </code></pre>
  
  <p>I would like to replace the values in the columns with a numeric value derived by multiplying a scalar (say 0.5) by all values in this dataframe:</p>
  
  <p>dataframe_b</p>
  
  <pre><code>Y2000   Y2001   Y2002    Y2003    Y2004    Item    Item Code
  34        43      10      20        25     Test      Val
  </code></pre>
  
  <p>So, in dataframe_a value for column Y2002 should be 10 * 0.5 and value for column Y2003 should be 20 * 0.5</p>
  
  <p>Currently, I am doing this:</p>
  
  <pre><code>df = dataframe_a[dataframe_a == 0]
  df = df * dataframe_b * 0.5
  </code></pre>
  
  <p>However, not sure how I can update dataframe_a with the new values</p>