title: Multiplying boolean and string pandas series
body: <p>i'm looking to merge two series by "multiplying" them. One series is boolean and one String. Let's say this is df:</p>
  
  <pre><code>df=
          a     b
      0   1     abc
      1   1     cde
      2   1     NaN
      3   0     a123
      4   0     b123
      5   0     NaN
  </code></pre>
  
  <p>I want to get a Series c that looks like this:</p>
  
  <pre><code>      c
  0    abc
  1    cde
  2    NaN
  3    0
  4    0
  5    0 
  </code></pre>
  
  <p>i've tried </p>
  
  <pre><code>df['c']=df['a'].multiply(df['b'])
  </code></pre>
  
  <p>and it did not work. i got:</p>
  
  <blockquote>
    <p>TypeError: can't multiply sequence by non-int of type 'float'</p>
  </blockquote>
  
  <p>Any ideas?</p>