title: Constract DF from a logical DF and a Boolean DF
body: <p>I have two DF 
  the folowing one is Boolean </p>
  
  <pre><code>&gt; Portfolio 1 
  
  Name | X1992 | X1993 | X1994 
  XHHD   False   False   True
  Jdqd   False   False   False
  Jhds   False   True    False
  lkaz   False   False   True                      
  nqb    True    False   False
  jbqs   False   False   False
  jbq    False   True    False
  knd    True    False   True
  njvd   False   True    False
  kjiz   True    False   True
  khza   False   False   False
  akhd   False   False   True
  jkaze  True    True    False
  bzae   True    False   False
  </code></pre>
  
  <p>the Second DF have a monthly frequence </p>
  
  <blockquote>
    <p>Return</p>
  </blockquote>
  
  <pre><code>Name | 1992/01 | 1992/02 ..... 1994/12 
  XHHD   0.23       0.564
  Jdqd   0.3        0.654
  Jhds   0.234      0.456
  lkaz   0.54       0.472               
  nqb    0.99       0.761
  jbqs   0.01       1.765
  jbq    0.23       0.002
  knd    0.59       2.32
  njvd   0.123      0.43
  kjiz   0.987      -0.12
  khza   1.34       0.12
  akhd   0.76       0.23
  jkaze  0.654      0.98
  bzae   0.43       0.73
  </code></pre>
  
  <p>I want to have a DF that calculte me the mean monthly return of companies based on the boolean DF if the Companies TRUE then include her return on the calculaion. 
  the results will be </p>
  
  <pre><code>Date     Portfolio 1
  1992/01   mean
  1992/02   mean
  1992/03   mean
  1992/04   mean
  </code></pre>