title: Pandas - find rows with matching values in two columns and multiply value in another column
body: <p>First suppose we have a dataframe below:</p>
  
  <pre><code>import pandas as pd
  data = pd.DataFrame({'id':['1','2','3','4','5','6','7','8'], 
                       'A':['foo', 'bar', 'foo', 'bar','foo', 'bar', 'foo', 'foo'],  
                       'C':['10','10','10','30','50','60','50','8'], 
                       'D':['9','8','7','6','5','4','3','2']})
  print(data)
  
      A   C   D   id
  0   foo 10  9   1
  1   bar 10  8   2
  2   foo 10  7   3
  3   bar 30  6   4
  4   foo 50  5   5
  5   bar 60  4   6
  6   foo 50  3   7
  7   foo 8   2   8
  </code></pre>
  
  <p>What I would like to do is find match rows and then do some calculation.</p>
  
  <pre><code>for any two ids(idx, idy) in data.iterrows():
         if idx.A == idy.A and idx.C = idy.C:
         result = idx.D * idy.D
  </code></pre>
  
  <p>and then generate a new dataframe with three columns <code>['id']</code>, <code>['A']</code> and <code>['result']</code>.</p>
  
  <p>So a few rows of expected result is:</p>
  
  <pre><code>     id   A   result   
  0    1   foo   63   
  1    3   foo   63   
  2    5   foo   15
  3    7   foo   15
  </code></pre>
  
  <p>I have tried but the results are either wrong logic or wrong code/data format.
  Can someone give me a hand please?</p>