title: how to replace dataframe in pandas
body: <p>sample dataframes are</p>
  
  <pre><code>df1= 
  A  B  C
  a0 b0 c0
  a1 b1 c1
  a2 b2 c2
  ...
  an bn cn
  
  
  df2=
  A   B 
  x0 y0
  x1 y1
  x2 y2
  ...
  xm ym
  </code></pre>
  
  <p>What I want to replace is,</p>
  
  <p>if (ai,bi) = (xj,yj) for some j, then ci = 0</p>
  
  <p>For example,</p>
  
  <pre><code>df3= 
  A  B  C
  1  4 c0
  2  8 c1
  2  9 c2
  3 12 c3
  3 16 c4
  4 16 c5
  
  
  df4=
  A   B 
  2   8
  3  12
  </code></pre>
  
  <p>and I would like to get below</p>
  
  <pre><code>result=
  A  B  C
  1  4 c0
  2  8  0
  2  9 c2
  3 12  0
  3 16 c4
  4 16 c5
  </code></pre>
  
  <p>I couldn't find how to replace by using <code>np.where</code>. I need your help!</p>