title: pandas dataframe adding value to a cell by matching column header and value
body: <p>I have a dataframe like this :</p>
  
  <pre><code>Num  A    B   C   D
  1    wer
  1    df
  1    ffg
  2    asd
  2    vb
  3    ncnc
  3    oiji
  4    qwe
  </code></pre>
  
  <p>I will filter out based on <code>Num</code> first, Say I filter for <code>Num = 1</code></p>
  
  <pre><code>    Num  A    B   C   D
      1    wer
      1    df
      1    ffg
  </code></pre>
  
  <p>Now I need to add value from a variable the cell where <code>wer</code> and <code>C</code> meet, similarly with <code>ffg</code> and <code>D</code></p>
  
  <pre><code>Num  A    B   C   D
  1    wer      5
  1    df
  1    ffg          7
  </code></pre>
  
  <p>If there is some value already existing, new value should replace it</p>