title: How to calculate correlation for one column vs rest of the dataframe columns grouped by index
body: <p>I have a data frame like this:</p>
  <pre><code>df:
           Col-1   Col-2   Col-3 ..................Col-100
  index   
    A        23      33     1                        5                       
    A        33      22     4                        29
    A        1       5      3                        24
    B        65      12     4                        42
    B        2       24     4                        24
    B        3       12     24                       5
  </code></pre>
  <p>I want to calculate the correlation for Col-1 against all the other columns and group them by Index.</p>
  <p>So my output looks like this:</p>
  <pre><code>       Col-1  Col-2  Col-3 ............Col-100
    A    1      0.24    0.2               0.45
    B    1      0.30    0.25              0.18
  </code></pre>
  <p>Can anyone help me with this?</p>