title: In PANDAS, how to get the index of a known value?
body: <p>If we have a known value in a column, how can we get its index-value? For example:  </p>
  
  <pre><code>In [148]: a = pd.DataFrame(np.arange(10).reshape(5,2),columns=['c1','c2'])
  In [149]: a
  Out[149]:   
     c1  c2
  0   0   1
  1   2   3
  2   4   5
  ........
  </code></pre>
  
  <p>As we know, we can get a value by the index corresponding to it, like this.</p>
  
  <pre><code>In [151]: a.ix[0,1]    In [152]: a.c2[0]   In [154]: a.c2.ix[0]   &lt;--  use index
  Out[151]: 1            Out[152]: 1         Out[154]: 1            &lt;--  get value
  </code></pre>
  
  <p>But how to get the index by value?</p>