title: Get index when looping through one column of pandas
body: <p>I have a simple dataframe:</p>
  
  <p>index, a, y
  0    , 1, 2
  1    , 4, 6
  2    , 5, 8</p>
  
  <p>I want to loop through the "a" column, and print out its index for a specific value.</p>
  
  <pre><code>for x in df.a:
      if x == 4:
          print ("Index of that row")
  </code></pre>
  
  <p>What syntax should I use to obtain the index value when the for loop hits the specific value in the "a" column that I am seeking?</p>
  
  <p>Thank You</p>