title: zero index for all rows in python dataframe
body: <p>I have problem with indexing python <code>dataframe</code>. I have dataframe which I fill it with loop. I simplified it like this :</p>
  
  <pre><code>d = pd.DataFrame(columns=['img', 'time', 'key'])
  for i in range(5):
      image = i
      timepoint = i+1
      key = i+2
      temp = pd.DataFrame({'img':[image], 'timepoint':[timepoint], 'key': [key]})
      d = pd.concat([d, temp])
  </code></pre>
  
  <p>The problem is since it shows <code>0</code> as and index for all rows, I can not access to the specific row based on <code>.loc[]</code>. Does anybody have any idea how can I fix the problem and get normal index column?</p>