title: Error: 'BlockManager' object has no attribute 'T' issue while using df.at function in a loop
body: <p>When i am trying to use df.at fuction without loop it works fine and change the data for a perticular column but it is giving error while using this in a loop.
  Code is here.</p>
  
  <pre><code>import pandas as pd
  
  data1 = {'Name': ['Jai', 'Princi', 'Gaurav', 'Anuj'],
          'Height': [5.1, 6.2, 5.1, 5.2]}
  df1 = pd.DataFrame(data1)
  data2 = {'Name': ['Jai', 'Princi', 'Gaurav', 'Anuj'],
           'Height': [4.1, 3.4, 7.1, 9.2]}
  df2 = pd.DataFrame(data2)
  df3 = pd.concat([df1, df2], axis=1)
  for i in range(int(len(df1))):
      for j in range(int(len(df2))):
          if df1['Name'][i] != df2['Name'][j]:
              continue
          else:
              out = (df1['Height'][i] - df2['Height'][j])
              df3.at[i, 'Height_Comparison'] = out
              break
  print(df3)
  </code></pre>