title: Creating numpy array from pandas dataframe
body: <pre><code>import pandas as pd
  import numpy as np
  df = pd.read_csv('~/test.txt')
  list(df.columns.values)
  </code></pre>
  
  <p>I get the following output :</p>
  
  <pre><code>['time', 'Res_fs1', 'angle1', 'Res_fs2', 'angle2', 'Res_ps1', 'Force1', 
  'Res_ps2', 'Force2', 'object']
  </code></pre>
  
  <p>when i try to create a numppy array using Res_fs1,Res_fs2,Res_ps1,Res_ps2</p>
  
  <pre><code>X=np.array(df['Res_fs1','Res_fs2','Res_ps1','Res_ps2'])
  </code></pre>
  
  <p>I get this error message saying key error although the keys exist:</p>
  
  <pre><code>Traceback (most recent call last):
  File "&lt;stdin&gt;", line 1, in &lt;module&gt;
  File "/usr/lib/python2.7/dist-packages/pandas/core/frame.py", line 1969, in 
  __getitem__
  return self._getitem_column(key)
  File "/usr/lib/python2.7/dist-packages/pandas/core/frame.py", line 1976, in 
  _getitem_column
  return self._get_item_cache(key)
  File "/usr/lib/python2.7/dist-packages/pandas/core/generic.py", line 1091, 
  in _get_item_cache
  values = self._data.get(item)
  File "/usr/lib/python2.7/dist-packages/pandas/core/internals.py", line 3211, 
  in get
  loc = self.items.get_loc(item)
  File "/usr/lib/python2.7/dist-packages/pandas/core/index.py", line 1759, in 
  get_loc
  return self._engine.get_loc(key)
  File "pandas/index.pyx", line 137, in pandas.index.IndexEngine.get_loc 
  (pandas/index.c:3979)
  File "pandas/index.pyx", line 157, in pandas.index.IndexEngine.get_loc 
  (pandas/index.c:3843)
  File "pandas/hashtable.pyx", line 668, in 
  pandas.hashtable.PyObjectHashTable.get_item (pandas/hashtable.c:12265)
  File "pandas/hashtable.pyx", line 676, in 
  pandas.hashtable.PyObjectHashTable.get_item (pandas/hashtable.c:12216)
  KeyError: ('Res_fs1', 'Res_fs2', 'Res_ps1', 'Res_ps2')
  </code></pre>