Use idxmax for indexing in pandas - python-3.x

Question

Here is what I am trying to do:
In [7]: from pandas import DataFrame, Series
In [8]: import pandas as pd
In [9]: import numpy as np
In [10]: df = DataFrame([[1.4, np.nan], [7.1, -4.5],
[np.nan, np.nan], [0.75, -1.3]],
index=['a', 'b', 'c', 'd'],
columns=['one', 'two'])
Out[10]:
one two
a 1.40 NaN
b 7.10 -4.5
c NaN NaN
d 0.75 -1.3
In [11]: df.idxmax()
Out[11]:
one b
two d
dtype: object
In [12]: df[df.idxmax()] = -9.99
---------------------------------------------------------------------------
KeyError Traceback (most recent call last)
<ipython-input-12-018b077daf48> in <module>()
----> 1 df[df.idxmax()] = -9.99
/usr/local/lib/python3.4/site-packages/pandas/core/frame.py in __setitem__(self, key, value)
2103
2104 if isinstance(key, (Series, np.ndarray, list, Index)):
-> 2105 self._setitem_array(key, value)
2106 elif isinstance(key, DataFrame):
2107 self._setitem_frame(key, value)
/usr/local/lib/python3.4/site-packages/pandas/core/frame.py in _setitem_array(self, key, value)
2131 self[k1] = value[k2]
2132 else:
-> 2133 indexer = self.ix._convert_to_indexer(key, axis=1)
2134 self._check_setitem_copy()
2135 self.ix._setitem_with_indexer((slice(None), indexer), value)
/usr/local/lib/python3.4/site-packages/pandas/core/indexing.py in _convert_to_indexer(self, obj, axis, is_setter)
1141 if isinstance(obj, tuple) and is_setter:
1142 return {'key': obj}
-> 1143 raise KeyError('%s not in index' % objarr[mask])
1144
1145 return _values_from_object(indexer)
KeyError: "['b' 'd'] not in index"
Intuitively this should work, but it doesn't. Any workarounds?

Answer 1

You should iterate over the series and access the index and col name to set the values:
In [30]:
for items in df.idxmax().iteritems():
print(items)
df.loc[items[1], items[0]] = -9.9
df
('one', 'b')
('two', 'd')
Out[30]:
one two
a 1.40 NaN
b -9.90 -4.5
c NaN NaN
d 0.75 -9.9
I've printed the items to show what the contents are