import pandas as pd

import numpy as np
dates=pd.date_range('20130201',periods=6)

a=pd.DataFrame(np.arange(24).reshape((6,4)),index=dates,columns=['a','b','c','d'])
print(a)
a.loc[a['a']==12,'b']=5
print(a)
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a b c d
2013-02-01 0 1 2 3
2013-02-02 4 5 6 7
2013-02-03 8 9 10 11
2013-02-04 12 13 14 15
2013-02-05 16 17 18 19
2013-02-06 20 21 22 23
a b c d
2013-02-01 0 1 2 3
2013-02-02 4 5 6 7
2013-02-03 8 9 10 11
2013-02-04 12 5 14 15
2013-02-05 16 17 18 19
2013-02-06 20 21 22 23