浅谈Pandas中map, applymap and apply的区别

In [116]: frame = DataFrame(np.random.randn(4, 3), columns=list('bde'), index=['Utah', 'Ohio', 'Texas', 'Oregon'])
In [117]: frame
Out[117]: 
        b     d     e
Utah  -0.029638 1.081563 1.280300
Ohio  0.647747 0.831136 -1.549481
Texas  0.513416 -0.884417 0.195343
Oregon -0.485454 -0.477388 -0.309548
In [118]: f = lambda x: x.max() - x.min()
In [119]: frame.apply(f)
Out[119]: 
b  1.133201
d  1.965980
e  2.829781
dtype: float64

In [120]: format = lambda x: '%.2f' % x
In [121]: frame.applymap(format)
Out[121]: 
      b   d   e
Utah  -0.03  1.08  1.28
Ohio   0.65  0.83 -1.55
Texas  0.51 -0.88  0.20
Oregon -0.49 -0.48 -0.31

In [122]: frame['e'].map(format)
Out[122]: 
Utah    1.28
Ohio   -1.55
Texas   0.20
Oregon  -0.31
Name: e, dtype: object