Question

您好我有一个具有2级多索引和一列的DataFrame / Series。我想采用二级索引并将其用作列。例如（代码取自multi-index docs）：

import pandas as pd
import numpy as np

arrays = [['bar', 'bar', 'baz', 'baz', 'foo', 'foo', 'qux', 'qux'],
          ['one', 'two', 'one', 'two', 'one', 'two', 'one', 'two']]
tuples = list(zip(*arrays))
index = pd.MultiIndex.from_tuples(tuples, names=['first', 'second'])
s = pd.DataFrame(np.random.randn(8), index=index, columns=["col"])

看起来像：

first  second
bar    one      -0.982656
       two      -0.078237
baz    one      -0.345640
       two      -0.160661
foo    one      -0.605568
       two      -0.140384
qux    one       1.434702
       two      -1.065408
dtype: float64

我希望拥有一个索引为[bar, baz, foo, qux]和列[one, two]的数据框。

Answer 1

你只需要unstack你的系列剧：

>>> s.unstack(level=1)
second       one       two
first                     
bar    -0.713374  0.556993
baz     0.523611  0.328348
foo     0.338351 -0.571854
qux     0.036694 -0.161852

Answer 2

这是使用数组重塑的解决方案 -

>>> idx = s.index.levels
>>> c = len(idx[1])
>>> pd.DataFrame(s.values.reshape(-1,c),index=idx[0].values, columns=idx[1].values)
          one       two
bar  2.225401  1.624866
baz  1.067359  0.349440
foo -0.468149 -0.352303
qux  1.215427  0.429146

如果您不关心索引顶部显示的名称 -

>>> pd.DataFrame(s.values.reshape(-1,c), index=idx[0], columns=idx[1])
second       one       two
first                     
bar     2.225401  1.624866
baz     1.067359  0.349440
foo    -0.468149 -0.352303
qux     1.215427  0.429146

给定数据集大小的计时 -

# @AChampion's solution
In [201]: %timeit s.unstack(level=1)
1000 loops, best of 3: 444 µs per loop

# Using array reshaping step-1
In [199]: %timeit s.index.levels
1000000 loops, best of 3: 214 ns per loop

# Using array reshaping step-2    
In [202]: %timeit pd.DataFrame(s.values.reshape(-1,2), index=idx[0], columns=idx[1])
10000 loops, best of 3: 47.3 µs per loop

Answer 3

另一个强大的解决方案是使用.reset_index和.pivot：

levels= [['bar', 'baz'], ['one', 'two', 'three']]
index = pd.MultiIndex.from_product(levels, names=['first', 'second'])
series = pd.Series(np.random.randn(6), index)

df = series.reset_index()
# Shorthand notation instead of explicitly naming index, columns and values
df = df.pivot(*df.columns)

结果：

second       one     three       two
first                               
bar     1.047692  1.209063  0.891820
baz     0.083602 -0.303528 -1.385458

使用第二个索引作为列

3 个答案: