如何转置pandas数据帧以交叉制表保存所有值的数据帧

Question

我们假设我们有这样的数据帧:

df = pd.DataFrame({'key' : ['one', 'two', 'three', 'four'] * 3,
                   'col' : ['A', 'B', 'C'] * 4,
                   'val1' : np.random.randn(12),
                   'val2' : np.random.randn(12),
                   'val3' : np.random.randn(12)})

key + col 是唯一的关键

我想让col值成为拆分列或交叉列表,最后看起来像这样:

第一个天真的方法pd.crosstab(df.key,df.col)在这里不起作用:

此代码pd.crosstab(df.key,df.col,values = df[['val1', 'val2', 'val3']], aggfunc = np.max)无法运行ValueError: Wrong number of items passed 3, placement implies 1

怎么运作？

Answer 1

使用pivot_table与swaplevel和sort_index使用聚合函数np.max:

df = (df.pivot_table(index='key', columns='col', aggfunc=np.max)
       .swaplevel(0,1,axis=1)
       .sort_index(axis=1))

替代方案是聚合GroupBy.max:

df = (df.groupby(['key', 'col'])
        .max()
        .unstack()
        .swaplevel(0,1,axis=1)
        .sort_index(axis=1))

print (df)
col           A                             B                             C  \
           val1      val2      val3      val1      val2      val3      val1   
key                                                                           
four  -0.225967  0.362041  0.040915 -1.227718 -0.879248 -1.279912 -1.577218   
one   -0.187167  1.530731 -1.112116 -0.871077 -2.099876 -0.069297 -0.351971   
three -0.165375 -0.378049 -0.390724  0.484519 -0.408990 -1.496042  0.590083   
two    1.923084 -0.688284  1.702659 -0.159921  0.635245  0.623821 -1.503893   

col                        
           val2      val3  
key                        
four  -1.135872  0.645371  
one    2.347472  0.129252  
three  0.402825  0.883710  
two   -0.132847  0.179476  

Answer 2

使用melt，set_index和unstack仅当您期望每个像元的值时才可以使用，否则，可以使用第二个选项来聚合值：

df.melt(['key','col'])\
  .set_index(['key','col','variable'])['value']\
  .unstack([1,2])\
  .sort_index(axis=1)

输出：

col              A                             B                             C                    
variable      val1      val2      val3      val1      val2      val3      val1      val2      val3
key                                                                                               
four     -1.964246  0.958854 -0.605128  0.055120 -1.144306 -0.800712 -0.917324 -0.581882 -0.152399
one       0.513347 -1.689448 -2.434481  0.990924 -1.014848  0.713703  1.344299  0.052877  1.174183
three    -0.156336 -0.156157 -2.253689  0.877726 -0.686758 -0.407892  0.816636  1.008870 -0.390872
two       1.942495  1.811712 -0.762283 -2.169613 -1.073372  0.201996 -1.073370 -0.902032 -0.168796

使用melt和的另一种选择pd.crosstab：

df1 = df.melt(['key','col'])
pd.crosstab(df1.key, [df1.col, df1.variable], df1.value, aggfunc=np.max)

输出：

col              A                             B                             C                    
variable      val1      val2      val3      val1      val2      val3      val1      val2      val3
key                                                                                               
four     -1.964246  0.958854 -0.605128  0.055120 -1.144306 -0.800712 -0.917324 -0.581882 -0.152399
one       0.513347 -1.689448 -2.434481  0.990924 -1.014848  0.713703  1.344299  0.052877  1.174183
three    -0.156336 -0.156157 -2.253689  0.877726 -0.686758 -0.407892  0.816636  1.008870 -0.390872
two       1.942495  1.811712 -0.762283 -2.169613 -1.073372  0.201996 -1.073370 -0.902032 -0.168796