Pandas从groupby的结果中添加新列

Question

具有下面的数据帧，并希望添加一个新列，该列指示与groupby操作中的max关联的值。因此，我的DF将有许多这样的记录，我只想通过类似于df.groupby('B')['C'].mean().idxmax()的方式遍历每个组，但是遍历整个DF，然后创建一个新的col，结果如下：

           A     B   C    D
    0   05641  KMPV  45  056
    1   05649  KMPV  45  056
    2   05650  KMPV  45  056
    3   05651  KMPV  45  056
    4   05653  KMPV  45  056
    5   05678  KMPV  45  056
    6   05654  KMPV  45  056
    7   05609  KMPV  45  056
    8   05620  KMPV  45  056
    9   05633  KMPV  45  056
    10  05601  KMPV  45  056
    11  05602  KMPV  45  056
    12  05603  KMPV  45  056
    13  05662  KMVL  23  056
    14  05665  KMVL  23  056
    15  05672  KMVL  23  056
    16  05677  KMVL  23  056
    17  05648  KMVL  23  056
    18  05652  KMVL  23  056
    19  05680  KMVL  23  056
    20  05655  KMVL  23  056
    21  05656  KMVL  23  056
    22  05657  KMVL  23  056
    23  05661  KMVL  23  056
    24  05681  KMVL  23  056
    25  05682  KMVL  23  056
    26  05647  K1V4  30  056
    27  05658  K1V4  30  056

新的数据帧，其中的列指示KMPV具有最高值。在

^{pr2}$

更简单地说。如何从下面的groupby中选择max？换句话说，我想过滤到056->；KMPV和100->；KJRB，因为它们的数字最高，依此类推。在

     A               B
    056            K1V4        30
                   KMPV        45
                   KMVL        23
    100            KJRB       130
                   KNYC        84
                   KTEB        80

Answer 1

又快又脏。在

数据：

{'A': {0: 5641, 1: 5649, 2: 5650, 3: 5651, 4: 5653, 5: 5678, 6: 5654, 7: 5609, 8: 5620, 9: 5633, 10: 5601, 11: 5602, 12: 5603, 13: 5662, 14: 5665, 15: 5672, 16: 5677, 17: 5648, 18: 5652, 19: 5680, 20: 10155, 21: 10156, 22: 10157, 23: 10161, 24: 10181, 25: 10182, 26: 10147, 27: 10158}, 'C': {0: 45, 1: 45, 2: 45, 3: 45, 4: 45, 5: 45, 6: 45, 7: 45, 8: 45, 9: 45, 10: 45, 11: 45, 12: 45, 13: 23, 14: 23, 15: 23, 16: 23, 17: 23, 18: 23, 19: 23, 20: 23, 21: 23, 22: 23, 23: 23, 24: 23, 25: 23, 26: 30, 27: 30}, 'B': {0: 'KMPV', 1: 'KMPV', 2: 'KMPV', 3: 'KMPV', 4: 'KMPV', 5: 'KMPV', 6: 'KMPV', 7: 'KMPV', 8: 'KMPV', 9: 'KMPV', 10: 'KMPV', 11: 'KMPV', 12: 'KMPV', 13: 'KMVL', 14: 'KMVL', 15: 'KMVL', 16: 'KMVL', 17: 'KMVL', 18: 'KMVL', 19: 'KMVL', 20: 'KMVL', 21: 'KMVL', 22: 'KMVL', 23: 'KMVL', 24: 'KMVL', 25: 'KMVL', 26: 'K1V4', 27: 'K1V4'}, 'D': {0: 56, 1: 56, 2: 56, 3: 56, 4: 56, 5: 56, 6: 56, 7: 56, 8: 56, 9: 56, 10: 56, 11: 56, 12: 56, 13: 56, 14: 56, 15: 56, 16: 56, 17: 56, 18: 56, 19: 56, 20: 101, 21: 101, 22: 101, 23: 101, 24: 101, 25: 101, 26: 101, 27: 101}}

代码：

^{pr2}$

结果：

        A     B   C    D newval
0    5641  KMPV  45   56   KMPV
...
13   5662  KMVL  23   56   KMPV
...
20  10155  KMVL  23  101   K1V4
...
26  10147  K1V4  30  101   K1V4
27  10158  K1V4  30  101   K1V4