Pandas:将一组值向下移动一行 [英] Pandas: Shift down values by one row within a group

问题描述

我有一个 Pandas 数据框，我想创建一个新列，其值是另一列的值，向下移动一行.最后一行应显示 NaN.

问题是我想按组执行此操作，每组的最后一行显示 NaN.不要让组的最后一行从恰好在数据帧中相邻的组中窃取"一个值.

我尝试的实现很可耻地被破坏了，所以我显然误解了一些基本的东西.

df['B_shifted'] = df.groupby(['A'])['B'].transform(lambda x:x.values[1:])

解决方案

Shift 作用于 groupby 子句的输出:

<预><代码>>>>df = pandas.DataFrame(numpy.random.randint(1,3, (10,5)), columns=['a','b','c','d','e'])>>>dfa b c d0 2 1 2 1 11 2 1 1 1 12 1 2 2 1 23 1 2 1 1 24 2 2 1 1 25 2 2 2 2 16 2 2 1 1 17 2 2 2 1 18 2 2 2 2 19 2 2 2 2 1对于 df.groupby('a') 中的 k, v:打印 k打印正常"打印 v打印移位"打印 v.shift(1)1普通的a b c d2 1 2 2 1 23 1 2 1 1 2移位a b c d2 南南南南南南南3 1 2 2 1 22普通的a b c d0 2 1 2 1 11 2 1 1 1 14 2 2 1 1 25 2 2 2 2 16 2 2 1 1 17 2 2 2 1 18 2 2 2 2 19 2 2 2 2 1移位a b c d0 南南南南南南南1 2 1 2 1 14 2 1 1 1 15 2 2 1 1 26 2 2 2 2 17 2 2 1 1 18 2 2 2 1 19 2 2 2 2 1

I have a Pandas dataframe, and I want to create a new column whose values are that of another column, shifted down by one row. The last row should show NaN.

The catch is that I want to do this by group, with the last row of each group showing NaN. NOT have the last row of a group "steal" a value from a group that happens to be adjacent in the dataframe.

My attempted implementation is quite shamefully broken, so I'm clearly misunderstanding something fundamental.

df['B_shifted'] = df.groupby(['A'])['B'].transform(lambda x:x.values[1:])

解决方案

Shift works on the output of the groupby clause:

>>> df = pandas.DataFrame(numpy.random.randint(1,3, (10,5)), columns=['a','b','c','d','e'])
>>> df
   a  b  c  d  e
0  2  1  2  1  1
1  2  1  1  1  1
2  1  2  2  1  2
3  1  2  1  1  2
4  2  2  1  1  2
5  2  2  2  2  1
6  2  2  1  1  1
7  2  2  2  1  1
8  2  2  2  2  1
9  2  2  2  2  1


for k, v in df.groupby('a'):
    print k
    print 'normal'
    print v
    print 'shifted'
    print v.shift(1)

1
normal
   a  b  c  d  e
2  1  2  2  1  2
3  1  2  1  1  2
shifted
    a   b   c   d   e
2 NaN NaN NaN NaN NaN
3   1   2   2   1   2
2
normal
   a  b  c  d  e
0  2  1  2  1  1
1  2  1  1  1  1
4  2  2  1  1  2
5  2  2  2  2  1
6  2  2  1  1  1
7  2  2  2  1  1
8  2  2  2  2  1
9  2  2  2  2  1
shifted
    a   b   c   d   e
0 NaN NaN NaN NaN NaN
1   2   1   2   1   1
4   2   1   1   1   1
5   2   2   1   1   2
6   2   2   2   2   1
7   2   2   1   1   1
8   2   2   2   1   1
9   2   2   2   2   1

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