pandas 离开加入并更新现有专栏 [英] pandas left join and update existing column

问题描述

我是熊猫新手，似乎无法通过合并功能使用它:

I am new to pandas and can't seem to get this to work with merge function:

>>> left       >>> right
   a  b   c       a  c   d 
0  1  4   9    0  1  7  13
1  2  5  10    1  2  8  14
2  3  6  11    2  3  9  15
3  4  7  12    

在列a上有一个左联接，我想通过联接键来更新公共列.请注意，c列中的最后一个值来自LEFT表，因为没有匹配项.

With a left join on column a, I would like to update common columns BY THE JOINED KEYS. Note last value in column c is from LEFT table since there is no match.

>>> final       
   a  b   c   d 
0  1  4   7   13
1  2  5   8   14
2  3  6   9   15
3  4  7   12  NAN 

如何使用Pandas合并功能执行此操作?谢谢.

How should I do this with Pandas merge function? Thank you.

推荐答案

一种方法是将一列设置为索引，然后

One way to do this is to set the a column as the index and update:

In [11]: left_a = left.set_index('a')

In [12]: right_a = right.set_index('a')

注意:update仅执行左连接(不合并)，因此，除了set_index之外，还需要包括left_a中不存在的其他列.

Note: update only does a left join (not merges), so as well as set_index you also need to include the additional columns not present in left_a.

In [13]: res = left_a.reindex(columns=left_a.columns.union(right_a.columns))

In [14]: res.update(right_a)

In [15]: res.reset_index(inplace=True)

In [16]: res
Out[16]:
   a   b   c   d
0  1   4   7  13
1  2   5   8  14
2  3   6   9  15
3  4   7  12 NaN

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