【发布时间】：2017-08-25 18:53:40
【问题描述】：
我正在尝试以下代码
df = pd.DataFrame([[23, 52], [36, 49], [52, 61], [75, 82], [97, 12]], columns=['A', 'B'])
df['C'] = np.where(df['A'] > df['C'].shift(), df['A'], df['C'].shift())
print(df)
假设第一个df['C].shift() 操作应该假设为0（因为df['C'] 不存在）
预期输出
A B C
0 23 52 23
1 36 49 36
2 12 61 36
3 75 82 75
4 70 12 75
但我遇到了 KeyError 异常。
Traceback (most recent call last):
File "C:\Program Files\Python36\lib\site-packages\pandas\core\indexes\base.py", line 2442, in get_loc
return self._engine.get_loc(key)
File "pandas\_libs\index.pyx", line 132, in pandas._libs.index.IndexEngine.get_loc (pandas\_libs\index.c:5280)
File "pandas\_libs\index.pyx", line 154, in pandas._libs.index.IndexEngine.get_loc (pandas\_libs\index.c:5126)
File "pandas\_libs\hashtable_class_helper.pxi", line 1210, in pandas._libs.hashtable.PyObjectHashTable.get_item (pandas\_libs\hashtable.c:20523)
File "pandas\_libs\hashtable_class_helper.pxi", line 1218, in pandas._libs.hashtable.PyObjectHashTable.get_item (pandas\_libs\hashtable.c:20477)
KeyError: 'C'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "C:\Users\Development\workspace\TestPython\TestPython.py", line 6, in <module>
df['C'] = np.where(df['A'] > df['C'].shift(), df['B'].shift(), df['A'])
File "C:\Program Files\Python36\lib\site-packages\pandas\core\frame.py", line 1964, in __getitem__
return self._getitem_column(key)
File "C:\Program Files\Python36\lib\site-packages\pandas\core\frame.py", line 1971, in _getitem_column
return self._get_item_cache(key)
File "C:\Program Files\Python36\lib\site-packages\pandas\core\generic.py", line 1645, in _get_item_cache
values = self._data.get(item)
File "C:\Program Files\Python36\lib\site-packages\pandas\core\internals.py", line 3590, in get
loc = self.items.get_loc(item)
File "C:\Program Files\Python36\lib\site-packages\pandas\core\indexes\base.py", line 2444, in get_loc
return self._engine.get_loc(self._maybe_cast_indexer(key))
File "pandas\_libs\index.pyx", line 132, in pandas._libs.index.IndexEngine.get_loc (pandas\_libs\index.c:5280)
File "pandas\_libs\index.pyx", line 154, in pandas._libs.index.IndexEngine.get_loc (pandas\_libs\index.c:5126)
File "pandas\_libs\hashtable_class_helper.pxi", line 1210, in pandas._libs.hashtable.PyObjectHashTable.get_item (pandas\_libs\hashtable.c:20523)
File "pandas\_libs\hashtable_class_helper.pxi", line 1218, in pandas._libs.hashtable.PyObjectHashTable.get_item (pandas\_libs\hashtable.c:20477)
KeyError: 'C'
据我了解，这种情况正在发生，因为 C 列第一次不存在，因此移动列会引发此异常。
我的问题是否有解决此问题的替代方法？
【问题讨论】：
-
Er.. 你创建了一个没有“C”列的 df，所以你得到一个
KeyError也就不足为奇了。什么是预期的输出？您不应该使用“B”列吗？ -
你的意思是
np.where(df['A'] > df['B'].shift(), df['B'].shift(), df['A'])？ -
我需要这种方式。所以，
df.loc[0: 'C']可以是0或df['A']，然后从df.loc[1: 'C']df['C'] = np.where(df['A'] > df['C'].shift(), df['C'].shift(), df['A'])接管 -
对于上面的df，请给我们一些想要的输出。换班可能不是办法。
-
@cᴏʟᴅsᴘᴇᴇᴅ 我已经更新了原始帖子以显示预期的输出应该是什么。如果
shift不是这样，那么另一个建议将不胜感激。谢谢