python - 如何使用 "loc"和 "for"在 pandas 中添加行？

Question

我想通过“loc”将数据帧的数据添加到新数据帧中。我使用“loc”但发生了错误。我可以添加数据吗？

>>> import pandas as pd    
>>> df = pd.DataFrame({'A': [1.0, 1.2, 3.4, 4.1, 8.2]})
>>> import pandas as pd
>>> df_new = pd.DataFrame(columns=['A'])
>>> for i in df:
...     df_new.loc[i] = df.loc[i]
... 
Traceback (most recent call last):
  File "/Users/Hajime/anaconda3/lib/python3.6/site-packages/pandas/core/indexing.py", line 1434, in _has_valid_type
    error()
  File "/Users/Hajime/anaconda3/lib/python3.6/site-packages/pandas/core/indexing.py", line 1429, in error
    (key, self.obj._get_axis_name(axis)))
KeyError: 'the label [A] is not in the [index]'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/Users/Hajime/anaconda3/lib/python3.6/site-packages/pandas/core/indexing.py", line 1328, in __getitem__
    return self._getitem_axis(key, axis=0)
  File "/Users/Hajime/anaconda3/lib/python3.6/site-packages/pandas/core/indexing.py", line 1551, in _getitem_axis
    self._has_valid_type(key, axis)
  File "/Users/Hajime/anaconda3/lib/python3.6/site-packages/pandas/core/indexing.py", line 1442, in _has_valid_type
    error()
  File "/Users/Hajime/anaconda3/lib/python3.6/site-packages/pandas/core/indexing.py", line 1429, in error
    (key, self.obj._get_axis_name(axis)))
KeyError: 'the label [A] is not in the [index]'

但是下面的代码是成功的。

>>> df_new.loc[1] = df.loc[1]
>>> df_new
     A
1  1.2

Answer 1

为什么不看看 for 在这里迭代了什么？

In [353]: for i in df:
     ...:     print(i)
     ...:     
A

结论 - 对 df 的迭代会导致对列名称的迭代。您正在寻找类似于 df.iterrows 的内容，或者迭代 df.index 的内容。

例如，

for i, r in df.iterrows():
     df_new.loc[i, :] = r

df_new

     A
0  1.0
1  1.2
2  3.4
3  4.1
4  8.2