python數組切片

numpy 切片

In [8]: arr = np.array( [[1, 2], [3, 4]])
In [9]: arr
Out[9]:
array([[1, 2],
       [3, 4]])

In [10]: arr[0]  # 取第一行
Out[10]: array([1, 2])

In [11]: arr[:, 1]  # 取第二列, 用逗號隔開表示第二個切片或者索引，就像一個tuple2，第二個切片來表示y軸
Out[11]: array([2, 4])

pandas 如果直接對DataFrame進行切片，只能操作行，如果還要操作列需要使用pd.DataFrame.iloc:

In [24]: df = pd.DataFrame(data=[[1,2], [3,4]])
In [25]:
In [25]: df[:1]
Out[25]:
   0  1
0  1  2

In [27]: df[:1]  # 取第一行
Out[27]:
   0  1
0  1  2

In [28]: df[:1, 1]  # 如果要取列則報錯，需要使用iloc
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-28-e53f18cf1f5d> in <module>
----> 1 df[:1, 1]

~/miniconda3/envs/cooka/lib/python3.7/site-packages/pandas/core/frame.py in __getitem__(self, key)
   3022             if self.columns.nlevels > 1:
   3023                 return self._getitem_multilevel(key)
-> 3024             indexer = self.columns.get_loc(key)
   3025             if is_integer(indexer):
   3026                 indexer = [indexer]

~/miniconda3/envs/cooka/lib/python3.7/site-packages/pandas/core/indexes/range.py in get_loc(self, key, method, tolerance)
    352                 except ValueError as err:
    353                     raise KeyError(key) from err
--> 354             raise KeyError(key)
    355         return super().get_loc(key, method=method, tolerance=tolerance)
    356

KeyError: (slice(None, 1, None), 1)

In [29]: df.iloc[:1, 1]  # 使用iloc可以操作行或者列
Out[29]:
0    2
Name: 1, dtype: int64