python - 如何获取 pandas 系列的最后一列

Question

我正在尝试计算数组的频率。我读过这个post ，我正在使用 DataFrame 并得到一个系列。

>>> a = np.array([1, 1, 5, 0, 1, 2, 2, 0, 1, 4])
>>> df = pd.DataFrame(a, columns=['a'])
>>> b = df.groupby('a').size()
>>> b
a
0    2
1    4
2    2
4    1
5    1
dtype: int64
>>> b.iloc[:,-1]

当我尝试获取最后一列时，出现此错误。

Traceback (most recent call last):   File "<stdin>", line 1, in <module>   File "/Users/pan/anaconda3/lib/python3.6/site-packages/pandas/core/indexing.py", line 1472, in __getitem__
    return self._getitem_tuple(key)   File "/Users/pan/anaconda3/lib/python3.6/site-packages/pandas/core/indexing.py", line 2013, in _getitem_tuple
    self._has_valid_tuple(tup)   File "/Users/pan/anaconda3/lib/python3.6/site-packages/pandas/core/indexing.py", line 220, in _has_valid_tuple
    raise IndexingError('Too many indexers') pandas.core.indexing.IndexingError: Too many indexers

如何获取b的最后一列？

Answer 1

因为pandas.Series是一个

One-dimensional ndarray with axis labels

如果您只想获取频率列，即您的系列，使用:

b.tolist()

或者，或者:

b.to_dict()

保留标签和频率。

附注:

对于您的特定任务，还可以考虑集合包:

>>> from collections import Counter
>>> a = [1, 1, 5, 0, 1, 2, 2, 0, 1, 4]
>>> c = Counter(a)
>>> list(c.values())
[2, 4, 2, 1, 1]