Question

在熊猫＆＃39; print incremented value is 3 Returning 42 t3 returned 42方法，sort_values参数仅在对单个列或标签进行排序时应用。为什么这样，以及在未应用kind参数的情况下使用什么排序算法？它是稳定的吗？

（有关文档，请参阅https://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.sort_values.html。）

Answer 1

这是docstring from the source file，声明get_group_index_sorter(group_index, ngroups)：

algos.groupsort_indexer implements `counting sort` and it is at least
O(ngroups), where
    ngroups = prod(shape)
    shape = map(len, keys)
that is, linear in the number of combinations (cartesian product) of unique
values of groupby keys. This can be huge when doing multi-key groupby.
np.argsort(kind='mergesort') is O(count x log(count)) where count is the
length of the data-frame;

Both algorithms are `stable` sort and that is necessary for correctness of

groupby operations. e.g. consider:
    df.groupby(key)[col].transform('first')

PS这里是一个“调用链”：

pandas.core.frame.DataFrame.sort_values() -> \
  pandas.core.sorting.lexsort_indexer() ->  \
    pandas.core.sorting.indexer_from_factorized() -> \
      pandas.core.sorting.get_group_index_sorter()

当未应用kind参数时，Panda的sort_values使用的排序算法

1 个答案: