Grouping and Sorting

Table of contents

1. Grouping (groupby())
   1. useful Groupby methods
2. Sorting
   1. sort by values of rows/columns
   2. sort by index of row/column

import pandas as pd

Grouping (groupby())

1. groupby() created a group of (primitive) dataframe which allotted the same column values to the rows. (if axis='index')
2. return 되는 객체(pandas.core.groupby)의 구조 : We generate each group as being a slice of our DataFrame containing only data with values that match. : 기준열(행)의 값에 따라 dataframe 을 분류(분리)해 놓은 것.
3. method 작동 구조 (roughly) : 원래의 dataframe 을 기준에 따라 여러 dataframe 덩어리로 나누고, 각각의 dataframe 에 대하여 덩어리씩 method() 적용.

groupby() 의 return 인 GroupBy 자체보다는 이 객체가 가지고 있는 수많은 method 가 중요하다.
(ex) dataframe.groupby('column_name').column_name.count() == dataframe.column_name.value_counts() : just shortcut

DataFrame.groupby(by, axis)         # df.groupby('column_name')

pandas document - pandas.DataFrame.groupby

1. by : mapping, function, label, or list of labels
   • list of labels : multiple classification criteria -> multi-index dataframe
2. axis: {0 or ‘index’, 1 or ‘columns’}, default 0

return : pandas.core.groupby.generic.DataFrameGroupBy

useful Groupby methods

Groupby.agg([callable, callable, ...])              # 한꺼번에 적용
DataFrameGroupBy.coloumn_name.count() == DataFrameGroupBy.size()
DataFrameGroupBy.min()
DataFrameGroupBy.describe()
DataFrameGroupBy.first()
DataFrameGroupBy.last()
DataFrameGroupBy.apply()

Sorting

sort by values of rows/columns

DataFrame.sort_values(by, axis, ascending, inplace)

pandas document - pandas.DataFrame.sort_values

1. by : str or list of str
   • list of str -> multiple criteria
2. axis : {0 or ‘index’, 1 or ‘columns’}, default 0
3. ascending : bool or list of bool, default True
   • list of bool -> must match the length of the by
4. inplace : bool, default False

sort by index of row/column

DataFrame.sort_index(axis, level, ascending, inplace)

pandas document - pandas.DataFrame.sort_index

1. axis : {0 or ‘index’, 1 or ‘columns’}, default 0
2. level : int or level name or list of ints or list of level names
3. ascending : bool or list-like of bools, default True
   • list-like of bool -> for multiindex, indivisually
4. inplace : bool, default False