【python】详解pandas.DataFrame.loc函数

官方函数

DataFrame.loc
Access a group of rows and columns by label(s) or a boolean array.
.loc[] is primarily label based, but may also be used with a boolean array.
# 可以使用label值，但是也可以使用布尔值

Allowed inputs are: # 可以接受单个的label，多个label的列表，多个label的切片

•A single label, e.g. 5 or ‘a’, (note that 5 is interpreted as a label of the index, and never as an integer position along the index). #这里的5不是数值指定的位置，而是label值
•A list or array of labels, e.g. [‘a’, ‘b’, ‘c’].
•A slice object with labels, e.g. ‘a’:’f’.

Warning: #如果使用多个label的切片，那么切片的起始位置都是包含的

Note that contrary to usual python slices, both the start and the stop are included
•A boolean array of the same length as the axis being sliced, e.g. [True, False, True].

实例详解

一、选择数值

1、生成df

df = pd.DataFrame([[1, 2], [4, 5], [7, 8]],
...
index=['cobra', 'viper', 'sidewinder'],
...
columns=['max_speed', 'shield'])
df
Out[15]:
max_speed
shield
cobra
1
2
viper
4
5
sidewinder
7
8

2、Single label. 单个 row_label 返回的Series

 df.loc['viper']
Out[17]:
max_speed
4
shield
5
Name: viper, dtype: int64

2、List of labels. 列表 row_label 返回的DataFrame

df.loc[['cobra','viper']]
Out[20]:
max_speed
shield
cobra
1
2
viper
4
5

3、Single label for row and column 同时选定行和列

df.loc['cobra', 'shield']
Out[24]: 2

4、Slice with labels for row and single label for column. As mentioned above, note that both the start and stop of the slice are included. 同时选定多个行和单个列，注意的是通过列表选定多个row label 时，首位均是选定的。

df.loc['cobra':'viper', 'max_speed']
Out[25]:
cobra
1
viper
4
Name: max_speed, dtype: int64

5、Boolean list with the same length as the row axis 布尔列表选择row label

布尔值列表是根据某个位置的True or False 来选定，如果某个位置的布尔值是True，则选定该row

df
Out[30]:
max_speed
shield
cobra
1
2
viper
4
5
sidewinder
7
8
df.loc[[True]]
Out[31]:
max_speed
shield
cobra
1
2
df.loc[[True,False]]
Out[32]:
max_speed
shield
cobra
1
2
df.loc[[True,False,True]]
Out[33]:
max_speed
shield
cobra
1
2
sidewinder
7
8

6、Conditional that returns a boolean Series 条件布尔值

df.loc[df['shield'] > 6]
Out[34]:
max_speed
shield
sidewinder
7
8

7、Conditional that returns a boolean Series with column labels specified 条件布尔值和具体某列的数据

df.loc[df['shield'] > 6, ['max_speed']]
Out[35]:
max_speed
sidewinder
7

8、Callable that returns a boolean Series 通过函数得到布尔结果选定数据

df
Out[37]:
max_speed
shield
cobra
1
2
viper
4
5
sidewinder
7
8
df.loc[lambda df: df['shield'] == 8]
Out[38]:
max_speed
shield
sidewinder
7
8

二、赋值

1、Set value for all items matching the list of labels 根据某列表选定的row 及某列 column 赋值

df.loc[['viper', 'sidewinder'], ['shield']] = 50
df
Out[43]:
max_speed
shield
cobra
1
2
viper
4
50
sidewinder
7
50

2、Set value for an entire row 将某行row的数据全部赋值

df.loc['cobra'] =10
df
Out[48]:
max_speed
shield
cobra
10
10
viper
4
50
sidewinder
7
50

3、Set value for an entire column 将某列的数据完全赋值

df.loc[:, 'max_speed'] = 30
df
Out[50]:
max_speed
shield
cobra
30
10
viper
30
50
sidewinder
30
50

4、Set value for rows matching callable condition 条件选定rows赋值

df.loc[df['shield'] > 35] = 0
df
Out[52]:
max_speed
shield
cobra
30
10
viper
0
0
sidewinder
0
0

三、行索引是数值

df = pd.DataFrame([[1, 2], [4, 5], [7, 8]],
...
index=[7, 8, 9], columns=['max_speed', 'shield'])
df
Out[54]:
max_speed
shield
7
1
2
8
4
5
9
7
8

通过 行 rows的切片的方式取多个：

df.loc[7:9]
Out[55]:
max_speed
shield
7
1
2
8
4
5
9
7
8

四、多维索引

1、生成多维索引

tuples = [
...
('cobra', 'mark i'), ('cobra', 'mark ii'),
...
('sidewinder', 'mark i'), ('sidewinder', 'mark ii'),
...
('viper', 'mark ii'), ('viper', 'mark iii')
... ]
index = pd.MultiIndex.from_tuples(tuples)
values = [[12, 2], [0, 4], [10, 20],
...
[1, 4], [7, 1], [16, 36]]
df = pd.DataFrame(values, columns=['max_speed', 'shield'], index=index)
df
Out[57]:
max_speed
shield
cobra
mark i
12
2
mark ii
0
4
sidewinder mark i
10
20
mark ii
1
4
viper
mark ii
7
1
mark iii
16
36

2、Single label. 传入的就是最外层的row label，返回DataFrame

df.loc['cobra']
Out[58]:
max_speed
shield
mark i
12
2
mark ii
0
4

3、Single index tuple.传入的是索引元组，返回Series

df.loc[('cobra', 'mark ii')]
Out[59]:
max_speed
0
shield
4
Name: (cobra, mark ii), dtype: int64

4、Single label for row and column.如果传入的是row和column，和传入tuple是类似的，返回Series

df.loc['cobra', 'mark i']
Out[60]:
max_speed
12
shield
2
Name: (cobra, mark i), dtype: int64

5、Single tuple. Note using [[ ]] returns a DataFrame.传入一个数组，返回一个DataFrame

df.loc[[('cobra', 'mark ii')]]
Out[61]:
max_speed
shield
cobra mark ii
0
4

6、Single tuple for the index with a single label for the column 获取某个colum的某row的数据，需要左边传入多维索引的tuple，然后再传入column

df.loc[('cobra', 'mark i'), 'shield']
Out[62]: 2

7、传入多维索引和单个索引的切片：

df.loc[('cobra', 'mark i'):'viper']
Out[63]:
max_speed
shield
cobra
mark i
12
2
mark ii
0
4
sidewinder mark i
10
20
mark ii
1
4
viper
mark ii
7
1
mark iii
16
36
df.loc[('cobra', 'mark i'):'sidewinder']
Out[64]:
max_speed
shield
cobra
mark i
12
2
mark ii
0
4
sidewinder mark i
10
20
mark ii
1
4
df.loc[('cobra', 'mark i'):('sidewinder','mark i')]
Out[65]:
max_speed
shield
cobra
mark i
12
2
mark ii
0
4
sidewinder mark i
10
20

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