Pandas index操作索引

索引（index）是 Pandas 的重要工具，通過索引可以從 DataFame 中選擇特定的行數和列數，這種選擇數據的方式稱為“子集選擇”。

在 Pandas 中，索引值也被稱為標簽（label），它在 Jupyter 筆記本中以粗體字進行顯示。索引可以加快數據訪問的速度，它就好比數據的書簽，通過它可以實現數據的快速查找。

創建索引

通過示例對 index 索引做進一步講解。下面創建一個帶有 index 索引的數據，并使用 read_csv() 這些讀取數據：

import pandas as pd
df = pd.read_excel("C:\\Users\\qwy\\Desktop\\data\\empdata.xlsx")
print(df)

輸出結果：

    EMPNO   ENAME        JOB     MGR    HIREDATE   SAL    COMM  DEPTNO
0    7369   SMITH      CLERK  7902.0  1980-12-17   800     NaN      20
1    7499   ALLEN   SALESMAN  7698.0  1981-02-20  1600   300.0      30
2    7521    WARD   SALESMAN  7698.0  1981-02-22  1250   500.0      30
3    7566   JONES    MANAGER  7839.0  1981-04-02  2975     NaN      20
4    7654  MARTIN   SALESMAN  7698.0  1981-09-28  1250  1400.0      30
5    7698   BLAKE    MANAGER  7839.0  1981-05-01  2850     NaN      30
6    7782   CLARK    MANAGER  7839.0  1981-06-09  2450     NaN      10
7    7788   SCOTT    ANALYST  7566.0  1987-04-19  3000     NaN      20
8    7839    KING  PRESIDENT     NaN  1981-11-17  5000     NaN      10
9    7844  TURNER   SALESMAN  7698.0  1981-09-08  1500     0.0      30
10   7876   ADAMS      CLERK  7788.0  1987-05-23  1100     NaN      20
11   7900   JAMES      CLERK  7698.0  1981-12-03   950     NaN      30
12   7902    FORD    ANALYST  7566.0  1981-12-03  3000     NaN      20
13   7934  MILLER      CLERK  7782.0  1982-01-23  1300     NaN      10

通過列索引（標簽）讀取多列數據。

import pandas as pd
df = pd.read_excel("C:\\Users\\qwy\\Desktop\\data\\empdata.xlsx")print('讀取EMPNO、ENAME、JOB、MGR：\n',df[['EMPNO','ENAME','JOB','MGR']])
# 或使用loc函數
#print('讀取EMPNO、ENAME、JOB、MGR：\n',df.loc[:,['EMPNO','ENAME','JOB','MGR']])

輸出結果：

讀取EMPNO、ENAME、JOB、MGR：EMPNO   ENAME        JOB     MGR
0    7369   SMITH      CLERK  7902.0
1    7499   ALLEN   SALESMAN  7698.0
2    7521    WARD   SALESMAN  7698.0
3    7566   JONES    MANAGER  7839.0
4    7654  MARTIN   SALESMAN  7698.0
5    7698   BLAKE    MANAGER  7839.0
6    7782   CLARK    MANAGER  7839.0
7    7788   SCOTT    ANALYST  7566.0
8    7839    KING  PRESIDENT     NaN
9    7844  TURNER   SALESMAN  7698.0
10   7876   ADAMS      CLERK  7788.0
11   7900   JAMES      CLERK  7698.0
12   7902    FORD    ANALYST  7566.0
13   7934  MILLER      CLERK  7782.0

再看一組簡單的示例：

import pandas as pd
df = pd.read_excel("C:\\Users\\qwy\\Desktop\\data\\empdata.xlsx",index_col='EMPNO')print('讀取ENAME、JOB、MGR：\n',df[['ENAME','JOB','MGR']])

輸出結果：

讀取EMPNO、ENAME、JOB、MGR：ENAME        JOB     MGR
EMPNO                           
7369    SMITH      CLERK  7902.0
7499    ALLEN   SALESMAN  7698.0
7521     WARD   SALESMAN  7698.0
7566    JONES    MANAGER  7839.0
7654   MARTIN   SALESMAN  7698.0
7698    BLAKE    MANAGER  7839.0
7782    CLARK    MANAGER  7839.0
7788    SCOTT    ANALYST  7566.0
7839     KING  PRESIDENT     NaN
7844   TURNER   SALESMAN  7698.0
7876    ADAMS      CLERK  7788.0
7900    JAMES      CLERK  7698.0
7902     FORD    ANALYST  7566.0
7934   MILLER      CLERK  7782.0

設置索引

set_index() 將已存在的列標簽設置為 DataFrame 行索引。除了可以添加索引外，也可以替換已經存在的索引。比如您也可以把 Series 或者一個 DataFrme 設置成另一個 DataFrame 的索引。示例如下：

import pandas as pd
df = pd.read_excel("C:\\Users\\qwy\\Desktop\\data\\empdata.xlsx")
print('讀取ENAME、JOB、MGR：\n',df[['EMPNO','ENAME','JOB','MGR']].set_index(['EMPNO']))

輸出結果：

讀取ENAME、JOB、MGR：ENAME        JOB     MGR
EMPNO                           
7369    SMITH      CLERK  7902.0
7499    ALLEN   SALESMAN  7698.0
7521     WARD   SALESMAN  7698.0
7566    JONES    MANAGER  7839.0
7654   MARTIN   SALESMAN  7698.0
7698    BLAKE    MANAGER  7839.0
7782    CLARK    MANAGER  7839.0
7788    SCOTT    ANALYST  7566.0
7839     KING  PRESIDENT     NaN
7844   TURNER   SALESMAN  7698.0
7876    ADAMS      CLERK  7788.0
7900    JAMES      CLERK  7698.0
7902     FORD    ANALYST  7566.0
7934   MILLER      CLERK  7782.0

重置索引

您可以使用 reset_index() 來恢復初始行索引，示例如下：

import pandas as pd
df = pd.read_excel("C:\\Users\\qwy\\Desktop\\data\\empdata.xlsx")
print('讀取ENAME、JOB、MGR：\n',df[['EMPNO','ENAME','JOB','MGR']].set_index(['EMPNO']))
print('重置后的索引：\n',df[['EMPNO','ENAME','JOB','MGR']].reset_index())

輸出結果：

讀取ENAME、JOB、MGR：ENAME        JOB     MGR
EMPNO                           
7369    SMITH      CLERK  7902.0
7499    ALLEN   SALESMAN  7698.0
7521     WARD   SALESMAN  7698.0
7566    JONES    MANAGER  7839.0
7654   MARTIN   SALESMAN  7698.0
7698    BLAKE    MANAGER  7839.0
7782    CLARK    MANAGER  7839.0
7788    SCOTT    ANALYST  7566.0
7839     KING  PRESIDENT     NaN
7844   TURNER   SALESMAN  7698.0
7876    ADAMS      CLERK  7788.0
7900    JAMES      CLERK  7698.0
7902     FORD    ANALYST  7566.0
7934   MILLER      CLERK  7782.0
重置后的索引：index  EMPNO   ENAME        JOB     MGR
0       0   7369   SMITH      CLERK  7902.0
1       1   7499   ALLEN   SALESMAN  7698.0
2       2   7521    WARD   SALESMAN  7698.0
3       3   7566   JONES    MANAGER  7839.0
4       4   7654  MARTIN   SALESMAN  7698.0
5       5   7698   BLAKE    MANAGER  7839.0
6       6   7782   CLARK    MANAGER  7839.0
7       7   7788   SCOTT    ANALYST  7566.0
8       8   7839    KING  PRESIDENT     NaN
9       9   7844  TURNER   SALESMAN  7698.0
10     10   7876   ADAMS      CLERK  7788.0
11     11   7900   JAMES      CLERK  7698.0
12     12   7902    FORD    ANALYST  7566.0
13     13   7934  MILLER      CLERK  7782.0