我想从索引i开始添加（最好是求和的平均值）几个列值，
investmentlength=list(range(1,13,1))
returns=list()
for i in range(0,len(stocks2)):
if stocks2['Startpoint'][i]==1:
nextmonth=nextmonth+stocks2['RET'][i+1]+stocks2['RET'][i+2]+stocks2['RET'][i+3]+....
counter+=1
有没有一种方法可以给出开始索引，结束索引和概率步长，然后在一个命令中将其全部求和，而不是复制并粘贴到死机？我想遍历所有不同的投资期限，并将平均收益率放在空列表中。
SHRCD EXCHCD SICCD PRC VOL RET SHROUT \
DATE PERMNO
1970-08-31 10559.0 10.0 1.0 5311.0 35.000 1692.0 0.030657 12048.0
12626.0 10.0 1.0 5411.0 46.250 926.0 0.088235 6624.0
12749.0 11.0 1.0 5331.0 45.500 5632.0 0.126173 34685.0
13100.0 11.0 1.0 5311.0 22.000 1759.0 0.171242 15107.0
13653.0 10.0 1.0 5311.0 13.125 141.0 0.220930 1337.0
13936.0 11.0 1.0 2331.0 11.500 270.0 -0.053061 3942.0
14322.0 11.0 1.0 5311.0 64.750 6934.0 0.024409 154187.0
16969.0 10.0 1.0 5311.0 42.875 1069.0 0.186851 13828.0
17072.0 10.0 1.0 5311.0 14.750 777.0 0.026087 5415.0
17304.0 10.0 1.0 5311.0 24.875 1939.0 0.058511 8150.0
MV XRET IB ... PE2 \
DATE PERMNO ...
1970-08-31 10559.0 421680.000 0.025357 NaN ... 13.852692
12626.0 306360.000 0.082935 NaN ... 13.145312
12749.0 1578167.500 0.120873 NaN ... 25.970466
13100.0 332354.000 0.165942 NaN ... 9.990711
13653.0 17548.125 0.215630 NaN ... 6.273570
13936.0 45333.000 -0.058361 NaN ... 6.473123
14322.0 9983608.250 0.019109 NaN ... 22.204047
16969.0 592875.500 0.181551 NaN ... 11.948061
17072.0 79871.250 0.020787 NaN ... 8.845526
17304.0 202731.250 0.053211 NaN ... 8.641655
lagPE1 lagPE2 lagMV lagSEQ QUINTILE1 \
DATE PERMNO
1970-08-31 10559.0 13.852692 13.852692 412644.000 264.686 4
12626.0 13.145312 13.145312 281520.000 164.151 4
12749.0 25.970466 25.970466 1404742.500 367.519 5
13100.0 9.990711 9.990711 288921.375 414.820 3
13653.0 6.273570 6.273570 14372.750 24.958 1
13936.0 6.473123 6.473123 48289.500 76.986 1
14322.0 22.204047 22.204047 9790874.500 3439.802 5
16969.0 11.948061 11.948061 499536.500 NaN 4
17072.0 8.845526 8.845526 77840.625 NaN 3
17304.0 8.641655 8.641655 191525.000 307.721 3
QUINTILE2 avgvol avg Startpoint
DATE PERMNO
1970-08-31 10559.0 4 9229.057592 1697.2 0
12626.0 4 3654.367470 894.4 0
12749.0 5 188206.566860 5828.6 0
13100.0 3 94127.319048 3477.2 0
13653.0 1 816.393162 268.8 0
13936.0 1 71547.050633 553.2 0
14322.0 5 195702.521519 6308.8 0
16969.0 4 3670.297872 2002.0 0
17072.0 3 3774.083333 3867.8 0
17304.0 3 12622.112903 1679.4 0