把数据按acc（年份），weight2（权重的平方）排序，然后如何取每个年份中的前4个weight2？
就是取2000年的前四个weight2，2001年的前四个weight2。
谢谢各位大神指教！
| stkcd | acc | inc | tinc | weight | weight2 | HHI |
| 600097 | 2000 | 26978754 | 6.28E+09 | 0.004299 | 1.85E-05 | 0.136708 |
| 600257 | 2000 | 74635242 | 6.28E+09 | 0.011893 | 0.000141 | 0.136708 |
| 000588 | 2000 | 79943236 | 6.28E+09 | 0.012739 | 0.000162 | 0.136708 |
| 000592 | 2000 | 1.44E+08 | 6.28E+09 | 0.022906 | 0.000525 | 0.136708 |
| 600265 | 2000 | 1.54E+08 | 6.28E+09 | 0.024595 | 0.000605 | 0.136708 |
| 000998 | 2000 | 1.63E+08 | 6.28E+09 | 0.025902 | 0.000671 | 0.136708 |
| 600762 | 2000 | 1.94E+08 | 6.28E+09 | 0.030974 | 0.000959 | 0.136708 |
| 000735 | 2000 | 2.26E+08 | 6.28E+09 | 0.03605 | 0.0013 | 0.136708 |
| 000769 | 2000 | 2.74E+08 | 6.28E+09 | 0.043576 | 0.001899 | 0.136708 |
| 000798 | 2000 | 3.29E+08 | 6.28E+09 | 0.052474 | 0.002754 | 0.136708 |
| 200992 | 2000 | 3.50E+08 | 6.28E+09 | 0.055767 | 0.00311 | 0.136708 |
| 000713 | 2000 | 4.08E+08 | 6.28E+09 | 0.064983 | 0.004223 | 0.136708 |
| 600672 | 2000 | 6.35E+08 | 6.28E+09 | 0.101252 | 0.010252 | 0.136708 |
| 600359 | 2000 | 6.86E+08 | 6.28E+09 | 0.109263 | 0.011938 | 0.136708 |
| 600108 | 2000 | 6.90E+08 | 6.28E+09 | 0.109969 | 0.012093 | 0.136708 |
| 600709 | 2000 | 1.84E+09 | 6.28E+09 | 0.293356 | 0.086058 | 0.136708 |
| 600709 | 2001 | 55683712 | 6.13E+09 | 0.009084 | 8.25E-05 | 0.136523 |
| 000592 | 2001 | 78314483 | 6.13E+09 | 0.012776 | 0.000163 | 0.136523 |
| 600506 | 2001 | 99011673 | 6.13E+09 | 0.016152 | 0.000261 | 0.136523 |
| 600097 | 2001 | 1.15E+08 | 6.13E+09 | 0.018704 | 0.00035 | 0.136523 |
| 600762 | 2001 | 1.15E+08 | 6.13E+09 | 0.01877 | 0.000352 | 0.136523 |
| 600257 | 2001 | 1.62E+08 | 6.13E+09 | 0.026397 | 0.000697 | 0.136523 |
| 600265 | 2001 | 1.86E+08 | 6.13E+09 | 0.030313 | 0.000919 | 0.136523 |
| 000769 | 2001 | 1.86E+08 | 6.13E+09 | 0.030344 | 0.000921 | 0.136523 |
| 000735 | 2001 | 2.29E+08 | 6.13E+09 | 0.037295 | 0.001391 | 0.136523 |
| 000998 | 2001 | 2.68E+08 | 6.13E+09 | 0.043704 | 0.00191 | 0.136523 |
| 000798 | 2001 | 3.03E+08 | 6.13E+09 | 0.049464 | 0.002447 | 0.136523 |
| 200992 | 2001 | 3.41E+08 | 6.13E+09 | 0.055637 | 0.003095 | 0.136523 |
| 000713 | 2001 | 3.98E+08 | 6.13E+09 | 0.064993 | 0.004224 | 0.136523 |
| 600359 | 2001 | 5.38E+08 | 6.13E+09 | 0.087711 | 0.007693 | 0.136523 |
| 600672 | 2001 | 5.43E+08 | 6.13E+09 | 0.088542 | 0.00784 | 0.136523 |
| 600108 | 2001 | 6.43E+08 | 6.13E+09 | 0.104856 | 0.010995 | 0.136523 |
| 600313 | 2001 | 1.87E+09 | 6.13E+09 | 0.305258 | 0.093183 | 0.136523 |
----------------------- copy starting from the next line -----------------------
------------------ copy up to and including the previous line -------
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