1.例如我想查询全世界人口数最少的前33个城市：

world.cities[world.cities$pop<=sort(world.cities$pop)[33],]

输出结果为：

[data]

name country.etc pop lat long capital

672 Ajmanol Marshall Islands 0 7.22 171.65 0

2444 Avigaat Greenland 0 62.23 -49.82 0

3451 Barorkan Marshall Islands 0 10.28 169.98 0

4831 Boken Marshall Islands 2 7.08 171.83 0

9129 Denmeo Marshall Islands 2 7.12 171.32 0

9144 Depil Faroe Islands 2 62.28 -6.52 0

10703 Eneweto Marshall Islands 0 7.20 171.65 0

11061 Ewo Marshall Islands 1 5.83 169.55 0

15506 Ipiutaq Greenland 1 59.88 -44.21 0

15713 Iterlak Greenland 2 60.92 -45.29 0

16720 Kaijen Marshall Islands 0 9.53 169.88 0

17056 Kangerluarsorujuk Greenland 2 60.63 -45.80 0

18122 Kingitooq Greenland 1 60.73 -46.37 0

18556 Kolalen Marshall Islands 0 7.03 171.78 0

18606 Koltur Faroe Islands 2 61.98 -6.95 0

18752 Kora Marshall Islands 0 7.12 171.35 0

22781 Marib Marshall Islands 0 10.25 169.98 0

24092 Mjorkadalur Faroe Islands 1 62.07 -6.92 0

25363 Nanij Marshall Islands 0 6.22 169.52 0

25446 Narsalik Greenland 0 61.62 -49.34 0

25791 Nesvik Faroe Islands 2 62.22 -7.02 0

26094 Ningi Marshall Islands 1 8.87 167.75 0

26737 Nzara Sudan 0 4.64 28.25 0

29252 Plymouth Montserrat 0 16.68 -62.30 0

30258 Pyramiden Svalbard and Jan Mayen 0 78.65 16.42 0

30345 Qeqertarsuaq Greenland 2 77.43 -70.18 0

30349 Qernertuarssuit Greenland 1 65.72 -37.29 0

30407 Qooqqut Greenland 1 64.27 -50.89 0

30408 Qoornoq Greenland 2 64.53 -51.09 0

30529 Qullissat Greenland 0 70.08 -53.01 0

31189 Ribon Marshall Islands 0 6.07 169.60 0

35919 Souskiou Cyprus 2 34.73 32.60 0

36738 Svea Svalbard and Jan Mayen 0 77.90 16.68 0

39758 Uunartoq Greenland 0 70.40 -21.96 0

40622 Vikarbyrgi Faroe Islands 2 61.43 -6.72 0

[/data]</p>

那些人口数只有0或者1、2的是怎么回事？。。。

china[china$capital==2,]

[data]

name country.etc pop lat long capital

7595 Chongqing China 4133663 29.57 106.58 2

34723 Shanghai China 15017783 31.23 121.47 2

38080 Tianjin China 3761272 39.13 117.20 2

[/data]</p>

china[china$capital==3,]

[data]

name country.etc pop lat long capital

7108 Changchun China 2602574 43.87 125.35 3

7118 Changsha China 2131620 28.20 112.97 3

7271 Chengdu China 3961709 30.67 104.07 3

11988 Fuzhou China 1202136 26.08 119.30 3

13236 Guangzhou China 3158125 23.12 113.25 3

13387 Guiyang China 1183692 26.58 106.72 3

13640 Haikou China 631179 20.05 110.32 3

13836 Hangzhou China 1894655 30.25 120.17 3

13879 Harbin China 3279454 45.75 126.65 3

14132 Hefei China 1406741 31.85 117.28 3

14547 Hohhot China 783645 40.82 111.64 3

16302 Jinan China 2109987 36.67 117.00 3

19383 Kunming China 1044356 25.05 102.70 3

20222 Lanzhou China 1431171 36.05 103.68 3

20322 Lasa China 120488 29.65 91.10 3

25333 Nanchang China 1934445 28.68 115.88 3

25364 Nanjing China 3160179 32.05 118.78 3

25369 Nanning China 805718 22.82 108.32 3

34806 Shenyang China 3538726 41.80 123.45 3

34842 Shijiazhuang China 2080120 38.05 114.48 3

37065 Taiyuan China 2752516 37.87 112.55 3

39658 Urumqi China 1538778 43.80 87.58 3

41911 Wuhan China 4236023 30.58 114.27 3

42022 Xining China 770872 36.62 101.77 3

42376 Yinchuan China 480916 38.47 106.32 3

42871 Zhengzhou China 2023495 34.75 113.67 3

[/data]</p>

我想知道capital==2 or 3的是什么意思？。。。谢谢各位了。。