> x
学号 数学分析 高等代数 运筹学
1 201501 62 72 73
2 201502 71 69 95
3 201503 75 78 100
4 201504 69 86 81
5 201505 72 79 92
6 201506 91 29 72
7 201507 80 80 61
8 201508 80 95 61
9 201509 64 98 96
10 201510 92 88 66
11 201511 97 82 69
12 201512 76 63 89
13 201513 76 85 85
14 201514 93 70 100
15 201515 67 94 87
16 201516 95 68 72
17 201517 96 77 95
18 201518 75 58 100
19 201519 86 80 74
20 201520 97 100 100
21 201521 71 72 99
22 201522 84 100 93
23 201523 80 58 100
24 201524 98 53 100
25 201525 61 87 76
26 201526 63 83 83
27 201527 83 74 82
28 201528 81 88 79
29 201529 92 69 80
30 201530 71 54 77
31 201531 88 68 71
32 201532 66 65 79
33 201533 66 96 81
34 201534 73 86 82
35 201535 93 88 92
36 201536 75 90 79
37 201537 99 64 76
38 201538 66 69 76
39 201539 85 94 83
40 201540 61 68 64
41 201541 100 96 100
42 201542 64 100 76
43 201543 61 89 86
44 201544 82 92 71
45 201545 66 74 74
46 201546 68 73 65
47 201547 68 61 71
48 201548 94 93 75
49 201549 94 86 87
50 201550 72 82 67
> apply(x["数学分析","高等代数","运筹学"],1,sum)
错误于drop && length(x) == 1L : 'x && y'中'x'种类不对
> apply(x["数学分析","高等代数","运筹学"],1,sum)
错误于drop && length(x) == 1L : 'x && y'中'x'种类不对
> apply(x["数学分析","高等代数","运筹学"],1,sum)
错误于drop && length(x) == 1L : 'x && y'中'x'种类不对


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