如何根据R的data.table对象中的多列查找列的行数总和？
要根据Rdata.table对象中的多列查找列的行数总和，我们可以按照以下步骤操作：
首先，创建一个data.table对象。
然后，使用聚合函数根据多列查找一列的行数总和。
示例
创建data.table对象
让我们创建一个data.table对象，如下所示-
library(data.table)
F1<-sample(c("Male","Female"),25,replace=TRUE)
F2<-sample(c("Low","Medium","High"),25,replace=TRUE)
F3<-sample(c("I","II","III"),25,replace=TRUE)
Response<-sample(1:100,25)
DT<-data.table(F1,F2,F3,Response)
DT输出结果执行时，上述脚本生成以下内容output(thisoutputwillvaryonyoursystemduetorandomization)-
F1 F2 F3 Response
1: Male Medium I 2
2: Female Low I 32
3: Female High II 43
4: Male Medium I 24
5: Male Medium III 88
6: Female High I 95
7: Male Low II 68
8: Female High I 74
9: Female High II 12
10: Male High III 42
11: Male High I 62
12: Male High III 67
13: Female High III 38
14: Male Low III 73
15: Female Low III 3
16: Male Medium III 98
17: Female Low I 100
18: Male Medium II 89
19: Female High II 81
20: Female Low I 53
21: Female Medium II 91
22: Female High II 63
23: Female High II 60
24: Male Medium II 72
25: Female Medium III 56
F1 F2 F3 Response根据多列查找一列的行数总和
使用聚合函数根据列F1、F2和F3查找列Response的行总和，如下所示-
library(data.table)
F1<-sample(c("Male","Female"),25,replace=TRUE)
F2<-sample(c("Low","Medium","High"),25,replace=TRUE)
F3<-sample(c("I","II","III"),25,replace=TRUE)
Response<-sample(1:100,25)
DT<-data.table(F1,F2,F3,Response)
DT[,sum(Response),by=.(F1,F2,F3)]输出结果F1 F2 F3 V1 1: Male Medium I 26 2: Female Low I 185 3: Female High II 259 4: Male Medium III 186 5: Female High I 169 6: Male Low II 68 7: Male High III 109 8: Male High I 62 9: Female High III 38 10: Male Low III 73 11: Female Low III 3 12: Male Medium II 161 13: Female Medium II 91 14: Female Medium III 56