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> v <- c(40,2,83,28,58)
> f <- factor(c("A","C","C","B","C"))

Splitting a Vector into Groups

> library(MASS)
Warning message:
패키지 ‘MASS’는 R 버전 3.2.5에서 작성되었습니다 
> split(Cars93$MPG.city, Cars93$Origin)
$USA
 [1] 22 19 16 19 16 16 25 25 19 21 18 15
[13] 17 17 20 23 20 29 23 22 17 21 18 29
[25] 20 31 23 22 22 24 15 21 18 17 18 23
[37] 19 24 23 18 19 23 31 23 19 19 19 28

$`non-USA`
 [1] 25 18 20 19 22 46 30 24 42 24 29 22
[13] 26 20 17 18 18 29 28 26 18 17 20 19
[25] 29 18 29 24 17 21 20 33 25 23 39 32
[37] 25 22 18 25 17 21 18 21 20

Comparison between (among) groups

> g <- split(Cars93$MPG.city, Cars93$Origin)
> g
$USA
 [1] 22 19 16 19 16 16 25 25 19 21 18 15
[13] 17 17 20 23 20 29 23 22 17 21 18 29
[25] 20 31 23 22 22 24 15 21 18 17 18 23
[37] 19 24 23 18 19 23 31 23 19 19 19 28

$`non-USA`
 [1] 25 18 20 19 22 46 30 24 42 24 29 22
[13] 26 20 17 18 18 29 28 26 18 17 20 19
[25] 29 18 29 24 17 21 20 33 25 23 39 32
[37] 25 22 18 25 17 21 18 21 20

> mean(g$USA)
[1] 20.95833
> mean(g$`non-USA`)
[1] 23.86667
>