#对向量的子集进行操作
#tapply(参数)：tapply(向量,因子/因子列表,函数/函数名)

> x <- c(rnorm(5),runif(5),rnorm(5,1))

> f <- gl(3,5)
> f
 [1] 1 1 1 1 1 2 2 2 2 2 3 3 3 3 3
Levels: 1 2 3

> tapply(x,f,mean)
         1          2          3 
-0.5004154  0.4044779  0.9769996 

> tapply(x,f,mean,simplify = FALSE)
$`1`
[1] -0.5004154

$`2`
[1] 0.4044779

$`3`
[1] 0.9769996


> tapply(x,f,sum)
        1         2         3 
-2.502077  2.022389  4.884998