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第一次写简书，用Rmarkdown试试水！
木䬕——于2019.4.2

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apply族函数

apply(X, MARGIN, FUN, ...)，MARGIN为维度，一般对数据框或矩阵，1代表行，2 代表列

> apply(iris[,1:4],2,mean) -> a
 >a
Sepal.Length  Sepal.Width Petal.Length  Petal.Width 
    5.843333     3.057333     3.758000     1.199333 
> class(a)#numeric
[1] "numeric"

lapply(X, FUN, ...)，X可以为list类型数据,输出返回项也是list

> lapply(iris[,1:4],mean) -> b
> b
$Sepal.Length
[1] 5.843333

$Sepal.Width
[1] 3.057333

$Petal.Length
[1] 3.758

$Petal.Width
[1] 1.199333
> class(b)#list
[1] "list"
> unlist(b) -> c
> c
Sepal.Length  Sepal.Width Petal.Length  Petal.Width 
    5.843333     3.057333     3.758000     1.199333
> class(c)
[1] "numeric"
> split(iris$Sepal.Length,iris$Species) -> d
> d
$setosa
 [1] 5.1 4.9 4.7 4.6 5.0 5.4 4.6 5.0 4.4 4.9 5.4 4.8 4.8 4.3 5.8 5.7 5.4 5.1 5.7 5.1 5.4 5.1 4.6
[24] 5.1 4.8 5.0 5.0 5.2 5.2 4.7 4.8 5.4 5.2 5.5 4.9 5.0 5.5 4.9 4.4 5.1 5.0 4.5 4.4 5.0 5.1 4.8
[47] 5.1 4.6 5.3 5.0

$versicolor
 [1] 7.0 6.4 6.9 5.5 6.5 5.7 6.3 4.9 6.6 5.2 5.0 5.9 6.0 6.1 5.6 6.7 5.6 5.8 6.2 5.6 5.9 6.1 6.3
[24] 6.1 6.4 6.6 6.8 6.7 6.0 5.7 5.5 5.5 5.8 6.0 5.4 6.0 6.7 6.3 5.6 5.5 5.5 6.1 5.8 5.0 5.6 5.7
[47] 5.7 6.2 5.1 5.7

$virginica
 [1] 6.3 5.8 7.1 6.3 6.5 7.6 4.9 7.3 6.7 7.2 6.5 6.4 6.8 5.7 5.8 6.4 6.5 7.7 7.7 6.0 6.9 5.6 7.7
[24] 6.3 6.7 7.2 6.2 6.1 6.4 7.2 7.4 7.9 6.4 6.3 6.1 7.7 6.3 6.4 6.0 6.9 6.7 6.9 5.8 6.8 6.7 6.7
[47] 6.3 6.5 6.2 5.9
> class(d)#list
[1] "list"
> lapply(d,mean)
$setosa
[1] 5.006

$versicolor
[1] 5.936

$virginica
[1] 6.588

sapply(X, FUN, ..., simplify = TRUE, USE.NAMES = TRUE)，与lapply用法相似，但返回项不同，非list类型

> sapply(d,mean)-> e
> e
    setosa versicolor  virginica 
     5.006      5.936      6.588 
> class(e)#numeric
[1] "numeric"

tapply(X, INDEX, FUN = NULL, ..., default = NA, simplify = TRUE)，返回项array类型

> tapply(iris$Sepal.Length,list(iris$Species),mean) -> f
> f
    setosa versicolor  virginica 
     5.006      5.936      6.588 
> class(f)#"array"
[1] "array"

mapply(FUN, ..., MoreArgs = NULL, SIMPLIFY = TRUE,USE.NAMES = TRUE)，函数项FUN在前，与sapply类似

> mapply(mean,iris[,1:4])-> g
> g
Sepal.Length  Sepal.Width Petal.Length  Petal.Width 
    5.843333     3.057333     3.758000     1.199333 
> class(g)#numeric
[1] "numeric"