1.apply（）函数
apply的输入为data.frame或matrix，输出为 vector, list or array。
apply(X, MARGIN, FUN)
Here:
-x: an array or matrix
-MARGIN: take a value or range between 1 and 2 to define where to apply the function:
-MARGIN=1: the manipulation is performed on rows -MARGIN=2: the manipulation is performed on columns
-MARGIN=c(1,2)` the manipulation is performed on rows and columns
-FUN: tells which function to apply. Built functions like mean, median, sum, min, max and even user-defined functions can be applied
2.lapply() function
lapply（）函数输入为 list, vector or data.frame,输出为list。
lapply(X, FUN)
Arguments:
-X: A vector or an object
-FUN: Function applied to each element of x
3.sapply() function
sapply() 输入为 list, vector or data.frame ，输出为 vector or matrix. sapply()与 lapply() 可以完成相同的工作，但是输出的格式不一样。
sapply(X, FUN)
Arguments:
-X: A vector or an object
-FUN: Function applied to each element of x
一张总结的图：
4.tapply() function
tapply() 用于对一个vector做一个函数运算 (如：mean, median, min, max, etc..) 。
tapply(X, INDEX, FUN = NULL)
Arguments:
-X: An object, usually a vector
-INDEX: A list containing factor
-FUN: Function applied to each element of x
