R List

WHAT IS LIST
A list is an object which can store any object of any dimension.
A list can store a vector, matrix, array, data frame or list inside it. There is no limitation of size. A list will return a list when accessed by single square bracket. i.e [ ]. But when accessed via [[ ]], then it simplifies the output i.e we get output in the form of a vector or matrix. (in the form it was inserted in list)
METHOD OF CREATION
##A list can be created by list function()
l1<-list(name="Amy",num=1001,marks=c(70,75,80,68,79),mat=matrix(1:10,2,5))
l2<-list(name="Sam",num=1001,marks=c(56,78,76,69,89),mat=matrix(1:10,2,5))
l3<-list(name="Dan",num=1001,marks=c(69,86,75,87,65),mat=matrix(1:10,2,5))
## we have created three list, each one with four elements, name, number, and marks and at. Name is character type, num is a number, marks is a vector and mat is a matrix. It is like a structure which helps to store different data types in one place. Now we can make a composite list consisting of above three list.
l<-list(Amy=l1,Sam=l2,Dan=l3)
ACCESSING LIST
l1[1] ## return first element of list l1 in the form of list.
$name
[1] "Amy"
l1[2] ## return second element of list l1in the form of list.
$num
[1] 1001
l1[3] ## return third element of list l1 in the form of list.
$marks
[1] 70 75 80 68 79
## Now if we want to access the first element of marks( marks is a vector). Then first of all we have to use [[]]. By placing [[3]] output get simplified and we will get a vector. Now by placing another set of square bracket with element number 1,we will get first element of third element of list.
l1[[3]]   ## return third element of list in the form of vector.
[1] 70 75 80 68 79
l1[[3]][1]  ## return first element of third element of list in the form of vector.
[1] 70
l1[[4]]  ## return fourth element of list in the form of matrix.
       [,1] [,2] [,3] [,4] [,5]
[1,]    1    3    5    7    9
[2,]    2    4    6    8   10