Lists

Lists are very much similar to associative arrays. It comes very close in similarity to a PHP programmer.

Lets see ##Definition Just declare a variable using list() function. Note that list has a property to hold objects of different class/types. Lets do the same example of two students and their attributes

>team = list(c("Vineet", "Maheshwari", 5.6, "Gurgaon"), c("Rakesh", "Sharma", 6.0, "Gurgaon"))
> team
[[1]]
[1] "Vineet"     "Maheshwari" "5.6"        "Gurgaon"   

[[2]]
[1] "Rakesh"  "Sharma"  "6"       "Gurgaon"
> 
>

Lets name them

team = list(Vineet = c("Vineet", "Maheshwari", 5.6, "Gurgaon"), Rakesh = c("Rakesh", "Sharma", 6.0, "Gurgaon"))
> team
$Vineet
[1] "Vineet"     "Maheshwari" "5.6"        "Gurgaon"   

$Rakesh
[1] "Rakesh"  "Sharma"  "6"       "Gurgaon"

>

Note [[]] brackets and use of $ in front of the label while capturing the content of list member. These are used to access members of the list.

Accessing them

> team["Vineet"]
$Vineet
[1] "Vineet"     "Maheshwari" "5.6"        "Gurgaon"   

> team$Vineet
[1] "Vineet"     "Maheshwari" "5.6"        "Gurgaon"   
> 
> team[[1]]
[1] "Vineet"     "Maheshwari" "5.6"        "Gurgaon"   
>
> team[["Vineet"]]
[1] "Vineet"     "Maheshwari" "5.6"        "Gurgaon"

Note here four ways to access. Use of $ and Use of square brackets - both single and double.

Answer to the difference lies in following example.

> team[1][3]
$<NA>
NULL

> team[[1]][3]
[1] "5.6"
>

Note that though we can access the list member by using team[1], but we can’t access the member of a member. On the other hand using [[]], we are able to access. So also, we can modify.

Lets check the type of team[1]

> class(team[1])
[1] "list"
> class(team[[1]])
[1] "character"
>

It is again a list, though we had inserted a character type.

Attach and detach with lists

This is new concept. Something similar was there in VBA, where one selects and object and there is no need to repeat that name in the enclosing block. Similarly, attach(team) shall eliminate the need for repeating team list name. After this statement, list members within itself are available for direct access.

> attach(team)
> Vineet
[1] "Vineet"     "Maheshwari" "5.6"        "Gurgaon"   
> detach(team)
> Vineet
Error: object 'Vineet' not found
>