R - List

The list in R may contain elements of the different class (just like a data frame)

data.tables and data.frames are internally lists with all its columns of equal length and with a class attribute.

myList = list(object,object,...)  

• List

> v = list("Nico has",40,"year")
> v
[[1]]
[1] "Nico has"
 
[[2]]
[1] 40
 
[[3]]
[1] "year"

• List of vector (for the dimension of a matrix

dimnames = list(c("Row1", "Row2"), c("Col1", "Col2", "Col3"))
> dimnames
[[1]]
[1] "Row1" "Row2"
 
[[2]]
[1] "Col1" "Col2" "Col3"

names: Functions to get or set the variable name of a value.

l = list(name = "Nico", age = 40)
l

$name
[1] "Nico"

$age
[1] 40

> names(l)
[1] "name" "age" 

• You can access the value then by name

l$name

[1] "Nico"

Example using the subset operators

> l = list(name = c("Nico","Mad","Mel"), age = c(40,7,4), colour=c("Red","Blue","Parse"))
> l
$name
[1] "Nico" "Mad"  "Mel" 
 
$age
[1] 40  7  4
 
$colour
[1] "Red"   "Blue"  "Parse"

• Indexing by key (return a list)

> l[1]
$name
[1] "Nico" "Mad"  "Mel" 

• Indexing by Name (return a list)

> l["name"]
$name
[1] "Nico" "Mad"  "Mel" 

• Indexing by vector (return a list)

> name <- "name"
> l[name]
$name
[1] "Nico" "Mad"  "Mel"

• Indexing by key and returning a vector

> l[[1]]
[1] "Nico" "Mad"  "Mel" 

• Indexing by name and returning a vector

> l$name
[1] "Nico" "Mad"  "Mel" 
# of
> l[["name"]]
[1] "Nico" "Mad"  "Mel" 

• Indexing by vector and returning a vector

> name <- "name"
> l[[name]]
[1] "Nico" "Mad"  "Mel"

• Retrieving the first and third elements in the list

> l[c(1,3)]
$name
[1] "Nico" "Mad"  "Mel" 
 
$colour
[1] "Red"   "Blue"  "Parse"

• Retrieving the third element of the first element

l[[c(1,3)]]
[1] "Mel"
# of
> l[[1]][[3]]
[1] "Mel"

• Partial Matching when retrieving a vector

> l[["n"]]
NULL
> l[["n", exact=FALSE]]
[1] "Nico" "Mad"  "Mel" 

lapply apply a function over an list.

> class(v)
[1] "list"

> l = list("Nico has",40,"year")
> length(l)
[1] 3

Data Structure

> l = list("Nico has",40,"year")
> str(v)
List of 3
 $ : chr "Nico has"
 $ : num 40
 $ : chr "year"