why the object is vector?

Question

> x=c(1,2,3,4,5)  
> x1=list(n1=1,n2=2,n3=x)  
> is.vector(x1)  

[1] TRUE

> is.list(x1)  

[1] TRUE

Answer 1

From ?is.vector

If mode = "any", is.vector may return TRUE for the atomic modes, list and expression.

You can specify the mode if you do not want is.vector to return TRUE for a list

> is.vector(x1, mode='numeric')
[1] FALSE

> is.vector(x, mode='numeric')
[1] TRUE

Answer 2

A vector in R is an ordered collection of stuff. Stuff in this case is

> mode(x1)
[1] "list"

from the help file

is.vector returns TRUE if x is a vector of the specified mode having no attributes other than names.

> attributes(x1)
$names
[1] "n1" "n2" "n3"

if we were to give x1 another attribute:

levels(x1)<-1:3

> x1
$n1
[1] 1

$n2
[1] 2

$n3
[1] 1 2 3 4 5

attr(,"levels")
[1] 1 2 3

> is.list(x1)
[1] TRUE

> is.vector(x1)
[1] FALSE

it would still be a list but not now a vector

From A brief history of S "The basic data structure in S is a vector of like­elements: numbers, character strings, or logical val­ ues. Although the notion of an attribute for an S object wasn't clearly implemented until the 1988 release, from the beginning S recognized that the primary vector of data was often accompanied by other values that described special properties of the data. For example, a matrix is just a vector of data along with an auxil­ iary vector named Dim that tells the dimensionality (number of rows and columns). Similarly, a time series has a Tsp attribute to tell the start time, end time, and number of observations per cycle. These vectors with attributes are known as vector structures, and this distinguishes S from most other systems."

Presumably it is similar in R which is an implementation of S so these vector structures are not designated as vectors.

Answer 3

> is.atomic(x1)
[1] FALSE

From the R language definition, lists are generic vectors, but not atomic vectors.