FAQ
The elements of matrix MUST be of the same type.
You can convert a list into a matrix if and only if all the elements
of the list are of the same type. For example:
milista <- list(a1=1:3, a2=4:6,a31:103)
milista
$a1
[1] 1 2 3

$a2
[1] 4 5 6

$a3
[1] 101 102 103

You can convert milista into milista.matriz:
First, to vector
milista.matriz
a11 a12 a13 a21 a22 a23 a31 a32 a33
1 2 3 4 5 6 101 102 103

Then to matrix (as you quote from Prof Brian D Ripley
"> >and R matrices are just vectors with a dimension attribute."
dim(milista.matriz) <- c(3,3)
milista.matriz
[,1] [,2] [,3]
[1,] 1 4 101
[2,] 2 5 102
[3,] 3 6 103

BUT if milista just had one element with different length, then it
would be impossible to convert it into a matrix.
Also, if milista had elements of different type, such as:
milista <- list(a1=1:3, a2=4:6,a31:103,a4=c("a","b","c"))
you would not be able, by definition, to convert it into
matrix, although you could convert it into a dataframe. Let's see:
midataframe <- data.frame(milista.matriz)
midataframe
X1 X2 X3
1 1 4 101
2 2 5 102
3 3 6 103
midataframe$X1 [1] 1 2 3
is.numeric(midataframe$X1) [1] TRUE
midataframe <- cbind(midataframe,X4=c("a","b","a"))
midataframe
X1 X2 X3 X4
1 1 4 101 a
2 2 5 102 b
3 3 6 103 a
is.numeric(midataframe$X1) [1] TRUE
is.numeric(midataframe$X4)
[1] FALSE

The data frame keeps each column with its own type.
Instead:
cbind(milista.matriz,c("a","b","a"))
[,1] [,2] [,3] [,4]
[1,] "1" "4" "101" "a"
[2,] "2" "5" "102" "b"
[3,] "3" "6" "103" "a"

would automatically convert all elements to the same
type (char)).

(In other words, it would seem "as if you had converted" to a matrix,
but you have done more than just changing the organization of the
information:
you have changed the information itself, as 1 != "1". Beware of that,
as R would not warn you).

So:

vector: chain of elements, all of the same type (1 dimension). Note
that:
mivector <- c(1,2,3,"a","b","c")
would produce:
mivector
[1] "1" "2" "3" "a" "b" "c"
(numbers automatically converted to chars)

matrix: vector with more than 1 dimension (actually, we normally call
them matrices if they are 2D, and arrays for >3D).

dataframe: matrix which columns can be of different type (but same length)

lists: organized assemblage of elements that can be of different types,
lengths and dimensions, i.e.:
milista
$a1
[1] 1 2 3

$a2
[,1] [,2] [,3]
[1,] 1 5 9
[2,] 2 6 10
[3,] 3 7 11
[4,] 4 8 12

$a3
[1] "A ver si me hago un tutorial"

$a4
function(){print("patatin patatan")}


(Well, as far as I know...)

Agus

Dr. Agustin Lobo
Instituto de Ciencias de la Tierra (CSIC)
Lluis Sole Sabaris s/n
08028 Barcelona SPAIN
tel 34 93409 5410
fax 34 93411 0012
alobo at ija.csic.es

On Mon, 25 Feb 2002, Gabor Grothendieck wrote:



--- Prof Brian D Ripley wrote:
A matrix list? R lists are just vectors with elements of different types,
and R matrices are just vectors with a dimension attribute.
When I saw the above I tried to create a matrix from a list but
could not get it to work:

my.lm <- lm( rnorm(10) ~ I(1:10) )
my.list <- list(1, 2, 3, 4, 5, 6, my.lm, my.lm, my.lm)
my.mat <- matrix( my.list, nrow=3 )

gives the following error in R 1.4.0 on Windows 2000:

Error in matrix(my.list, nrow = 3) : Unimplemented feature in
copyVector

Is there a way to create a matrix out of elements of different types?

Thanks.


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