[R] Matrix of Elements of Different Types (was Interfacing pre-existing C++ library from R)

[Agustin Lobo]

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,a3=101:103)

> 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,a3=101:103,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 <ripley at stats.ox.ac.uk> 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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