How to make a 2-dim array being "interpreted" as an array rather than as a matrix

Dear List

A 2-dimensional array seems to be "interpreted" as a matrix whereas higher dimensional arrays are interpreted as arrays, e.g.
a1 <- array(1:8,c(2,2,2))
class(a1)
[1] "array"
a2 <- array(1:4,c(2,2))
class(a2)
[1] "matrix"

If I write a generic function (apologies if this is the wrong word) on arrays as

foo <- function(x) UseMethod("foo")
foo.array <- function(x) sum(x)

then this fails on a 2-dimensional array:
foo(a1)
[1] 36
foo(a2)
Error in UseMethod("foo") : no applicable method for "foo"

Is there any way in which I can "force" a2 to being "interpreted" as having class "array" rather than "matrix" so that foo will work on a2???

Thanks in advance
S?ren
I don't know if is the best way:

a3 <- array(a2, dim=c(nrow(a2), ncol(a2), 1))

foo(a3)
Dear List

 A 2-dimensional array seems to be "interpreted" as a matrix whereas higher dimensional arrays are interpreted as arrays, e.g.

 > a1 <- array(1:8,c(2,2,2))
 > class(a1)
 [1] "array"
 > a2 <- array(1:4,c(2,2))
 > class(a2)
 [1] "matrix"

 If I write a generic function (apologies if this is the wrong word) on arrays as

 foo <- function(x) UseMethod("foo")
 foo.array <- function(x) sum(x)

 then this fails on a 2-dimensional array:

 > foo(a1)
 [1] 36
 > foo(a2)
 Error in UseMethod("foo") : no applicable method for "foo"

 Is there any way in which I can "force" a2 to being "interpreted" as having class "array" rather than "matrix" so that foo will work on a2???

 Thanks in advance
 S?ren

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Henrique Dallazuanna
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25? 25' 40" S 49? 16' 22" O
Hello -
Dear List

A 2-dimensional array seems to be "interpreted" as a matrix whereas
higher dimensional arrays are interpreted as arrays, e.g.

a1 <- array(1:8,c(2,2,2)) class(a1)
[1] "array"
a2 <- array(1:4,c(2,2)) class(a2)
[1] "matrix"

Note that is.matrix(a2) and is.array(a2) are both TRUE though.
If I write a generic function (apologies if this is the wrong word)
on arrays as

foo <- function(x) UseMethod("foo") foo.array <- function(x) sum(x)

then this fails on a 2-dimensional array:

foo(a1)
[1] 36
foo(a2)
Error in UseMethod("foo") : no applicable method for "foo"

Is there any way in which I can "force" a2 to being "interpreted" as
having class "array" rather than "matrix" so that foo will work on
a2???
I don't know about that.  However, I see that both of the functions
duplicated.matrix and unique.matrix are copies of duplicated.array and 
unique.array, respectively.

Perhaps that's all you need to do?

foo.matrix <- foo.array

Maybe there is a better suggestion?

Best,
Erik Iverson
One more note:
Is there any way in which I can "force" a2 to being "interpreted" as
having class "array" rather than "matrix" so that foo will work on
a2???

Maybe try?

a3 <- structure(1:10, dim = c(2,5), class = "array")

Then class(a3) gives "array", and is.matrix and is.array are both still 
TRUE.

Erik