[R] Transfer a 3-dimensional array to a matrix in R

[Dénes Tóth]

Hi,

Bill was faster than me in suggesting aperm() instead of apply(), 
however, his solution is still suboptimal. Try to avoid array(), and
set the dimensions directly if possible.

----

fn1 <- function(x) {
     apply(x, 3, t)
}


fn2 <- function(x) {
     array(aperm(x, c(2, 1, 3)), c(prod(dim(x)[1:2]), dim(x)[3]))
}

fn3 <- function(x) {
     x <- aperm(x, c(2, 1, 3))
     dim(x) <- c(prod(dim(x)[1:2]), dim(x)[3])
     x
}

# check that the functions return the same
x <- array(1:18, dim=c(3, 2, 3))
stopifnot(identical(fn1(x), fn2(x)))
stopifnot(identical(fn1(x), fn3(x)))

# create two larger arrays, play with the size of the 3rd dimension
x <- array(1:18e4, dim=c(3, 2e1, 3e3))
y <- array(1:18e4, dim=c(3e3, 2e1, 3))

# and the timing:
library(microbenchmark)
microbenchmark(fn1(x), fn2(x), fn3(x), fn1(y), fn2(y), fn3(y), times = 100L)

---

Conclusion:
fn3() is about 3x as fast as fn2(), and fn1() can be extremely 
inefficient if dim(x)[3] is large.


HTH,
   Denes




On 10/20/2015 08:48 PM, William Dunlap wrote:
> Or use aperm() (array index permuation):
>    > array(aperm(x, c(2,1,3)), c(6,3))
>         [,1] [,2] [,3]
>    [1,]    1    7   13
>    [2,]    4   10   16
>    [3,]    2    8   14
>    [4,]    5   11   17
>    [5,]    3    9   15
>    [6,]    6   12   18
>
> Bill Dunlap
> TIBCO Software
> wdunlap tibco.com
>
>
> On Tue, Oct 20, 2015 at 11:31 AM, John Laing <john.laing at gmail.com> wrote:
>>> x <- array(1:18, dim=c(3, 2, 3))
>>> x
>> , , 1
>>
>>       [,1] [,2]
>> [1,]    1    4
>> [2,]    2    5
>> [3,]    3    6
>>
>> , , 2
>>
>>       [,1] [,2]
>> [1,]    7   10
>> [2,]    8   11
>> [3,]    9   12
>>
>> , , 3
>>
>>       [,1] [,2]
>> [1,]   13   16
>> [2,]   14   17
>> [3,]   15   18
>>
>>> apply(x, 3, t)
>>       [,1] [,2] [,3]
>> [1,]    1    7   13
>> [2,]    4   10   16
>> [3,]    2    8   14
>> [4,]    5   11   17
>> [5,]    3    9   15
>> [6,]    6   12   18
>>
>>
>> On Tue, Oct 20, 2015 at 12:39 PM, Chunyu Dong <dongchunyu2004 at 163.com>
>> wrote:
>>
>>> Hello!
>>>
>>>
>>> Recently I am trying to transfer a large 3-dimensional array to a matrix.
>>> For example, a array like:
>>> , , 1
>>>       [,1] [,2]
>>> [1,]    1    4
>>> [2,]    2    5
>>> [3,]    3    6
>>> , , 2
>>>       [,1] [,2]
>>> [1,]    7   10
>>> [2,]    8   11
>>> [3,]    9   12
>>> , , 3
>>>       [,1] [,2]
>>> [1,]   13   16
>>> [2,]   14   17
>>> [3,]   15   18
>>>
>>>
>>> I would like to transfer it to a matrix like:
>>> 1        7          13
>>> 4        10        16
>>> 2        8          14
>>> 5        11        17
>>> 3        9          15
>>> 6        12        18
>>>
>>>
>>> Could you tell me how to do it in R ? Thank you very much!
>>>
>>>
>>> Best regards,
>>> Chunyu
>>>
>>>
>>>
>>>
>>>
>>>          [[alternative HTML version deleted]]
>>>
>>> ______________________________________________
>>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide
>>> http://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>>
>>
>>          [[alternative HTML version deleted]]
>>
>> ______________________________________________
>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>