# [Rd] surprised matrix (1:256, 8, 8) doesn't cause error/warning

Hervé Pagès hp@ge@@on@g|thub @end|ng |rom gm@||@com
Mon Feb 22 20:34:38 CET 2021

```Hi Martin,

It kind of does make sense to issue the warning when **recycling** (and
this is consistent with what happens with recycling in general):

> matrix(1:4, 6, 6)
[,1] [,2] [,3] [,4] [,5] [,6]
[1,]    1    3    1    3    1    3
[2,]    2    4    2    4    2    4
[3,]    3    1    3    1    3    1
[4,]    4    2    4    2    4    2
[5,]    1    3    1    3    1    3
[6,]    2    4    2    4    2    4

> matrix(1:4, 5, 6)
[,1] [,2] [,3] [,4] [,5] [,6]
[1,]    1    2    3    4    1    2
[2,]    2    3    4    1    2    3
[3,]    3    4    1    2    3    4
[4,]    4    1    2    3    4    1
[5,]    1    2    3    4    1    2
Warning message:
In matrix(1:4, 5, 6) :
data length [4] is not a sub-multiple or multiple of the number of
rows [5]

(Note that the warning is misleading. matrix() is happy to take data
with a length that is not a sub-multiple of the number of rows or cols
as long as it's a sub-multiple of the length of the matrix.)

However I'm not sure that **truncating** the data is desirable behavior:

> matrix(1:6, 1, 3)
[,1] [,2] [,3]
[1,]    1    2    3

> matrix(1:6, 1, 5)
[,1] [,2] [,3] [,4] [,5]
[1,]    1    2    3    4    5
Warning message:
In matrix(1:6, 1, 5) :
data length [6] is not a sub-multiple or multiple of the number of
columns [5]

Maybe you get a warning sometimes, if you are lucky, but still.

Finally note that you never get any warning with array():

> array(1:4, c(5, 6))
[,1] [,2] [,3] [,4] [,5] [,6]
[1,]    1    2    3    4    1    2
[2,]    2    3    4    1    2    3
[3,]    3    4    1    2    3    4
[4,]    4    1    2    3    4    1
[5,]    1    2    3    4    1    2

> array(1:6, c(1, 5))
[,1] [,2] [,3] [,4] [,5]
[1,]    1    2    3    4    5

Cheers,
H.

On 2/1/21 1:08 AM, Martin Maechler wrote:
>>>>>> Abby Spurdle (/əˈbi/)
>>>>>>      on Mon, 1 Feb 2021 19:50:32 +1300 writes:
>
>      > I'm a little surprised that the following doesn't trigger an error or a warning.
>      > matrix (1:256, 8, 8)
>
>      > The help file says that the main argument is recycled, if it's too short.
>      > But doesn't say what happens if it's too long.
>
> It's somewhat subtler than one may assume :
>
>> matrix(1:9, 2,3)
>       [,1] [,2] [,3]
> [1,]    1    3    5
> [2,]    2    4    6
> Warning message:
> In matrix(1:9, 2, 3) :
>    data length [9] is not a sub-multiple or multiple of the number of rows [2]
>
>> matrix(1:8, 2,3)
>       [,1] [,2] [,3]
> [1,]    1    3    5
> [2,]    2    4    6
> Warning message:
> In matrix(1:8, 2, 3) :
>    data length [8] is not a sub-multiple or multiple of the number of columns [3]
>
>> matrix(1:12, 2,3)
>       [,1] [,2] [,3]
> [1,]    1    3    5
> [2,]    2    4    6
>>
>
> So it looks to me the current behavior is quite on purpose.
> Are you sure it's not documented at all when reading the docs
> carefully?  (I did *not*, just now).
>
> ______________________________________________
> R-devel using r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

--
Hervé Pagès

Bioconductor Core Team
hpages.on.github using gmail.com

```