[R] strange matrix behaviour: is there a matrix with one row?

Thomas Lumley tlumley at u.washington.edu
Fri Apr 7 16:59:11 CEST 2006

```On Fri, 7 Apr 2006, Dimitris Rizopoulos wrote:

> this is documented: check ?"[". You need to specify drop = FALSE,
> i.e.,>
> is.matrix(y[-c(1,2,3), , drop = FALSE])

It's not only documented, it's a FAQ.

-thomas

>
> Best,
> Dimitris
>
> ----
> Dimitris Rizopoulos
> Ph.D. Student
> Biostatistical Centre
> School of Public Health
> Catholic University of Leuven
>
> Address: Kapucijnenvoer 35, Leuven, Belgium
> Tel: +32/(0)16/336899
> Fax: +32/(0)16/337015
> Web: http://www.med.kuleuven.be/biostat/
>     http://www.student.kuleuven.be/~m0390867/dimitris.htm
>
>
> ----- Original Message -----
> From: <ryszard.czerminski at novartis.com>
> To: "Gabor Grothendieck" <ggrothendieck at gmail.com>
> Cc: "Brian Quinif" <bquinif at gmail.com>; <r-help at stat.math.ethz.ch>;
> <r-help-bounces at stat.math.ethz.ch>
> Sent: Friday, April 07, 2006 3:16 PM
> Subject: [R] strange matrix behaviour: is there a matrix with one row?
>
>
>> Consider this:
>>
>>> y <- matrix(1:8, ncol=2)
>>> is.matrix(y[-c(1,2),])
>> [1] TRUE
>>> is.matrix(y[-c(1,2,3),])
>> [1] FALSE
>>> is.matrix(y[-c(1,2,3,4),])
>> [1] TRUE
>>
>> It seems like an inconsistent behaviour:
>> - with 2 or more rows we have a matrix
>> - with 1 row we do not have a matrix and
>> - with 0 rows we have a matrix again
>>
>> I just stumbled on this behaviour, because I had a problem
>> with my program in which I have assumed that matrix with
>> some rows removed is still a matrix, which seems to be mostly
>> true, but it is not true if only one row is left.
>>
>> How to work around this problem - without to many "if" statements?
>>
>> Best regards,
>> Ryszard
>>
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>>
>
>
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