R help - Apr 2006 - 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.

Comments? Suggestions?
How to work around this problem - without to many "if" statements?

Best regards,
Ryszard

[ryszard.czerminski at novartis.com]
>> 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
?'[' explains it.  Using your example:
> is.matrix(y[-c(1, 2), , drop=FALSE])
[1] TRUE
> is.matrix(y[-c(1, 2, 3), , drop=FALSE])
[1] TRUE
> is.matrix(y[-c(1, 2, 3, 4), , drop=FALSE])
[1] TRUE

-- 
Fran?ois Pinard   http://pinard.progiciels-bpi.ca

this is documented: check ?"[". You need to specify drop = FALSE, 
i.e.,

is.matrix(y[-c(1,2,3), , drop = FALSE])


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.
>
> Comments? Suggestions?
> How to work around this problem - without to many "if"
statements?
>
> Best regards,
> Ryszard
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! 
> http://www.R-project.org/posting-guide.html
> 

Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm

Use drop = FALSE in your subscripting calls.  That will retain
"matrixness".

For example:
y <- matrix(1:8, ncol = 2)
is.matrix(y[-c(1,2,3),,drop = FALSE]

More info is on the help page for "[".  You can type:  ?"["
to get it from
the command line.

Hope this helps,

Matt Wiener

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of
ryszard.czerminski at novartis.com
Sent: Friday, April 07, 2006 9:16 AM
To: Gabor Grothendieck
Cc: Brian Quinif; r-help at stat.math.ethz.ch; r-help-bounces at
stat.math.ethz.ch
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.

Comments? Suggestions?
How to work around this problem - without to many "if" statements?

Best regards,
Ryszard

______________________________________________
R-help at stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html