[R] logical masking of a matrix converts it to a vector

[Ravi Varadhan]

Check out `drop'

?drop

a <- matrix(c(1, 2, 3, 4), nrow = 2)

rmask <- c(TRUE, FALSE)

b <- a[rmask, , drop=FALSE]

colSums(b)

This will maintain the matrix structure.

Ravi.

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Ravi Varadhan, Ph.D.

Assistant Professor, The Center on Aging and Health

Division of Geriatric Medicine and Gerontology 

Johns Hopkins University

Ph: (410) 502-2619

Fax: (410) 614-9625

Email: rvaradhan at jhmi.edu

Webpage:
http://www.jhsph.edu/agingandhealth/People/Faculty_personal_pages/Varadhan.h
tml

 

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-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On
Behalf Of Austin Huang
Sent: Friday, December 04, 2009 3:52 PM
To: R-help at r-project.org
Subject: [R] logical masking of a matrix converts it to a vector

One problem I've been having is the special case in which only one
row/column remains and the variable gets converted into a vector when
entries are removed by logical masking. This is a problem because subsequent
code may rely on matrix operations (apply, colsums, dim, etc) For example:

> a <- matrix(c(1, 2, 3, 4), nrow = 2)
> a
     [,1] [,2]
[1,]    1    3
[2,]    2    4
> rmask <- c(TRUE, FALSE)
> b <- a[rmask, ]
> colSums(b)
Error in colSums(b) : 'x' must be an array of at least two dimensions

To ensure the code works regardless of how the matrix gets modified, I need
to explicitly recast results to a matrix, for example:

b <- matrix(a[rmask, ], ncol = 2)

This can get messy, requiring extra work to maintain column/row names or
additional commands to determine the correct dimensions of the matrix (for
example, if I'm masking both rows and columns simultaneously). Is there a
more elegant way to deal with this in R?

~A

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