On Mon, 26 Apr 2010, Stu wrote:
Hi all,
One subtlety is that the drop argument only works if you specify 2 or
more indices e.g. [i, j, ..., drop=F]; but not for a single index e.g
[i, drop=F].
Wrong.
a <- structure(1:5,dim=5)
dim(a)
[1] 5
dim(a[2:3,drop=F]) # don't drop regardless
[1] 2
dim(a[2,drop=F]) # dont' drop regardless
[1] 1
dim(a[2:3,drop=T]) # no extent of length 1
[1] 2
dim(a[2,drop=T]) # drop, extent of length 1
NULL
Why doesn't R complain about the unused "drop=F" argument in the
single index case?
In the example you give (one index for a two-dimension array), vector
indexing is assumed. For vector indexing, drop is irrelevant.
HTH,
Chuck
Cheers,
- Stu
a = matrix(1:10, nrow=1)
b = matrix(10:1, ncol=1)
# a1 is an vector w/o dim attribute (i.e. drop=F is ignored silently)
(a1 = a[2:5, drop=F])
dim(a1)
# a2 is an vector WITH dim attribute: a row matrix (drop=F works)
(a2 = a[, 2:5, drop=F])
dim(a2)
# b1 is an vector w/o dim attribute (i.e. drop=F is ignored silently)
(b1 = b[2:5, drop=F])
dim(b1)
# b2 is an vector WITH dim attribute: a column matrix (drop=F works)
(b2 = b[2:5, , drop=F])
dim(b2)
On Mar 30, 4:08 pm, lith <minil...@gmail.com> wrote:
Reframe the problem. Rethink why you need to keep dimensions. I never ever had
to use drop.
The problem is that the type of the return value changes if you happen
to forget to use drop = FALSE, which can easily turn into a nightmare:
m <-matrix(1:20, ncol=4)
for (i in seq(3, 1, -1)) {
print(class(m[1:i, ]))}
[1] "matrix"
[1] "matrix"
[1] "integer"
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PLEASE do read the posting guidehttp://www.R-project.org/posting-guide.html
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______________________________________________
R-help@r-project.org mailing list
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
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Charles C. Berry (858) 534-2098
Dept of Family/Preventive Medicine
E mailto:cbe...@tajo.ucsd.edu UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/ La Jolla, San Diego 92093-0901
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
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