Thank you.
That was exactly what I need.
Looking at '?[' I see...
drop: For matrices and arrays. If TRUE the result is coerced to
the lowest possible dimension (see the examples). This only
works for extracting elements, not for the replacement. See
drop for further details.
But that implies that in the case where 0 rows are returned it should be
coerced into zero dimensions. I am not quite sure what it would mean to be
coerced into zero dimensions, but if I read that without having seen the
actual behavior (impossible now) I would assume...
M[M[,"a"]==1000,] (from my example below)
would return NULL, which has class "NULL" rather than a matrix with zero
rows.
thanks
Worik
On Wed, Apr 11, 2012 at 11:54 AM, David Winsemius <dwinsem...@comcast.net>wrote:
>
> On Apr 10, 2012, at 7:33 PM, Worik R wrote:
>
> Friends
>>
>> I am extracting sub-sets of the rows of a matrix. Generally the result is
>> a matrix. But there is a special case. When the result returned is a
>> single row it is returned as a vector (in the example below an integer
>> vector). If there are 0, or more than 1 rows returned the result is a
>> matrix.
>>
>> I am doing this in a function and I cannot be sure how many rows I am
>> removing. How can I do this in a general way that always returns a
>> matrix?
>>
>>
> ?"["
>
> > M[1, , drop=FALSE]
>
> a b c d
> a1 0 3 2 1
> > class( M[1, , drop=FALSE] )
> [1] "matrix"
>
>
>> M <- matrix(0:3, nrow=3, ncol=4)
>>> colnames(M) <- c('a','b','c','d')
>>> rownames(M) <- c('a1','b2','c3')
>>> N <- M[M[,"a"]==0,]
>>> O <- M[M[,"a"]!=0,]
>>> P <- M[M[,"a"]==100,]
>>> c(class(M), class(N), class(O), class(P))
>>>
>> [1] "matrix" "integer" "matrix" "matrix"
>>
>>> M
>>>
>> a b c d
>> a1 0 3 2 1
>> b2 1 0 3 2
>> c3 2 1 0 3
>>
>>> N
>>>
>> a b c d
>> 0 3 2 1
>>
>>> O
>>>
>> a b c d
>> b2 1 0 3 2
>> c3 2 1 0 3
>>
>>> P
>>>
>> a b c d
>>
>>>
>>>
>> cheers
>> Worik
>>
>> [[alternative HTML version deleted]]
>>
>> ______________________________**________________
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>> PLEASE do read the posting guide http://www.R-project.org/**
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>> and provide commented, minimal, self-contained, reproducible code.
>>
>
> David Winsemius, MD
> West Hartford, CT
>
>
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