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 <- 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

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