On Apr 21, 2009, at 8:33 AM, onyourmark wrote:
Thanks very much.
I don't really understand the row() function. I looked in the
reference but
I don't really get it. It says:
Description
Returns a matrix of integers indicating their row number in a matrix-
like
object, or a factor indicating the row labels.
Usage
row(x, as.factor = FALSE)
Arguments
x a matrix-like object, that is one with a two-dimensional dim.
I don't understand what row() does.
It returns a matrix of the same size as its arguments populated with
the row numbers instead of the matrix elements.
And in the example in the documentation it says:
x <- matrix(1:12, 3, 4)
Now try row(x)
# extract the diagonal of a matrix
dx <- x[row(x) == col(x)]
dx
[1] 1 5 9
I thought the single square bracket notation accepts a pair
separated by a
comma but I don't see how
row(x)==col(x) produces that?
The single square bracket notation can be used as [r,c] or without
the comma which requires a logical index. In the second method the
matrix entries get processed serially, column-wise.
> x <- matrix(1:12, 3, 4)
> x[TRUE]
[1] 1 2 3 4 5 6 7 8 9 10 11 12
> row(x)
[,1] [,2] [,3] [,4]
[1,] 1 1 1 1
[2,] 2 2 2 2
[3,] 3 3 3 3
--
David Winsemius, MD
Heritage Laboratories
West Hartford, CT
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