Vector comparison to matrix
On Mon, 15 Aug 2005, Prof Brian Ripley wrote:
Probably you use the idea from unique.matrix, that is 1) form a string from each row and 2) call match() to see which strings match your pattern row.
If your matrix A really does have short rows like c(1,2,3) and millions of them, another idea is to do target <- rep(c(1,2,3), each= nrow(A)) rowSums(A != target) == 0 For wider rows my first suggestion is probably faster.
On Sun, 14 Aug 2005, Todd Remund wrote:
I am looking for a fast way to count the number of rows in a matrix are identical to a pattern vector. For example, if I am interested in counting the number of row vectors in a matrix that are identical to (1,2,3) what would I do? I have tried the identical statement in a loop but this is far too slow. I have a very large matrix and need to avoid loops at all costs.
Brian D. Ripley, ripley at stats.ox.ac.uk Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595