[R] Vector comparison to matrix

[Prof Brian Ripley]

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