Message-ID: <Pine.LNX.4.61.0508150829020.22285@gannet.stats>
Date: 2005-08-15T07:33:50Z
From: Brian Ripley
Subject: Vector comparison to matrix
In-Reply-To: <Pine.LNX.4.61.0508150739170.19359@gannet.stats>
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