[R] an efficient pairwise matrix cell's comparison function

[Prof Brian Ripley]

C <- B
C[A==0] <- 0

would be somewhat more efficient.

On Sun, 2 Mar 2008, jim holtman wrote:

> Does this do what you want?
>
>> A <- matrix(sample(0:2, 25, TRUE), ncol=5)
>> B <- matrix(1:25, ncol=5)
>> C <- ifelse(A == 0, 0, B)
>> A
>     [,1] [,2] [,3] [,4] [,5]
> [1,]    1    1    1    2    1
> [2,]    1    0    1    1    0
> [3,]    0    0    1    0    2
> [4,]    0    1    2    0    0
> [5,]    1    2    1    2    2
>> B
>     [,1] [,2] [,3] [,4] [,5]
> [1,]    1    6   11   16   21
> [2,]    2    7   12   17   22
> [3,]    3    8   13   18   23
> [4,]    4    9   14   19   24
> [5,]    5   10   15   20   25
>> C
>     [,1] [,2] [,3] [,4] [,5]
> [1,]    1    6   11   16   21
> [2,]    2    0   12   17    0
> [3,]    0    0   13    0   23
> [4,]    0    9   14    0    0
> [5,]    5   10   15   20   25
>>
>
>
> On Sun, Mar 2, 2008 at 7:11 AM, Diogo André Alagador
> <mcnda839 at mncn.csic.es> wrote:
>> To all,
>>
>>
>>
>> I am undergoing an analysis involving big matrices of about 30000x200 which
>> I have to handle in a more efficient way. So I would like some advice to
>> build such efficient function to deliver the following result:
>>
>>
>>
>> -          starting with 2 matrices of the same dimension (eg. A and B)
>>
>>
>>
>>       0  0  3  5                      6  0  0  5
>>
>> A=   0  0  6  4              B=   0  4  3  5
>>
>>             0  0  5  0                      1  0  0  9
>>
>>
>>
>> -          the function should deliver a C matrix (same dimension too),
>> where at each position C(i,j), compares A and B.
>>
>>      if A(i,j)=0, than C(i,j)=0,
>>
>>      if A(i,j)!=0, than C(i,j)=B(i,j)
>>
>>
>>
>>      6  0  0  5
>>
>> C= 0  0  3  5
>>
>>  0  0  0  0
>>
>>
>>
>> Although not an expert I could build a function with 2 cycles (reading
>> columns and rows) which is not quick. Maybe you can help me in this
>> "challenge".
>>
>>
>>
>> Much thanks in advance,
>>
>>
>>
>>
>> Diogo André Alagador
>> Biodiversity & Global Change Lab, Museo Nacional de Ciencias Naturales,
>> CSIC, Madrid, España
>> Forest Research Centre, Instituto Superior de Agronomia, Universidade
>> Técnica de Lisboa, Lisboa, Portugal
>>
>>
>>        [[alternative HTML version deleted]]
>>
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>>
>
>
>
> --
> Jim Holtman
> Cincinnati, OH
> +1 513 646 9390
>
> What is the problem you are trying to solve?
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

-- 
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