[R] how to superimpose 2 matrices of different sizes

[David Winsemius]

On Sep 15, 2010, at 7:40 PM, Dennis Murphy wrote:

> Hi:
>
> Try something like this:
>
> ind <- as.matrix(expand.grid(rownames(U), colnames(U)))
> Uempty[ind] <- U[ind]
>
> ind should be a two column matrix of (row, col) indices. Then just  
> replace
> the values of Uempty associated with those indices with the  
> corresponding
> entries of U.

The two-column matrix indexing strategy is one of those truly powerful  
devices in R. Thanks for reminding us of it, Dennis. It would allow  
one even more complex operations such as taking the "same section" of  
two matrices and add or multiplying. I had composed a clunky rbind/ 
cbind approach, but threw it away when I saw this one.

Note to self: It's also a powerful lookup strategy into tables.

-- 
David
>
> HTH,
> Dennis
>
> On Wed, Sep 15, 2010 at 4:31 PM, Nicolas Gutierrez <nicolasg at uw.edu>  
> wrote:
>
>> Hi All,
>>
>> I’m trying to superimpose (or "add")  two matrices:
>>
>> 1. resulting from a table function with frequencies:
>>
>>> U=table(pop$xloc, pop$yloc))
>>
>>     14 15 16 17 18 19 20 21 22
>> 5   0  0  0  0  0  0  0  0  0
>> 7   0  0  0  0  0  0  0  0  0
>> 8   0  0  0  0  0  0  0  0  0
>> 9   0  0  0  0  0  0  9  0  5
>> 10  0  0  0  0  0  0  0  0  0
>> 11  0  0  0  0  0  0  0  0  0
>>
>> 2. The second created to match my spatial grid (20x25):
>>
>>> Uempty=matrix(data = 0, nrow = 20, ncol = 25)
>>
>> I want to place each element of U (e.g. U[22,9]=5) in the  
>> corresponding
>> cell of Uempty and end up with this final matrix like this:
>>
>>     1  2  3 ...  22 23 24 25
>> 5   0  0  0  ...  0  0  0  0
>> :   :  :  :  ...  :  :  :  :
>> 7   0  0  0  ...  0  0  0  0
>> 8   0  0  0  ...  0  0  0  0
>> 9   0  0  0  ...  5  0  0  0
>> :   :  :  :  ...  :  :  :  :
>> 20  0  0  0  ...  0  0  0  0
>>
>> Any ideas?
>>
>> Thanks!!!
>>
>> Nico
>>
>> ______________________________________________
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>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>
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>
> ______________________________________________
> 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.

David Winsemius, MD
West Hartford, CT