[R] How to extract from a matrix based on indices in a vector?

[Logickle]

Thanks, guys. I'll take at a look at these solutions. BTW, my apologies, by
A,B,C,D,... I meant "just some real number", not characters "A","B",..., but
I wasn't clear about that. If that effects the solution approach please let
me know, but I don't think it does. Thanks again, Doug


Marc Schwartz-3 wrote:
> 
> On Jun 10, 2009, at 7:05 PM, William Dunlap wrote:
> 
>> Subscripting by a 2-column matrix
>>   M[cbind(v, seq_len(ncol(M)))]
>> uses much less space (hence time) than making
>> the ncol(M) by ncol(M) intermediate matrix just
>> to extract its diagonal.  E.g.
>>
>> test <- function(n, seed) {
>>   if (!missing(seed))
>>      set.seed(seed)
>>   M <- matrix(sample(LETTERS, 2*n, replace = TRUE), 2)
>>   v <- sample(2, n, replace=T)
>>   t1<-system.time(r1<-M[cbind(v,seq_len(ncol(M)))])
>>   t2<-system.time(r2<-diag(M[v, 1:ncol(M)]))
>>   list(identical=identical(r1,r2), "time(matrix subscript)"=t1,
>> "time(diag(big matrix))"=t2)
>> }
>>
>>> test(100)
>> $identical
>> [1] TRUE
>>
>> $`time(matrix subscript)`
>>   user  system elapsed
>>  0.000   0.000   0.001
>>
>> $`time(diag(big matrix))`
>>   user  system elapsed
>>  0.001   0.000   0.001
>>
>>> test(1000)
>> $identical
>> [1] TRUE
>>
>> $`time(matrix subscript)`
>>   user  system elapsed
>>  0.000   0.000   0.001
>>
>> $`time(diag(big matrix))`
>>   user  system elapsed
>>  0.082   0.021   0.103
>>
>>> test(5000)
>> $identical
>> [1] TRUE
>>
>> $`time(matrix subscript)`
>>   user  system elapsed
>>  0.001   0.000   0.001
>>
>> $`time(diag(big matrix))`
>>   user  system elapsed
>>  3.379   0.552   3.932
> 
> 
> 
> Nicely done comparison Bill.
> 
> Thanks,
> 
> Marc
> 
> ______________________________________________
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> and provide commented, minimal, self-contained, reproducible code.
> 
> 

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