R help - Aug 2012 - hidden for() loop subsetting a matrix?

Hi,

I am subsetting a matrix thus:

test
     [,1] [,2] [,3]
[1,]    1    7   13
[2,]    2    8   14
[3,]    3    9   15
[4,]    4   10   16
[5,]    5   11   17
[6,]    6   12   18

test[cbind(c(1,3,5), c(2,1,3))]
[1]  7  3 17

This works fine, and is the equivalent of c(test[1,2], test[3,1], test[5,3]). 
cbind(c(1,3,5), c(2,1,3)) would obviously look like:

     [,1] [,2]
[1,]    1    2
[2,]    3    1
[3,]    5    3


My question is, since the subsetting is effectively extracting the values by
taking one line at a time in the cbind() coordinates matrix, is a for() loop at
work here?  Or is the subsetting action doing something smarter than just
extracting each value reading one set of coordinates at a time?  This is purely
an academic question, but I'm very curious about the answer.

Best wishes,

Federico



--
Federico C. F. Calboli
Neuroepidemiology and Ageing Research
Imperial College, St. Mary's Campus
Norfolk Place, London W2 1PG

Tel +44 (0)20 75941602   Fax +44 (0)20 75943193

f.calboli [.a.t] imperial.ac.uk
f.calboli [.a.t] gmail.com

On Wed, Aug 15, 2012 at 5:32 AM, Federico Calboli
<f.calboli at imperial.ac.uk> wrote:
> Hi,
>
> I am subsetting a matrix thus:
>
> test
>      [,1] [,2] [,3]
> [1,]    1    7   13
> [2,]    2    8   14
> [3,]    3    9   15
> [4,]    4   10   16
> [5,]    5   11   17
> [6,]    6   12   18
>
> test[cbind(c(1,3,5), c(2,1,3))]
> [1]  7  3 17
>
> This works fine, and is the equivalent of c(test[1,2], test[3,1],
test[5,3]).  cbind(c(1,3,5), c(2,1,3)) would obviously look like:
>
>      [,1] [,2]
> [1,]    1    2
> [2,]    3    1
> [3,]    5    3
>
>
> My question is, since the subsetting is effectively extracting the values
by taking one line at a time in the cbind() coordinates matrix, is a for() loop
at work here?  Or is the subsetting action doing something smarter than just
extracting each value reading one set of coordinates at a time?  This is purely
an academic question, but I'm very curious about the answer.
Canonically, see

$(R_HOME)/src/main/subset.c

lines 159ff and

$(R_HOME)/src/main/subscript.c

lines 316ff.

More directly, R does something akin to arrayInd() but at the C level
-- so yes, a for loop, but a good one ;-) -- and then regular
subsetting.

Cheers,
Michael
>
> Best wishes,
>
> Federico
>
>
>
> --
> Federico C. F. Calboli
> Neuroepidemiology and Ageing Research
> Imperial College, St. Mary's Campus
> Norfolk Place, London W2 1PG
>
> Tel +44 (0)20 75941602   Fax +44 (0)20 75943193
>
> f.calboli [.a.t] imperial.ac.uk
> f.calboli [.a.t] gmail.com
>
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