[R] hidden for() loop subsetting a matrix?

[Federico Calboli]

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

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