hidden for() loop subsetting a matrix?
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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