From: Bill Venables <wvenable@arcola.stats.adelaide.edu.au>
Date: Sat, 27 Mar 1999 13:00:54 +1030 (CST)
To: Peter Dalgaard BSA <p.dalgaard@biostat.ku.dk>
Subject: Re: [R] Array indexing
In-Reply-To: <x2g16saueq.fsf@blueberry.kubism.ku.dk>
<x2g16saueq.fsf@blueberry.kubism.ku.dk>
>>>>> "Peter" == Peter Dalgaard BSA <p.dalgaard@biostat.ku.dk> writes:
Peter> John Logsdon <j.logsdon@lancaster.ac.uk> writes:
>> The only way I can see of getting the vector result I want
>> is:
>>
>> z <- rep(0,4)
>> for (i in 1:4) z[i] <- a[i, b[i]]
>>
>> # 11 24 42 60
>>
Peter> Well, there's diag(a[,b]) or
Peter> sapply(1:4,function(i) a[i, b[i]]) or (double-UGH!! but
Peter> efficient) Fortran style a[(b-1)*nrow(a)+1:4]
Peter> (And probably an obvious one that I've forgotten for
Peter> the moment...)
Indeed there is and just about everyone does forget it:
> b <- c(1,2,4,5)
> z <- a[cbind(1:4, b)]
> z
[1] 11 24 42 60
You can also work it in the other direction:
> a
[,1] [,2] [,3] [,4] [,5]
[1,] 11 12 13 14 15
[2,] 22 24 26 28 30
[3,] 33 36 39 42 45
[4,] 44 48 52 56 60
> a[cbind(1:4, c(2,3,1,5))] <- 0
> a
[,1] [,2] [,3] [,4] [,5]
[1,] 11 0 13 14 15
[2,] 22 24 0 28 30
[3,] 0 36 39 42 45
[4,] 44 48 52 56 0
It's sometimes called a "matrix index" which can be a bit
confusing since the *index* is the matrix as well as the thing
it indexes (though the latter can be an arbitrary array).
Bill Venables.
-- The following contact details work now, but is only official after 1/5/1999: ----------------------------------------------------------------- Dr Bill Venables, Statistician, CMIS Environmetrics Project.Physical address: Postal address: CSIRO Marine Laboratories, PO Box 120, 233 Middle St, Cleveland, Queensland Cleveland, Qld, 4163 AUSTRALIA AUSTRALIA
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