[R] Array indexing

[Bill Venables]

>>>>> "Peter" == Peter Dalgaard BSA <p.dalgaard at biostat.ku.dk> writes:

    Peter> John Logsdon <j.logsdon at 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:
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Dr Bill Venables, Statistician, CMIS Environmetrics Project.

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