As with most things like this, you can trade memory for speed. Here is
an obfuscated solution that appears to eschew loops entirely.
> ma <- matrix(10:15, nr = 3)
> idx <- matrix(c(1,3,2, 2,3,1), nr = 3)
> mb <- ma
> mb[] <- as.vector(ma)[as.vector(idx +
outer(rep(nrow(ma), nrow(ma)), 1:ncol(ma)-1, '*'))]
> mb
[,1] [,2]
[1,] 10 14
[2,] 12 15
[3,] 11 13
Ordinarily, though, my preferred solution would be the for() loop.
Bill Venables
CMIS, CSIRO Laboratories,
PO Box 120, Cleveland, Qld. 4163
AUSTRALIA
Office Phone (email preferred): +61 7 3826 7251
Fax (if absolutely necessary): +61 7 3826 7304
Mobile (rarely used): +61 4 1963 4642
Home Phone: +61 7 3286 7700
mailto:[EMAIL PROTECTED]
http://www.cmis.csiro.au/bill.venables/
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Osorio Roberto
Sent: Friday, 19 January 2007 4:15 PM
To: [email protected]
Subject: [R] Vectorize rearrangement within each column
Consider a matrix like
> ma = matrix(10:15, nr = 3)
> ma
[,1] [,2]
[1,] 10 13
[2,] 11 14
[3,] 12 15
I want to rearrange each column according to row indexes (1 to 3)
given in another matrix, as in
> idx = matrix(c(1,3,2, 2,3,1), nr = 3)
> idx
[,1] [,2]
[1,] 1 2
[2,] 3 3
[3,] 2 1
The new matrix mb will have for each column the corresponding column
of ma indexed by the corresponding column of idx, as in
> mb = ma
> for (j in 1:2) mb[,j] = ma[idx[,j], j]
> mb
[,1] [,2]
[1,] 10 14
[2,] 12 15
[3,] 11 13
Can I avoid the for() loop? I'm specially interested to find out if a
fast implementation using lapply() would be feasible for large input
matrices (analogues of ma and idx) transformed into data frames.
Roberto Osorio
______________________________________________
[email protected] mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
______________________________________________
[email protected] mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
