[R] Vectorize rearrangement within each column

From: Osorio Roberto <roboso_at_gmail.com>
Date: Fri 19 Jan 2007 - 06:14:31 GMT


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



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