Re: [R-sig-Geo] obtaining row indexes from a list of column indexes

[Roger Bivand]

On Wed, 17 Nov 2010, Edzer Pebesma wrote:

List,

given an arbitrary n x m boolean matrix, say

     [,1]  [,2]  [,3]  [,4]  [,5]
[1,] FALSE FALSE FALSE FALSE FALSE
[2,] FALSE FALSE  TRUE FALSE FALSE
[3,] FALSE FALSE FALSE FALSE FALSE
[4,] FALSE FALSE FALSE  TRUE  TRUE

I have the TRUE entries (indexes) of each column, in a list

[[1]]
integer(0)

[[2]]
integer(0)

[[3]]
[1] 2

[[4]]
[1] 4

[[5]]
[1] 4

How can I, from this list, efficiently obtain the TRUE entries of each
row, in this case

[[1]]
integer(0)

[[2]]
[1] 3

[[3]]
integer(0)

[[4]]
[1] 4 5

without constructing the n x m boolean matrix?

Use sparse matrices?

library(Matrix)
i <- c(2, 4, 4)
j <- c(3, 4, 5)
x <- c(TRUE, TRUE, TRUE)
M <- sparseMatrix(i, j, x=x)
here <- which(M, arr.ind=TRUE)
split(here[,2], here[,1])

is almost there, what remains is to insert the empty list elements.

Roger

--
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: roger.biv...@nhh.no

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