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

[caspar hallmann]

Edzer,

say m is your matrix of TRUE's and FALSE's, then:

 which(m,arr.ind=TRUE)

will give you the column and row numbers of the matrix satisfying the
conditions supplied to which.

Hope it helps

Caspar


On Wed, Nov 17, 2010 at 8:29 PM, Edzer Pebesma
<edzer.pebesma at uni-muenster.de> 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?
> --
> Edzer Pebesma
> Institute for Geoinformatics (ifgi), University of Münster
> Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
> 8333081, Fax: +49 251 8339763  http://ifgi.uni-muenster.de
> http://www.52north.org/geostatistics      e.pebesma at wwu.de
>
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