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

[Edzer Pebesma]

Thanks -- I want to retrieve the one list of indexes from the other,
without constructing the matrix. m and n might be 1e6, so constructing
the full matrix is to be avoided.

On 11/17/2010 08:55 PM, caspar hallmann wrote:
> 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
>>
>> _______________________________________________
>> R-sig-Geo mailing list
>> R-sig-Geo at stat.math.ethz.ch
>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>>

-- 
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