Hi there, I have a binary matrix (dim 100x100) filled with values 0 and 1. I need select a record "n" positions of that matrix when values are 1. How can I do that? Thanks for all, Miltinho Brazil --------------------------------- [[alternative HTML version deleted]]
On 12/10/06, Milton Cezar Ribeiro <milton_ruser at yahoo.com.br> wrote:> I have a binary matrix (dim 100x100) filled with values 0 and 1. I need select a record "n" positions of that matrix when values are 1. How can I do that?Do you mean extracting the indexes of n randomly chosen elements of the matrix which are equal to one? Paul
Milton,
If I understand your problem correctly, you want to take (and store) a
random sample of the indexes of a matrix for values equal to 1. If
that's the case the following may work for you:
m=matrix(sample(c(0,1),100*100,replace=T),nrow=100,ncol=100) #Creates matrix
idxsample=function(m,n){
idx=which(m==1,F)#Index of positions of 1's in the matrix
pos=sample(idx,n,replace=F)#Random sample of matrix indexes
return(pos)
}
This call will sample and store 20 "positions" with value==1 in your
matrix
pos=idxsample(m,n=20)
you can then use the resulting object to retrieve the values from your
matrix i.e.
m[pos] (not surprisingly, all == 1)
I hope this helps,
Francisco
Dr. Francisco J. Zagmutt
College of Veterinary Medicine and Biomedical Sciences
Colorado State University
Milton Cezar Ribeiro wrote:> Hi there,
>
> I have a binary matrix (dim 100x100) filled with values 0 and 1. I need
select a record "n" positions of that matrix when values are 1. How
can I do that?
>
> Thanks for all,
> Miltinho
> Brazil
>
>
> ---------------------------------
>
> [[alternative HTML version deleted]]
>
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>
On Sun, 10 Dec 2006, Milton Cezar Ribeiro wrote:> Hi there, > > I have a binary matrix (dim 100x100) filled with values 0 and 1. I need > select a record "n" positions of that matrix when values are 1. How can > I do that? >Is this what you want?> mat.100 <- diag(100) # just for instance > n <- 7 # supposing you want 7 > which( mat.100==1, arr.ind=T )[ sample( 100, n ), ]row col [1,] 99 99 [2,] 93 93 [3,] 36 36 [4,] 74 74 [5,] 11 11 [6,] 3 3 [7,] 21 21>[...] Charles C. Berry (858) 534-2098 Dept of Family/Preventive Medicine E mailto:cberry at tajo.ucsd.edu UC San Diego http://biostat.ucsd.edu/~cberry/ La Jolla, San Diego 92093-0717
On Sun, 10 Dec 2006, Charles C. Berry wrote:> On Sun, 10 Dec 2006, Milton Cezar Ribeiro wrote: > >> Hi there, >> >> I have a binary matrix (dim 100x100) filled with values 0 and 1. I need >> select a record "n" positions of that matrix when values are 1. How can >> I do that? >> > > Is this what you want? > >> mat.100 <- diag(100) # just for instance >> n <- 7 # supposing you want 7 >> which( mat.100==1, arr.ind=T )[ sample( 100, n ), ] > row col > [1,] 99 99 > [2,] 93 93 > [3,] 36 36 > [4,] 74 74 > [5,] 11 11 > [6,] 3 3 > [7,] 21 21OOPS! That only works when there is exactly one one in each row. This is what is needed generally:> index.of.ones <- which(mat.100==1,arr.ind=T) > index.of.ones[ sample( nrow( index.of.ones ), n ), ]row col [1,] 82 82 [2,] 36 36 [3,] 54 54 [4,] 89 89 [5,] 88 88 [6,] 52 52 [7,] 65 65>>> > > [...] > > Charles C. Berry (858) 534-2098 > Dept of Family/Preventive Medicine > E mailto:cberry at tajo.ucsd.edu UC San Diego > http://biostat.ucsd.edu/~cberry/ La Jolla, San Diego 92093-0717 > > > >Charles C. Berry (858) 534-2098 Dept of Family/Preventive Medicine E mailto:cberry at tajo.ucsd.edu UC San Diego http://biostat.ucsd.edu/~cberry/ La Jolla, San Diego 92093-0717