sample "n" random positions from a matrix

  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

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

 		
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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, 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

OOPS! 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