[R] sample "n" random positions from a matrix

[Charles C. Berry]

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  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