# [R] changing equal values on matrix by same random number

William Dunlap wdunlap at tibco.com
Wed Aug 26 21:48:41 CEST 2009

```
Bill Dunlap
TIBCO Software Inc - Spotfire Division
wdunlap tibco.com

> -----Original Message-----
> From: r-help-bounces at r-project.org
> [mailto:r-help-bounces at r-project.org] On Behalf Of milton ruser
> Sent: Wednesday, August 26, 2009 9:54 AM
> To: r-help at r-project.org
> Subject: [R] changing equal values on matrix by same random number
>
> Dear all,
>
> I have about 30,000 matrix (512x512), with values from 1 to N.
> Each value on a matrix represent a habitat patch on my
> matrix (i.e. my landscape). Non-habitat are stored as ZERO.
> No I need to change each 1-to-N values for the same random
> number.
>
> Just supose my matrix is:
> mymat<-matrix(c(1,1,1,0,0,0,0,0,0,0,0,
> 0,0,0,0,2,2,2,0,0,0,0,
> 0,0,0,0,2,2,2,0,0,0,0,
> 3,3,0,0,0,0,0,0,0,4,4,
> 3,3,0,0,0,0,0,0,0,0,0), nrow=5)
>
> I would like that all cells with 1 come to be
> runif(1,min=0.4, max=0.7), and cells with 2
> be replace by another runif(...).
>
> I can do it using for(), but it is very time expensive.
> Any help are welcome.

Is the following what you want?  It uses the small
integers in mymat as indices into a vector of N random
numbers.

> mymat
[,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10] [,11]
[1,]    1    0    0    2    0    0    0    0    0     3     0
[2,]    1    0    0    2    0    2    0    0    0     0     0
[3,]    1    0    0    2    0    2    0    0    4     0     0
[4,]    0    0    0    0    0    2    3    0    4     0     0
[5,]    0    0    0    0    0    0    3    0    3     0     0
> range(unique(mymat))
[1] 0 4
> f<-function(mat){
N<-max(mat)
tmp <- mat[mat>0]
tmp <- runif(N, 0.4, 0.7)[tmp]
mat[mat>0] <- tmp
mat
}
> f(mymat)
[,1] [,2] [,3]      [,4] [,5]      [,6]      [,7] [,8]
[,9]
[1,] 0.5361427    0    0 0.6727491    0 0.0000000 0.0000000    0
0.0000000
[2,] 0.5361427    0    0 0.6727491    0 0.6727491 0.0000000    0
0.0000000
[3,] 0.5361427    0    0 0.6727491    0 0.6727491 0.0000000    0
0.5357566
[4,] 0.0000000    0    0 0.0000000    0 0.6727491 0.6417832    0
0.5357566
[5,] 0.0000000    0    0 0.0000000    0 0.0000000 0.6417832    0
0.6417832
[,10] [,11]
[1,] 0.6417832     0
[2,] 0.0000000     0
[3,] 0.0000000     0
[4,] 0.0000000     0
[5,] 0.0000000     0

The 3 lines involving tmp could be collapsed into one, making
things more obscure
mat[mat>0] <- runif(N, 0.4, 0.7)[mat[mat>0]]

Bill Dunlap
TIBCO Software Inc - Spotfire Division
wdunlap tibco.com

> cheers
>
> milton
>
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>
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