[R] Request for help

[Patrick Burns]

If I understand your problem properly, then your matrices have a
known number of zeros and ones in them.  So you can create a
matrix with just this constraint binding via:

mat <- matrix(sample(rep(0:1, c(nzeros, nones))), nr, nc)

That command first generates the appropriate number of zeros and ones
(via 'rep'), then does a random permutation of them (with 'sample') and
finally turns it into a matrix.

You could then test for the row and column constraints, and permute
the sub-matrix of rows and columns that do not obey their constraints.
It could look something like:

mat[bad.rows, bad.cols] <- sample(mat[bad.rows, bad.cols])

where 'bad.rows' and 'bad.cols' are logical vectors stating if the 
constraints
are satisfied or not.

You do not need to be a statistician to use R -- far from it.  The 
'Guide for the
Unwilling' gives you a  brief introduction.  There is also a lot of 
introductory
material in the contributed documentation section of  the R Project website.

It would be good to use a more descriptive subject for messages to R-help.

Patrick Burns

Burns Statistics
patrick at burns-stat.com
+44 (0)20 8525 0696
http://www.burns-stat.com
(home of S Poetry and "A Guide for the Unwilling S User")

Michela Marignani wrote:

> My name is Michela Marignani and I'm an ecologist trying to solve a 
> problem linked to knight' s tour algorithm.
> I need a program to create random matrices with presence/absence (i.e. 
> 1,0 values), with defined colums and rows sums, to create null models 
> for statistical comparison of species distribution phenomena.
> I've seen on the web many solutions of the problem, but none provides 
> the freedom to easily change row+colums constraint and none of them 
> produce matrices  with 1 and 0. Also, I've tryied to use R, but it is 
> too complicated for a not-statistician as I am....can you help me?
>
> Thank you for your attention,
> so long
>
> Michela Marignani
> University of Siena
> Environmental Science Dept.
> Siena, Italy
> michela.marignani at uniroma1.it
>
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