I want to gain thousands of random sampling data by randomizing the
presence-absence data. Meantime, one important limition is that the row and
column sums must be fixed. For example, the data "tst" is following:
   site1 site2 site3 site4 site5 site6 site7 site8 1 0 0 0 1 1 0 0 0 1 1 1 0
1 0 1 1 0 0 0 1 0 1 0 0 0 0 1 0 1 0 1 1 0 1 0 0 0 0 0 0 1 0 1 1 1 1 1 1 0 0
0 0 0 0 0 0 0 0 1 0 1 0 1

sum(tst[1,]) = 3, sum(tst[,1])=4, and so on. When I randomize the data, the
first row sums must equal to 3, and the first column sums must equal to 4.
The rules need to be applied to each row and column.
How to get the new random sampling data? I have no idea.
Thanks.

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