# Iterating over all matrices with fixed row and column sums

Can anyone suggest an algorithm for iterating once through all matrices with non-negative integer entries which are $2$ by $n$ with fixed row sums ($r_1$ and $r_2$) and fixed column sums ($c_1, c_2, \ldots c_n$)? For instance, for row and column sums $r_1= r_2 = c_1 = c_2 = 2$, (if I haven't make a mistake) the possible matrices would be

$\left(\begin{array}{cc} 2 & 0\\ 0 & 2 \end{array}\right)$

$\left(\begin{array}{cc} 1 & 1\\ 1 & 1 \end{array}\right)$

$\left(\begin{array}{cc} 0 & 2\\ 2 & 0 \end{array}\right)$

However, I would want to do this for general $2$ by $n$ matrices, not just $2$ by $2$.

I would also like to know if there is a quick way to compute the total number of matrices I would have to iterate over.

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It seems you're interested only in matrices with non-negative integer entries? –  joriki Oct 1 '11 at 13:01
@joriki, Good catch. –  Henry B. Oct 1 '11 at 13:04
It still doesn't say "integer". –  joriki Oct 1 '11 at 13:08