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I am given an M * N (M > 1, N > 1) matrix with all the numbers blackened but their row and column sums are visible.

For example, I am given this 3 * 3 matrix.

enter image description here

And one of the possible matrix values are

enter image description here

My question is, can one solve one set of possible M * N matrix values, depending on just the row and column sums, in polynomial time? For example, can one use those 6 red values in the picture to solve those 9 black values as a possible solution, in polynomial time? For a matrix of M * N values, there will be M + N sum values. M and N can be very large.

Thanks!

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  • $\begingroup$ Use max flow... $\endgroup$
    – Laakeri
    Aug 10 at 6:01
  • $\begingroup$ Thank you. Could you give an example? $\endgroup$
    – Billy Chen
    Aug 10 at 7:17

1 Answer 1

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It seems the following very simple algorithm works as long as $\sum_x \text{row}[x]= \sum_y \text{col}[y]$:

for x in 1..N:
  for y in 1..M:
    take = min(row[x],col[y])
    row[x] -= take
    col[y] -= take
    m[x][y] = take

Indeed the invariant $\sum_x \text{row}[x]= \sum_y \text{col}[y]$ is preserved after each iteration of the loop and when the inner loop is finished for some $x$ we can see that row$[x]=0$ which means (by invariant) that when the outer loop is finished we have $\sum_y \text{col}[y]=0$.

In the case of negative numbers we can get back to non-negative numbers by virtually adding some constant $c$ to all matrix cells (i.e. adding $cN$ to all columns and $cM$ to all rows).

Note that if $\sum_x \text{row}[x] \neq \sum_y \text{col}[y]$ then the problem is unsolvable.

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  • 1
    $\begingroup$ Well if the numbers are not required to be nonnegative, the problem is much simpler: you can put e.g. $m[1][y]=col[y]$ for $y\ge2$, $m[x][1]=row[x]$ for $x\ge2$, assign whatever is left to $m[1][1]$, and leave the remaining entries as $0$. $\endgroup$ Aug 10 at 9:45
  • $\begingroup$ Yes, indeed, that works :) $\endgroup$
    – Louis
    Aug 10 at 13:10
  • $\begingroup$ Thank you Louis and Emil for your answer! Yes, negative answers are allowed and the condition ∑𝑥row[𝑥] = ∑𝑦col[𝑦] is not required. It seems then this is an NP problem. $\endgroup$
    – Billy Chen
    Aug 10 at 19:49
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    $\begingroup$ It is not an NP problem because it is not a decision problem. If you were asking "is there a solution given row and col?" then it would be a decision problem and that decision problem would be in P because you just have to check that Σrow=Σcol (and since P is a subset of NP then it would be in NP but I suppose that was not what you meant). $\endgroup$
    – Louis
    Aug 11 at 8:32
  • $\begingroup$ Oh I see. Indeed, I revisited the definition of NP problems and they say "NP is the set of decision problems". And yes, my question is not a decision problem. Thank you again! $\endgroup$
    – Billy Chen
    Aug 11 at 18:46

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