Both of these work fine, the second being more compact. Thanks. After floundering around, however, I believe the easiest way to find the positions(s) of single element lists is to use pos=Position[ mat, {}]. For double element lists (and so on) use Position[mat, {,_}]. After you have found the position in the form {i,j} , you can recapture that element of matrix mat with Extract[ mat, pos]. KH

Hi,

if I understand correctly this should do the trick.

First I generate a matrix which is -hopefully- of the form you want to test my algorithm:

matrix = Table[Table[RandomInteger[8] + 1, {m, 1, RandomInteger[4] + 1}], {i, 1, 9}, {j, 1, 9}]

Then this function should do the trick:

Position[ArrayReshape[Length /@ Flatten[matrix, 1], {9, 9}], 1]

Cheers, Marco

PS: You might also like http://mathworld.wolfram.com/notebooks/RecreationalMath/Sudoku.nb and http://demonstrations.wolfram.com/SudokuGame/