This is not a precise answer to the actual question, but the list of answers is from 2012 and IMO does urgently lack a note about Association. Since version 10 (2014) these will solve the underlying problem by offering an alternative for such cases:
ClearAll@matM matM = Association[] matM[i] = ConstantArray[1, {10, 10}]; matM[[Key[i], 1, 10]] = 10 For every case where one uses downvalues as a hashtable using Association instead like shown will also have many other advantages. I have not made tests but I would be very surprised if the above wouldn't be much more efficient than any of the alternatives. Of course using an Association will not work in all cases, e.g. when one needs a mixture of patterns and constant "indices" in the downvalues of a symbol. But for a vast majority of appications where one runs into the mentioned problem nowadays Association will certainly be the best solution.
