JustSid's idea to save only the changed values and its offsets is a great idea and common practice in numerical mathematics to save and work with sparse matrices, see here if you want to know more about it:
http://en.wikipedia.org/wiki/Sparse_matrix#Compressed_sparse_row_.28CSR_or_CRS.29
The German version has some probably helpful images, too:
http://de.wikipedia.org/wiki/Compressed_Row_Storage


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