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How to Export/Write a SparseArray?
How to Import/Read a SparseArray?
Is it always stored as a normal array?
How can it be stored in a dense form?
Is a numerical SparseArray different to
Export/Import from a string one?

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What a nice poem –  Rojo May 14 '13 at 9:27
    
You can use the methods described in answer to this question: mathematica.stackexchange.com/q/1959/121 In fact I am inclined to close this question as I believe it has already been answered. –  Mr.Wizard May 14 '13 at 11:08
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marked as duplicate by Sjoerd C. de Vries, Artes, Ajasja, Yves Klett, m_goldberg May 14 '13 at 21:06

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1 Answer

The two file formats I am familiar with, with respect to sparse matrices, are the Harwell-Boeing format, and the Matrix Market format. I have linked to the docs on how you can Import[]/Export[] matrices in those formats, and you'll do well by reading up on them.

Sparse arrays are very much different from lists. For instance, consider these two representations of the $9\times 9$ identity matrix:

idDense = IdentityMatrix[9];
idSparse = SparseArray[Band[{1, 1}] -> 1, {9, 9}];

For the purposes of Equal[], they're the same:

idSparse == idDense
   True

SameQ[] shows that they aren't:

idSparse === idDense
   False

One can peek at heads to see the difference:

Head /@ {idSparse, idDense}
   {SparseArray, List}

If, for some reason, you need to convert your sparse array to a dense array, that's what Normal[] is for:

Normal[idSparse] === idDense
   True

See the docs for SparseArray[] and Normal[] for more details.

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OK. . .What if you have txt char data instead? . .As in your example idSparse above replace the 1 by "A"? . .It is still a SparseArry. . .How to export this one? . .Can't see it in Harwell-Boeing or Matrix Market. –  Hp Radojewski Schäfer Von May 15 '13 at 14:04
    
I don't believe any of those formats will work for data with text entries. You will have to use Normal[] on your sparse array and then export to your favorite format. –  J. M. May 15 '13 at 14:07
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