Sparse for arrays with lots of NaN.

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Laurent
Laurent 2013-9-4
编辑： Walter Roberson 2017-12-4
Hi,
I am using very large arrays containing a lot fo NaNs. If they were zeros, I could use 'sparse' to save a lot of memory, but since they are NaNs this does not work. Does a sparse datatype exist for arrays with a lot of NaNs?

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Matt J
Matt J 2013-9-4
No, but why do you insist on using NaNs instead of zeros?
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Matt J
Matt J 2017-12-4
编辑：Matt J 2017-12-4
How often does sensor failure occur? Is it really often enough to benefit from a sparse representation? That would mean that at least half of your records are invalid.
But in any case, you could, for example, add +1 to all the valid wind speed values so that they are encoded into the interval [1,101]. Then reserve 0 for missing data. Since you clearly don't plan to do linear algebra with this matrix, I don't think this should interfere with anything.
Walter Roberson
Walter Roberson 2017-12-4
编辑：Walter Roberson 2017-12-4
Construct a sparse logical array of locations that are nan. Zero those locations out in the other array and sparse that (assuming that less than about 1/3 of the entries remain as non-zero, to make it worth while to construct the sparse array.)
If more than about 2/3 of your locations are nan, then it might instead make sense to create a logical array of locations which were originally 0 and not nan, and zero out the nan values and sparse the result of that; then the entries in the logical array would say where the "true" zeros are, and the rest of the zeros in the second array would be assumed to be nan.

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