[julia-users] spzeros column vs row vector memory footprint

[Zouhair Mahboubi]

I have the following code:

NS = 23^3
TSA = Array(Dict, NS)

for from_i in 1:NS
           TSA[from_i] = Dict()
           for a in 1:6
               TSA[from_i][a] = spzeros(Float32, 1, NS)
           end
end

This gobbles up about 4G of memory. If instead, I use TSA[from_i][a] =
spzeros(Float32, NS, 1) it barely makes a dent in my memory usage.

FYI, I’m using dictionaries here because in my application a’s are actually
tuples of symbols, not integers, and it makes things easier.

The only reason I wrote it as a rowvector is because I need to dot it with
a non-sparse vector b, but unfortunately dot does not support
SparseMatrixCSC so I’m using a' * b instead. I can get away with using the
column vector for now by doing b' * a.

I was surprised at the difference in memory footprint, and I’m curious if
someone would care to explain why that is?

Thanks,
-z
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