trying to minimize the memory of a matrix with sparse

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the meaning of the sparse function is to reduce memory usage by ignoring the zero elements inside a matrix, for some reason when i use this function the memory increase what am i doing wrong?
>> whos('hologram_r1_r2')
Name Size Bytes Class Attributes
hologram_r1_r2 376x376 2262016 double complex
>> hologram_r1_r2(abs(hologram_r1_r2)<min(min(abs(hologram_r1_r2))))=0;
>> whos('hologram_r1_r2')
Name Size Bytes Class Attributes
hologram_r1_r2 376x376 2262016 double complex
>> hologram_r1_r2=sparse(hologram_r1_r2);
>> whos('hologram_r1_r2')
Name Size Bytes Class Attributes
hologram_r1_r2 376x376 3396040 double sparse, complex

Answers (2)

Iain
Iain on 13 Feb 2014
Edited: Iain on 13 Feb 2014
Sparse only saves memory if your array is dominated by 0. That is exactly zero, and not small numbers.
The memory used by sparse is: 4/8/16 bytes for the value of each nonzero element, plus some overhead to store the location of each nonzero element, so a full "sparse" array can take many times the memory of a genuine double array.

Matt J
Matt J on 13 Feb 2014
Edited: Matt J on 13 Feb 2014
If the number of zero elements isn't at least 2/3 of the matrix (or 1/2 for a complex matrix), you won't benefit in terms of memory. Each non-zero element consumes 3 times as much memory as an ordinary matrix due to the need to store (i,j) coordinate data as well.
  1 Comment
Matt J
Matt J on 13 Feb 2014
Edited: Matt J on 13 Feb 2014
Wide sparse matrices also tend to consume more than tall ones. Here's my favorite example of an all-zero sparse matrix, which nevertheless, consumes 80 MB of memory
>> At=sparse(1,1e7); >> At=sparse(1,1e7); whos At
Name Size Bytes Class Attributes
At 1x10000000 80000024 double sparse

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