trying to minimize the memory of a matrix with sparse

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

Answer 1

[Iain]

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.

Answer 2

[Matt J]

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.

[Matt J]

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