MATLAB: Do I receive an “Out of memory” error when comparing if the elements in a sparse matrix are smaller than a certain value

indexindexinglogical?MATLABsparcesparse

Having a very large sparse matrix (> 10000×10000), with most of the elements equal to zero, and then indexing the matrix by using a logical operator such as:

a = largeMatrix(largeMatrix < comparisonDouble);

Where "comparisonDouble" is a positive value, the operation may result in the following error:

 ??? Error using ==> lt
 Out of memory. Type HELP MEMORY for your options.

Best Answer

This error is caused because the condition "< comparisonDouble" also applies to all elements that are equal to zero. The resulting logical sparse matrix thus contains only a few zeros, and most elements are ones, so it's size approaches that of a full sparse matrix.

To work around this issue, first extract all elements that are "> 0", and then apply the condition "< comparisonDouble" to them, like this:

a = largeMatrix(largeMatrix(largeMatrix > 0) < comparisonDouble);

Related Solutions

MATLAB: Is this sparse multiplication failing

You could use the relevant indices to perform the multiplication:

b = sparse(17249876309, 29);
b(430: 490) = 1;
c = sparse(ones(29, 82));
[row, col] = find(b);
d = sparse(size(b, 1), size(c, 2));
d(row, 1:end) = b(row, :)*c; 
spy(d)
h=gcf; 
set(h.Children,'Ylim',[400 500]);

However it's probably not even necessary to construct the whole matrix again. Note that you have to use 1:end instead of : in the assignment, otherwise you get a memory error.

MATLAB: Query regarding a sparse complex matrix

Hi,

the resulting size of the sparse matrix depends from the number of zero entries. Consider this:

A = randi(10,100);
A(A<=7) = 0;
B = sparse(A);
whos A B
Name        Size             Bytes  Class     Attributes
A         100x100            80000  double              
B         100x100            49096  double    sparse

The code above produced a sparse matrix which has about 60% size then the normal one. There should be about 70% zeros in it.

If we only have about 30% zeros the situation changes:

A = randi(10,100);
A(A<=3) = 0;
B = sparse(A);
whos A B
Name        Size              Bytes  Class     Attributes
A         100x100             80000  double              
B         100x100            113240  double    sparse

Now the size of B is about 40% bigger than A.

EDIT - Now the same example using complex numbers:

about 70% zeros:

R = randi(10,100);
I = ones(100);
C = complex(R,I);
C(real(C)<=7) = 0;
D = sparse(C);
whos C D

gives:

Name        Size              Bytes  Class     Attributes     
C         100x100            160000  double    complex        
D         100x100             71944  double    sparse, complex

and the same with about 30% zeros:

R = randi(10,100);
I = ones(100);
C = complex(R,I);
C(real(C)<=3) = 0;
D = sparse(C);
whos C D

gives:

Name        Size              Bytes  Class     Attributes     
C         100x100            160000  double    complex        
D         100x100            168928  double    sparse, complex

The values differ, but the fundamental effect is the same.

Best regards

Stephan

Best Answer

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MATLAB: Is this sparse multiplication failing

MATLAB: Query regarding a sparse complex matrix

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