Maximum array size in MATLAB?

Question

I'm writing a MATLAB program that will generate a matrix with 1 million rows and an unknown amount of columns (at max 1 million).

I tried pre-allocating this matrix:

a=zeros(1000000,1000000)

but I received the error:

"Maximum variable size allowed by the program is exceeded."

I have a feeling that not pre-allocating this matrix will seriously slow the code down.

This made me curious: What is the maximum array size in MATLAB?

Update: I'm going to look into sparse matrices, because the result I am aiming for in this particular problem will be a matrix consisting for the larger part of zeros.

Answer 1

Take a look at this page, it lists the maximum sizes: Max sizes

It looks to be on the order of a few hundred million. Note that the matrix you're trying to create here is: 10e6 * 10e6 = 10e12 elements. This is many orders of magnitude greater than the max sizes provided and you also do not have that much RAM on your system.

My suggestion is to look into a different algorithm for what you are trying to accomplish.

Answer 2

To find out the real maximum array size (Windows only), use the command user = memory. user.maxPossibleArrayBytes shows how many bytes of contiguous RAM are free. Divide that by the number of bytes per element of your array (8 for doubles) and you know the max number of elements you can preallocate.

Note that as woodchips said, Matlab may have to copy your array (if you pass by value to a subfunction, for example). In my experience 75% of the max possible array is usually available multiple times.