Matlab out of memory when use index to access an element of an array

Here is my code:

 rows = zeros(round(a large number),1);     
 cols = rows;
 vals = rows;
 .....(calculation)
 rows(idx) = (i-1)*p + j;
 cols(idx) = col;
 vals(idx) = d;

Here, idx, pj, col, d are four arrays of the same size. The first indexing line (rows(idx) = (i-1)*p + j;) gets no problem. But when excuting the next line, it comes the problem "out of memory".

I set a breakpoint and debug here. After excuting "rows(idx) = (i-1)*p + j;", i tried "cols(1)=1", it also got the out of memory problem. Could someone pleas explain this problem to me? Thanks a lot!

Answer 1

[Matt Fig]

You are probably seeing copy-on-write behavior. When you first assign the value of rows to cols, MATLAB doesn't actually create cols, but rather a pointer to rows. It is only when you change a value in cols that MATLAB (tries to) create cols. This is when you run out of memory.

Here is more information: Loren's Blog.