MATLAB: Can I get a lower triangle matrix by multiplying a col vector and a row vector

MATLABmatrix manipulationsumming unit-by-unit

Now I have a vector and I need to sum it and save it, unit by unit, like:
x = [1,1,0,0,0,1,0]
and
sum = [1,2,2,2,2,3,3]
Because of the low efficiency of for-loops, an n-times-n lower triangle matrix comes to me:
com = [
1 0 0 0 0 0 0;
1 1 0 0 0 0 0;
1 1 1 0 0 0 0;
1 1 1 1 0 0 0;
1 1 1 1 1 0 0;
1 1 1 1 1 1 0;
1 1 1 1 1 1 1];
By multiplying this com matrix and x, I think I can get sum.
But when n gets big, like 100,000, it cannot be created for taking too much memory(or something like that, which I don't remember precisely).
Therefore, is there a way that I can get this lower triangle matrix by multiplying a col vector and a row vector like:
com = [x1; x2; x3; ... ] * [y1, y2, y3 ...]
PS. is there any other way to accomplish this summing task fast without this lower triangle matrix? Please tell me.
Thanks for viewing and answering!

Best Answer

PS. is there any other way to accomplish this summing task fast without this lower triangle matrix? Please tell me.
Just use CUMSUM,
    >> cumsum([1,1,0,0,0,1,0])
ans =
       1     2     2     2     2     3     3
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