How to define all elemnts of a large matrix as positive?

1

I have coded like z>=0 in solver cvx_sedumi. But it stops with a message “out-of memory”.
The matrix z is very large. But if I write the code
for j=1:n
for k=j:n
if(j<=k)
z(j,k)>=0;
z(j,k)<=1;
end
end
end
then it gives a wrong answer but memory problem does not arise.
Please suggest.
Thanks.