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GAMS uses all sparse structures and, in general, having a value of zero and not existing are the same. However GAMS has a special value that means the element in question is numerically zero but that it does exist. That value is EPS. So, if you run the following example:
Set i /i1 * i2/; Set j /j1 * j2/; Parameter x(i, j) / i1.j1 11 i1.j2 12 /; Display x; x(i, j)$(Not x(i, j)) = EPS; Display x;
you'll see that the second row of data is written by the second Display statement.
You could still use x with the EPS entries for computations but you'd lose efficiency because x would be treated as dense instead of sparse (no importance for such a tiny example, of course).
If you don't like EPS, you can use an acronym instead:
Acronym zero; x(i, j)$(Not x(i, j)) = zero; Display x;
but then you can't do computations with the result.
If you don't like the looks of
zero, you can make the acronym equal to lower case letter
o instead. If you don't like that, you'll have to switch to writing reports with the Put statement.