GAMS modelation: how do i set an identifier as the last value of a set (index.last) on an equation

Question

I'm modeling a VRP in GAMS language and one of my equations would ideally be:

SUM(i, x(i,n+1)) =e= 0;

with "n+1" being the last value of the set i /0*4/ (so it's 4)

I can't type x(i,"4") because this number (4) is just an example.

The software doesn't recognize this equation. the error says "unknown identifier set as index, which i understand is because "n" isn't a set. so i put n as a set, just like i did with i, but then I'd have to give it a number (3, so that n+1 = 4) and i don't want that.

I just need a way to put "n+1" as a valid index for x(i,n+1)

Answer 1

Assuming that x is declared as x(i,i), you can do something like this:

Alias (i,i2);
Equation eq;

eq.. SUM((i,i2)$i2.last, x(i,i2)) =e= 0;