How to formulate equations with three indices in GAMS?

Question

Xijk = Number of units of product k purchased from vendor i for DC j

ObjCost.. Sum(i,Sum(k,j), xijk*Procurement-Cost);

Is the ObjCost equation formulation alright?

Answer 1

The Gams Compiler tells you if it is at least syntactically correct, but it doesn't look correct:

The definition is done like this:

equation_name..
  lhs =E= rhs;

with =E= (equals) can easily be replaced by =G= (greater than or equal to), or =L= (less than or equal to).

So you might want something like this:

...
defObjCost..
  OBJCOST =E= sum((i,k,j), X(i,j,k)*PROCUREMENT_COST(i, k);
...

model some_model /all/;
solve some_model using nlp minimizing OBJCOST;