Forum Discussion
MPVAR mod integer
Hi all,
I'm wondering is it possible to modulo an mpvar.
I have a set of mpvars Xs which are binary and I want to have Sum(i in set) Xi mod 2 = 0 (in other words), the sum of the binary variable are even. However, I get the typical error saying that mpvar mod integer is not defined.
Is there anyway around this?
Thanks,
Cian
2 Replies
In a (linear) mixed integer problem formulation you won't be able to use the "mod" operator on decision variables to state the problem. What about introducing an integer variable, say "y", and rewrite your constraint as follows:
y is_integer
forall(i in aSet) x(i) is_binary
sum(i in aSet) x(i) = 2*y
Enclosed you find a full example (just for the sake of completeness):
model Modulo options noimplicit, keepassert uses "mmxprs"; declarations I = 1..3 x: array (I) of mpvar y: mpvar obj: linctr end-declarations procedure createModel ! all x variables are binary forall (i in I) do x(i) is_binary end-do ! y is an auxiliary integer variable y is_integer ! (sum (i in I) x(i)) mod 2 = 0 sum(i in I) x(i) = 2 * y ! objective: maximize sum of x(i) obj := sum(i in I) x(i) end-procedure procedure showSolution writeln("(sum(i in I) x(i)) mod 2 = ", (sum(i in I) x(i).sol) mod 2) forall (i in I) do writeln("x(",i,") = ", x(i).sol) end-do end-procedure procedure main createModel maximize(obj) showSolution end-procedure main end-model
