[FlopCpp] Issue with constraint formulation

[Alfredo Olivera]

I finally solved the issue this way:

1) Declare the source as a set:
    MP_subset<1> Sources(Nodes);
    Sources.insert(1);

2) Index the constraint on Nodes rather than Arcs:
    MP_constraint DemandOut;
    DemandOut = sum(Sources(i) * Nodes(j), x(Arcs(i,j))) == units;

Nice but dangerous feature is that, even though the sum ranges over 
pairs of nodes i and j which do not define arcs, these seem to be 
ignored when instantiating x(Arcs(i,j)). So, I still wonder how to range 
my sum on the Arcs and restrict the domain to those arcs which start on 
Source.

Many thanks

Alfredo

Alfredo Olivera escribió:
> Hi All.
>
> I am new to this library, and to give my first steps I decided to 
> formulate a min cost s-t flow problem in a capacitated network with 
> FlopC++. That is, I have to send U units of flow from s to t subject to 
> capacity constraints and with minimum cost.
>
> I have my sets of nodes and arcs:
>     int n = 10;
>     MP_set Nodes(n);
>     MP_subset<2> Arcs(Nodes, Nodes);
>
> I populate the arcs randomly.
>
> My parameters are:
>     MP_data Capacity(Arcs);
>     MP_data Cost(Arcs);
>
> And the variables are defined over the arcs (not over nodes*nodes):
>     MP_variable x(Arcs);
>
> I also have s and t defined as integer variables, and the amount of 
> units to send:
>     int s = ...; // source
>     int t = ...; // terminal
>     int U = ...; // units to send
>
> I am having several issues when trying to formulate the constraints. My 
> current status is the following:
>
> // flow conservation on inner nodes
>     MP_constraint flowCons(Nodes);
>     flowCons(i).such_that(i != s && i != t) = sum(Arcs(i,j), 
> x(Arcs(i,j))) == sum(Arcs(j,i), x(Arcs(j,i)));
>
> // capacity constraints
>     MP_constraint CapacityCons(Arcs);
>     CapacityCons(Arcs(i,j)) = x(Arcs(i,j)) <= Capacity(Arcs(i,j));
>
> // send exactly U units out of s
>     MP_constraint DemandOut;
>     DemandOut() = sum(Arcs(s,j), x(Arcs(s,j))) == U;
>
> Flow conservation and arc capacity seem to be working ok (I generated a 
> file with the LP format and checked).
>
> But the DemandOut constraint is not working. The version I put above 
> just makes the model generation to crash. I also tried:
>     DemandOut() = sum(Arcs(i,j).such_that(i==s), x(Arcs(i,j))) == units;
>
> which does not crash, but sums every variable of the problem instead of 
> only those which correspond to (s,i) arcs where s is fixed.
>
> For sure I am making a mistake... but I cannot figure out where. Any 
> help would be really appreciated.
>
> Regards
>
> Alfredo Olivera
>
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