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From: Ian Kelly <ian.g.kelly@gmail.com>
Date: Wed, 4 Jun 2014 01:18:58 -0600
Subject: Re: Having trouble in expressing constraints in Python
To: Python <python-list@python.org>
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On Tue, Jun 3, 2014 at 7:44 AM,  <varun7rs@gmail.com> wrote:
> I have a problem in writing a constraint in Python. Firstly, I wrote the =
code in AMPL and it was working and I'm using Python for the reason that it=
 is more suitable to handle large data. I managed to write the code quite f=
ine except for one constraint(Link Mapping Constraint). I've attached piece=
s of code from both AMPL and Python. First part of it is the Link Capacity =
Constraint in AMPl followed by Python. Second part of the code is the Link =
Mapping Constraint and I wish to write it in a similar fashion. But, I'm no=
t able to proceed with it. I really appreciate your help.
>
> subject to Link_Capacity_Constraints { (ns1, ns2) in PHY_LINKS}:
>         sum {dns in DEMAND} ((f_eN_SGW[dns, ns1, ns2] * (MME_bdw[dns] + I=
MS_bdw[dns] + PoP_bdw[dns])) + (f_SGW_PGW[dns, ns1, ns2] * PoP_bdw[dns]) + =
(f_SGW_IMS[dns, ns1, ns2] * IMS_bdw[dns]) + (f_SGW_MME[dns, ns1, ns2] * MME=
_bdw[dns]) + (f_PGW_PoP[dns, ns1, ns2] * PoP_bdw[dns])) <=3D capacity_bdw[n=
s1, ns2];
>
>         for edge in phy_network.edges:
>             varNames =3D []
>             varCoeffs =3D []
>             for demand in demands:
>                 varNames.append("f_eN_SGW_{}_{}_{}".format(demand.demandI=
D, edge.SourceID, edge.DestinationID))
>                 varCoeffs.append(demand.MME_bdw + demand.IMS_bdw + demand=
.PoP_bdw )
>                 varNames.append("f_SGW_PGW_{}_{}_{}".format(demand.demand=
ID, edge.SourceID, edge.DestinationID))
>                 varCoeffs.append(demand.PoP_bdw)
>                 varNames.append("f_SGW_IMS_{}_{}_{}".format(demand.demand=
ID, edge.SourceID, edge.DestinationID))
>                 varCoeffs.append(demand.IMS_bdw)
>                 varNames.append("f_SGW_MME_{}_{}_{}".format(demand.demand=
ID, edge.SourceID, edge.DestinationID))
>                 varCoeffs.append(demand.MME_bdw)
>                 varNames.append("f_PGW_PoP_{}_{}_{}".format(demand.demand=
ID, edge.SourceID, edge.DestinationID))
>                 varCoeffs.append(demand.PoP_bdw)
>             solver.add_constraint(varNames, varCoeffs, "L", edge.capacity=
_bdw, "Link_Capacity_Constraints{}_{}_{}".format(edge.SourceID, edge.Destin=
ationID, demand.demandID))
>
> #Link Mapping Constraint
> subject to Link_Mapping_Constraints_1{dns in DEMAND, ns1 in PHY_NODES}: s=
um {(ns1,w) in PHY_LINKS} (f_eN_SGW[dns, w, ns1] - f_eN_SGW[dns, ns1, w]) =
=3D x_eN[dns, ns1] - x_SGW[dns, ns1];

You didn't tell us what aspect of this is confounding you, so I'll
take a wild stab at it.  Looks like you want to iterate over the
Cartesian product of all the demands and all the nodes, and for each
such pair generate a constraint by iterating over all the edges that
have that node as the source.  You can do that by iterating over *all*
the edges and comparing the source node of each one.  Or you can
precompute the edges for each source node and store them in a dict
mapping each node to the list of edges it is the source of. Then all
you need to do in the loop is look up the source node in the dict and
iterate over the retrieved list of edges.