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Re: [Help-glpk] Problem in GLPK

From: glpk xypron
Subject: Re: [Help-glpk] Problem in GLPK
Date: Fri, 30 Apr 2010 07:41:39 +0200

Hello Aly,

>  s.t. Delivery_from_Same_Project{
>     j in J, k in K, p in P, t in T: j!=k}:
>     xx[j,k,p,t] ==0; 

all xx with j==k are always zero. Hence it is not necessary to
define them as variables. You could define instead:

var xx{j in J, k in K, p in P, t in T: j!=k}, >= 0;

You could define that a delivery from project j to k is modeled
as a positive number and a delivery from k to j is a negative
number and define the variable xx as:

var xx{j in J, k in K, p in P, t in T: j>k};

Of cause you have to adjust the constraints using xx.

Best regards


-------- Original-Nachricht --------

> Hi Xypron,
> Thanks for your reply.
> What do you mean by "exploiting the natural sparsity of my problem"? I
> mean, how is that done exactly in my example for instance?
> Aly
> ----- Original Message -----
> From: "glpk xypron" <address@hidden>
> To: "Aly Megahed" <address@hidden>, address@hidden
> Sent: Thursday, April 29, 2010 2:16:37 AM GMT -05:00 US/Canada Eastern
> Subject: Re: [Help-glpk] Problem in GLPK
> Hello Aly,
> >   glp_malloc: no memory available
> >   Error detected in file    \src\glpenv05.c at line 70
> > 
> > The constraint that caused that error is:
> > 
> >  s.t. Delivery_from_Same_Project{j in J, k in K, p in P, t in T: j!=k}:
> > xx[j,k,p,t] ==0; 
> >
> columns that are always zero should not be defined.
> Instead you should exploit the natural sparsity of your
> problem to reduce the memory usage.
> Best regards
> Xypron

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