[Help-glpk] [Fwd: Constraints on 2 successive elements of a set]

[Andrew Makhorin]

-------- Forwarded Message --------
From: paterijk <address@hidden>
To: address@hidden
Subject: [Help-glpk] Constraints on 2 successive elements of a set
Date: Tue, 20 Nov 2012 09:40:00 -0800 (PST)

Hi, 

I'm trying to model a constraint using the GMPL modeling language. 

To start, we have a set of n indices called J, and a variable vector x with
n components. The constraint then should look like :

x_i >= x_{i+1} for i=1..n-1 (obviously we do not want i+1 to go beyond the
limits of J)

What I do is the following, but I do not find it very elegant : 

set J;
set K;
var x{i in J};

s.t. c{i in K} x[i] >= x[i+1]

In the data section I then define : 

set J := 1 2 3;
set K := 1 2;

Does anybody know how to handle this without having to go through this set K
which I have to adapt in the data section when the set J changes? K is just
J minus the last element. Note that this also does not work when J (and K)
contain non numerical elements (because of the i+1 in the constraints). 

Thanks for any hint, 

Patrick
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