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Re: [Help-glpk] How to use less operator in gmpl?

From: Andrew Makhorin
Subject: Re: [Help-glpk] How to use less operator in gmpl?
Date: Thu, 9 Aug 2007 13:54:50 +0400

> Hi, I am new to GNU MathProg language.
> I have following model but glpk always complains:

> "test2.mod:10: operand preceding
> less has invalid type"

> The GMPL document says it has this less
> operator. How could I use it in modeling?

Being non-linear the less operator can be applied only to constants
and parameters, but not to variables, because glpk does not support
non-linear constraints.

Nevertheless, you can reformulate your model using SOS2 constraints,
since less is a piecewise linear operator. For more details see:

> var a {i in I}, binary;

> s.t. R4 {i in I diff J}: a[i]-a[i-1] !=0;

This constraint is also incorrectly formulated. The correct
formulation is the following:

s.t. R4 {i in I diff J}: a[i] + a[i-1] = 1;

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