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[Help-glpk] Re: logic conditions in MathProg

From: Andrew Makhorin
Subject: [Help-glpk] Re: logic conditions in MathProg
Date: Sun, 8 Feb 2004 11:19:47 +0300

>I want to do a very simple thing: I want to sum the hours of a time
>slot only when a given employee is assigned to it.
>this won't work:
>WorkedHrs[e]= sum {s in Slots: Assignments[s]=e} Hours[e];
>this won't work, too:
>WorkedHrs[e]= sum {s in Slots} 0+ (if Assignments[s]=e then Hours[s] );
>I get the error message
>Reading model section from bernardi.mod...
>bernardi.mod:44: operand preceding = has invalid type
>Context: ...} : WorkedHrs [ e ] = sum { s in Slots : Assignments [ s ] =
>Model processing error

In your model 'Assignments' is a variable, not a parameter, so the
MathProg translator cannot generate corresponding constraints (it needs
the value of Assignments[s], which is unknown on the generating phase).

You can find more detailed answer to your question in sci.op-research

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