Originally posted by: noneless

HI

I'm getting trouble with my project.I'm trying to implement an university course timetabling using CP with OPL. But I have no idea how to declare soft constraint.I know I can use penalty,but how????

Originally posted by: ol

Hello,

typically a soft constraint is expressed as a term in the objective.

For example, if a given course A should be teached before a course B if possible, you may express it as:

  - penaltyAB = 5 * (A>=B). The expression A>=B equals 1 if it is true ant 0 if it is false.

  - and the objective to be minimized would be obj = ... + penaltyAB + ...

Several soft constraints correspond to several subterms of the objective function.

You will also find a detailed timetabling example in the opl distribution.

Regards,

Olivier

Originally posted by: noneless

Thnx

But when I use this form, considered it as hard constraint.

Originally posted by: ol

Hello,

when you state the constraint

   penalty[19] == (penalty[19]+1);

you have an equation of the form x=x+1, with no solution.

In your case this will force the condition of the if to be false, then check[19] will be forced to be different from 1, as itis a bolean expression, it is thus set to 0, and then by the constraint check[19] ==(Start[i]>4), this will force Start[i] to be <= 4.

I assume this is what you mean by saying that your soft constraint becomes hard. You need to clean your model.

Regards,

Olivier  

Originally posted by: noneless

Thx buddy

Could you turn this hard constraint to soft constraint?

This constraint means if a course is in the serviceclass,start time must begins at b. I don't want to force the course begins at b.I want to declare if there is no choice to begin at b, a penalty allocates

Originally posted by: ol

Hello,

for a direct approach, you could replace:
    if(i.Course==a)    
    Start[i]==b;
by something like:  
    penalty[i] == (i.Course==a) *  (Start[i]!=b) * weight[i];
and then you need to sum the penalties.
Note you may have more efficient ways for doing this, depending on you exact problem.
Regards,
Olivier
 

Originally posted by: noneless

THnx

That was very helpful.

I've got another question.How can I use two or more if and for in a statement.I mean like this: