Sets

[Nikola]

Hi All,

I am working on the pickup and delivery problem and I encountered an
issue. I am extracting here a few lines of code that are giving me a
hard time:

Set:
I set of pickup and delivery nodes /p1*p5, d1*d5/
P(I) set of pickup nodes /p1*p5/

Positive Variable:
Y(i)
X(i)

Equation:
eq1(p).. Y(p) =e= X(p+5) - X(p) - 3;

The report says that the above equation is infeasible and when I look
at the listing I realize that X(p+5) was ignored. As a result, the
listing gives: Y(p) + X(p) =e= -3, which is infeasible since both Y(p)
and X(p) are positive variables. My question is why was X(p+5) ignored
and what can I do to fix this?

I would greatly appreciate your help.
Nikola

[Arne Stolbjerg Drud]

Nikola:

The problem is, that the lead operator works within the set that you specify
so p+5 means five elements further out within the set p and if this element
does not exist then nothing. And since you only have 5 elements in p you
will never get anything.

You must use the lead operator on the large set, I. The equation can be
written as:

eq1(i)$p(i).. Y(i) =e= X(i+5) - X(i) - 3;

Good luck.

Arne

-------------------------------------------
Arne Stolbjerg Drud
ARKI Consulting & Development A/S
Bagsvaerdvej 246A, DK-2880 Bagsvaerd, Denmark
Phone: (+45) 44 49 03 23, Fax: (+45) 44 49 03 33, email: ad...@arki.dk

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[Nikola]

Hi Arne,

Thanks a lot for your help!

Nikola