Hello,
I’m writing my master thesis about the “vehicle routing problem with pickup and delivery and time windows” and I am trying to realise my model in GAMS.

In order to get the time the truck arrives at a knot, I used the parameter w(i,k) which isn’t given in the beginning but can be computed through the other parameters. The formula says: (binary variable)(time of arrival at knot i + service time + transporting time  time of arrival at following knot j)=0.
In GAMS: x(i,j,k)(w(i,k)+1.75+t(i,j)w(j,k))=e=0; and to arrive between the earliest and latest possible time at the knot: a(i)=l=w(i,k) and w(i,k)=l=b(i). If I just write it down like this and define w(i,k) as a parameter I get “error 66: symbol shown has not been defined or assigned” after the solve statement. I get also an error if I define w(i,k) as a variable. I couldn’t find any command in the handbook or web or so to tell GAMS it should compute the parameter. Is there a command or something to tell GAMS that it should compute w(i,k)? If no, do you have any idea how to define w(i,k)? 
I also got the same problem with the load on the truck. The truck can be either empty or full, so l(i,k) is binary. It should be computed, so that the load before knot j is exactly the demand of knot j. After visiting knot j the truck is empty again. In GAMS: x(i,j,k)*(l(i,k)+d(j)l(j,k))=e=0.
Thank you very much for your help,
Julia
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