Originally posted by: lisaz

Hello, 

I have another question on one constraint of my model. Most probably, it's easy for to answer, but I just don't understand how to formulate it correctly. 

I have the following constraint: 

//Cons08: Departuring Flights to Ireland and Northern Ireland need to be assigned to gates: 2, 5, 16, 19, 20, 23, 25, 26, 27, 28 
Cons08: forall(f in Flights) forall(g in {2,5,16,19,20,23,25,26,27,28})
if ((Type[f]==0) && (Destination[f]>=1)) {
assignment[f][g] ==1 ;}

and I have another constraint which says:

//Cons02: each flight is assigned to only one gate
Cons02: forall(f in Flights) 
sum (g in Gates) assignment[f][g] ==1; 

I attached my mod. file.

Whenever I run the model it doesn't find a solution and I think Constraint 8 and 2 are "contradicting" each other. Without constraint 8, a solution is found. 

How can I properly formulate the "forall(g in {2,5,16,19,20,23,25,26,27,28})"? --> These numbers all belong to the set "Gates". The reasoning of Cons08 is that certain flights (e.g. to Ireland (--> indicated by "if...")) can only be assigned to the gates "2, 5, 16, 19,..." but not to other gates like "1,3,4,..." 

Thank you so much for your help!!!

Regards, Lisa 

Originally posted by: Philippe_Refalo

I think that you rather need to remove the possibility that these flight (to Ireland for instance) are assigned to the other gates (1,3,4 ..) by setting assignment[f][g] to 0 and not the reverse : you cannot assign any variable to  1 at this stage because you don't know what gate will be assigned to these special flights. It is the role of the solver engine to do this. 

Regards,

Philippe

Originally posted by: lisaz

Hello, 

I tried to do so and reformulated my constraint into: 

//Cons08: Departures to Ireland and Northern Ireland need to be assigned to gates:

//1A, 1H, 12, 14, 15, 18, 20, 21, 22, 23
Cons08: forall(f in Flights) forall(g in {1,3,4,6,7,8,9,10,11,12,13,14,15,17,18,21,22,24})
if ((Type[f]==0) && (Destination[f]>=1)) {
assignment[f][g] ==0 ;}

However, I still have the same problem. The solver doesn't find a solution - without constraint 08 the problem gets solved. 

And I don't try to assign my decision variable to 1, I just want to tell the solver that it can set "assignment == 1" only for one of the gates {2,5,16,...}. Isn't it similar to say "forall(g in Gates)" - except that "Gates" is not a range anymore. 

Do you have another idea how to formulate this constraint?

Originally posted by: lisaz

Hello again, 

I just tried the code again with a much smaller data set and it worked. Probably the reason why I don't get a solution needs to be searched in my data sheet. 

Thanks very much for your help!