ampl - Summing specific elements in AMPL

Question

Hello fellow optimizers

I'm intending to add a constraint which prohibits the flow between two specific cities to be too high.

I'm intending to this using the following in the .mod file:

 set N:= Kiruna Stockholm Boliden;
 var x{N,N} binary;

 minimize sum{i in N, j in N} x[i,j];

 subject to Constraint1:
      sum{i in N, j in N} x[Kiruna, Stockholm] <= 4;

However this gives me errors. How to do I properly conduct this summarization?

Thanks in advance!

Answer 1

代码中有几个错误：

1. 您使用数据语法来初始化N模型中的集合。您应该将 set 的声明N和设置数据的声明分开，或者使用模型语法对其进行初始化：

   set N:= {'Kiruna', 'Stockholm', 'Boliden'};
   

2. minimize后跟目标名称和冒号：

   minimize obj: sum{i in N, j in N} x[i,j];
   

3. 字符串Kiruna并且Stockholm应该被引用：

   x['Kiruna', 'Stockholm'] <= 4;
   

4. 如果x[i, j]表示城市之间的流量i并且j您希望它不超过 4，那么您需要一组约束而不是求和：

   subject to Constraint1{i in N, j in N}: x[i, j] <= 4;