Originally posted by: Guerlain

Hi

I try to formulate an inventory management problem and the program run but the demand should change and take the values as mentioned in the model

if (t<1) (demand[i][t]=demand_in[i]) else (demand[i][t]=level_sup[i]-stock[i][t]).

I think I should declare the demand as a variable but I don't know how to do it

Here my data and mod files.

Do you mean this (with Demand declared as 'dvar int')?

forall (i in 2..n) {
  forall (t in Periods) {
    if (t<1) {
      Demand[i][t]==Demand_in[i];
    }
    else {
      Demand[i][t]==level_sup[i]-stock[i][t];
    }       
  }
}

Originally posted by: Guerlain

when I use the demand as a dvar I've this error message (CPLEX Error  5002: Q in 'q2' is not positive semi-definite.)

Hi,

this is because of

forall(i in cities, t in Periods)   
sum(i in 2..n) Demand[i][t]*y[i][t] <= stock[1][t];

that you could linearize with Linearizing the product of a binary and a continuous variable at http://www.leandro-coelho.com/linearization-product-variables/

or you could use a logical constraint and write

dvar int Demandy[2..n][Periods];

and later

forall(i in cities,t in Periods)   
sum(i in 2..n)
Demandy[i][t] <= stock[1][t];

forall(i in 2..n, t in Periods) (y[i][t]==1) => (Demandy[i][t]==Demand[i][t]);
forall(i in 2..n, t in Periods) (y[i][t]==0) => (Demandy[i][t]==0);

regards

Originally posted by: Guerlain

Thanks Alex I correct the model and now it gives the right results