Re: Scheduling problem - cost function

[Heinrich Schuchardt]

On 01.04.21 17:07, Philippe Jugla wrote:
> Hello everyone,
>
>  
>
> I have some difficulties to add a cost parameter that depends on 2
> variables to a scheduling model in GLPK. 
>
>  
>
> The problem is as follows : 
>
>  
>
> If the unit has started up and been active for less than k hours than
> the cost follows a certain cost function, if it has been more than k
> hours then the cost follows another function. 
>
>  
>
> I have tried the following example but doesn’t seem to work : 
>
>  
>
> #set

T has to be defined before defining the set.

>
> set TIME:=0...T
>
>
> #binary variables (already defined and working) 
>
> var start_up {TIME} binary; 
>
> var unit_on {TIME} binary; 
>
>
> #parameters
>
> param T:=10;
>
> param price {TIME};
>
> param k:=2;
>
>
> #this is what I’ve tried :
>
> param cost {t in TIME} := 
>
>               if sum { s in TIME : s = t - k and s >= 0 } start_up[s] +
> unit_on[s] < k+1 then 2.5*price[t] + 5 else 3.0*price[t] + 10;

You cannot define a parameter as a function of variables. You will have
to make cost a variable and use a constraint.

In a constraint you cannot use "if then else". But you might use
multiple constraints using the binaries.

Best regards

Heinrich

>
>  
>
> The following error is thrown : « operand preceding < has invalid
> type ». I guess it is because of the variables ? I tried to define cost
> as a variable but with no success as well..
>
>
> I am also trying to workaround things by defining another binary
> variable X {TIME} that I would constrain, and then define the cost with
> something like cost[t] = (1-X)*(cost_function_1) + X*cost_function_2 but
> it seems quite complicated for this type of problem.
>
>
> Does anyone know what is the issue ? How could I work things out ?
>
>
> Any help or advice would be greatly appreciated !
>
>
> Best regards 
>
>  
>
> Philippe 
>
>