CODEWITHSUNDEEP
cplex
liu chao
liu chao

Reputation: 1

Cplex how to use variables for indexing in constraints

I am trying to use decision variables Y to update index of decision variable X in constraint. However, it does not seem to work in Cplex. Any help is appreciated, thanks a lot in advance. Here is my code:

// Parameters
int Tmax = ...;
int u[i][r][m] = ...;
int b[i][r][m][a] = ...;
float t[i][r][j][s] = ...;

// Variables
dvar boolean X[i][r][m][j][s][n][k];
dvar boolean Y[i][r][m][k]; 

// Objective Function
dexpr float TotalDuration =  sum(ci in i, cr in r, cm in m, cj in j, cs in s, cn in n, ck in k) t[ci][cr][cj][cs]*X[ci][cr][cm][cj][cs][cn][ck] + sum(ci in i, cr in r, cm in m, ck in k) u[ci][cr][cm]*Y[ci][cr][cm][ck];
minimize TotalDuration;

subject to {
    forall(ci in i, cr in r, cm in m, cj in j, cs in s, cn in n, ck in k)
    TimeConservative:
    X[ci][cr][cm][cj][cs][cn][ck] == X[ci][cr][cm][cj][cs][cm+u[ci][cr][cm]*Y[ci][cr][cm][ck] + t[ci][cr][cj][cs]][ck];
}

Upvotes: 0

Views: 558

Answers (1)

Daniel Junglas
Daniel Junglas

Reputation: 5930

With CPLEX you cannot use a decision variable as index. With CP this is possible. Since you have only boolean decision variables, you could try using CP to solve your problem. To do this you have to add using CP; at the top of your .mod file.

If you need/want to stick with CPLEX a common way to work around this limitation is to explicitly spell out the two cases for Y by using the logical "implies" constraint:

(Y[ci][cr][cm][ck] == 0) => (/* here comes the constraint that must be satisfied if Y==0);
(Y[ci][cr][cm][ck] == 1) => (/* here comes the constraint that must be satisfied if Y==1);

This is of course only useful if Y is boolean or integer with a relative small domain.

Upvotes: 1

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