Either a data table gave values for the parameter with incorrect subscripts, or the parameter’s indexing set changed, causing some previously valid subscripts to become invalid. For example, in the diet.mod + diet.dat example (Figures 2-1 and 2-2) of the AMPL book, values of parameter cost are supplied for all eight members of set FOOD:

ampl: display cost;
cost [*] :=
BEEF 3.19   FISH 2.29    MCH 1.89    SPG 1.99
 CHK 2.59    HAM 2.89    MTL 1.99    TUR 2.49
;

If you remove the member CHK from FOOD, using for example a let command, then you get a message that cost["CHK"] has also been dropped from the data:

ampl: let FOOD := FOOD diff {"CHK"};
ampl: display cost;
Error executing "display" command:
error processing param cost:
        invalid subscript cost['CHK'] discarded.
cost [*] :=
BEEF 3.19    HAM 2.89    MTL 1.99    TUR 2.49
FISH 2.29    MCH 1.89    SPG 1.99

Since cost["CHK"] has now been dropped, no further error message will appear if you type display cost again.

To avoid error messages of this sort, you can define a more flexible set structure for your model as shown in dietflex.mod and dietflex.dat. The auxiliary set DIET_DROP defaults to empty, so that the problem is solved for all foods; but you can change DIET_DROP to {"CHK"} to solve without member CHK:

ampl: model dietflex.mod;
ampl: data dietflex.dat;
ampl: option show_stats 1;
ampl: solve;
8 variables, all linear
6 constraints, all linear; 47 nonzeros
1 linear objective; 8 nonzeros.
MINOS 5.4: optimal solution found.
13 iterations, objective 118.0594032
ampl: let FOOD_DROP := {"CHK"};
ampl: solve;
7 variables, all linear
6 constraints, all linear; 42 nonzeros
1 linear objective; 7 nonzeros.
MINOS 5.4: optimal solution found.
3 iterations, objective 117.3218891

Changing FOOD_DROP does not affect the set FOOD_ALL, and consequently all of the subscripts in the data remain valid.

Posted in: Sets and Indexing