Why am I getting this syntax error in AMPL? (beginner)

Question

I am learning AMPL for a university project and i don't understand why i'm getting a syntax error, my code:

model:
set FOODS;
param calories;
param proteins;
param calcium;
param vitaminA;
param cost;

param minCalories;
param minProt;
param minCalcium;
param minVit;

var x {FOODS} >=0;

minimize prix:
 sum {f in FOODS} cost[f] * x[f];

subject to calories: sum {f in FOODS} calories[f] * x[f]>=minCalories;
subject to proteins: sum {f in FOODS} proteins[f] * x[f]>=minProt;
subject to vitaminA: sum {f in FOODS} vitaminA[f] * x[f]>=minVit;
subject to calcium: sum {f in FOODS} calcium[f] * x[f]>=minCalcium;

data:

data;

set FOODS := Bread Meat Potatoes Cabbage Milk Gelatine;
param calories{FOODS};
param proteins{FOODS};
param calcium{FOODS};
param vitaminA{FOODS};
param cost{FOODS};

param minCalories = 3000;
param minProt = 70;
param minCalcium = 800;
param minVit = 500;

calories:=
Bread   1254
Meat    1457
Potatoes 318
Cabbage 48
Milk    309
Gelatin 1725;

proteins :=
Bread   39
Meat    73
Potatoes 8
Cabbage 4
Milk    16
Gelatin 43;

calcium :=
Bread   418
Meat    41
Potatoes 42
Cabbage 141
Milk    536
Gelatin 0;

vitaminA :=
Bread   0
Meat    0
Potatoes 70
Cabbage 860
Milk    720
Gelatin 0;

cost :=
Bread   0.3
Meat    1
Potatoes 0.05
Cabbage 0.08
Milk    0.23
Gelatin 0.48;

The error i am getting: alimentation.mod, line 16 (offset 222): syntax error context: sum {f in FOODS} >>> cost[ <<< f] * x[f];

i tried different ways of writing it, and even asking chatgpt but nothing changed

Don't you need to declare that the param cost is indexed by food when you declare the parameter? (similar for other indexed params)? — AirSquid
– AirSquid, Commented Nov 13, 2024 at 20:19

marcos · Accepted Answer · 2024-11-14 11:52:49Z

There are actually several errors in your model and data:

Errors in the model:

Cost is an indexed parameter, but you have declared as an scalar parameter param cost;, yo should rather use param cost{FOODS};.
This happens with other parameters in your model.
You should not name parameters and constraints in the same way, so you have "proteins" as a parameter and also as a constraint.

Errors in the data: 4. In your data section, you are re-declaring parameters. Data section is to assign values rather than declaring new entities:

param calories{FOODS};
param proteins{FOODS};
param calcium{FOODS};
param vitaminA{FOODS};
param cost{FOODS};

All the previous lines in the data section shouldn't be there

The "param" keyword is missing before assigning values to the previous ones:

param calcium :=
Bread   418
Meat    41

"Gelatin" or "Gelatine"? You have 2 different names for a food.

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