Warehouse Problem with minimizing the net present value of the cost

GAMS
1

I try to minimize the total net present value of the cost of a simple ware house problem.
But when I run the prblem with NLP, it shows an error “Dimension different - The symbol is referenced with more/less indices as declared”.
Why this error and how can I solve the problem?

Set
t time /1*4/;
scalar i /0.01/;

parameter
SP(t) selling price
/
1 10
2 12
3 8
4 9
/;

parameter
IS(t) initial stock /1 50/ ;

Scalar
SC storing cost /1/
SCap store capacity /100/;

variable
buy(t)
sell(t)
stock(t)
TC;
positive variable buy, sell, stock;

equation
cost
inventory(t);

cost… TC =e= sum[t, [{SP(t)(buy(t)-sell(t))+SCstock(t)}/((1+i)**t)]];
inventory(t)…stock(t) =e= stock(t-1)+buy(t)-sell(t)+IS(t);
stock.up(t) = Scap;

Model swp ‘simple warehouse problem’ / all /;
solve swp maximizing TC using nlp;

2

Hi,

Problem is how you use set t at the end of equation

cost.. TC =e= sum[t, [{SP(t)*(buy(t)-sell(t))+SC*stock(t)}/((1+i)**t)]];

While you use set elements 1,2,3,4, set elements can in general be strings (e.g. apple, orange, cherry, …) which is why you cannot make calculations with set elements like this. There t is a set attribute (https://www.gams.com/latest/docs/UG_SetDefinition.html#UG_SetDefinition_SetAttributes) .val though which allows you to use set elements that are numbers in calculations. The following should do the trick:

cost.. TC =e= sum[t, [{SP(t)*(buy(t)-sell(t))+SC*stock(t)}/((1+i)**t.val)]];

I hope this helps!

Fred

3

Hi Fred,

Thank you. The problem is solved now.

Regards
Asadujjaman