 GAMS Support Wiki

Site Tools

gams:model_predecessor_successor_relations

How do I model predecessor/successor relations?

Q: I have variables with two indices, e.g. x(d,h), where d and h are sets of days and hours. My problem is to make a predecessor/sucessor relation between those x variables. E.g. I have to model the difference between two neighboring x variables like: delta(d,h) =e= x(d,h) - x(pred(d,h)) ; delta(d,h) =l= constant ;

The GAMS lingo for this is lag and lead operators: x(d,h-1) e.g. shows the previous hour.

The - is not a numerical minus but a lag. Trouble is that the predecessor for x(d,'h1') is x(d-1,'h24'). You also have to decide what predecessor of 'd1','h1' is.

If you do a steady state model you could have 'd365','h24' or you can decide that there is no predecessor (meaning delta('d1','h1') = x('d1','h1)). I am assuming your sets look like this

Set d / d1*d365 /
h / h1*h24  /;

Now here is what you can do:

delta(d,h) =e= x(d,h) - x(d-(1\$sameas('h1')),h--1)) ;

Not so elegant, but it will work. I prefer working with an additional set of all hours in the year and a map between d,h and all hours in the year:

Set h /h1*h24/,   d          /d1*d365/, dh(d,h) /#d.#h/
Set t /t1*t8760/, tdh(t,d,h) /#t:#dh/,  dht     /#dh:#t/;

. and : are called matching operators. Now I would have the variable x and delta over t and then the constraint looks simple:

delta(t) =e= x(t) - x(t-1); (or x(t--1) for steady state)

In case you have data by d,h you can use the map tdh, for example: delta(t) =l= sum(tdh(t,d,h), maxdeviation(d,h)); 