Any element from a set, urgent

Problems with syntax of GAMS
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betulkayisoglu
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Any element from a set, urgent

Post by betulkayisoglu » 3 months ago

Hello,

In my gams code, by using loops I have iterations and I have a set p which has different elements in each iteration.
p(i) = y.l(i) ;
I define the set p as above, I have a binary variable y and at each iteration 3 of them take the value 1 so I create my set p according to these binary variables.
i.e.
p=(65,75,82) at 1st iteration
p=(22,35,95) at 2nd iteration...
my aim is to select only a single element from this set p at each iteration, I am trying to create a subset of p which only contains a single element from it.
but unfortunately I am not able to.

I am waiting for your help
thanks

Hussain_TUT
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Re: Any element from a set, urgent

Post by Hussain_TUT » 3 months ago

Hi,
If i am understand it corrctly, you want to get a single any single value (let's say frist) from the parameter p that contains three values. If this is the case then you may try this:
set i /1*3/;
** here i assume that p has only 3 three elements
parameter p1
j;

loop(i$(j=0),
p1 (i)=p(i);
j=1$(p1(i) ge 1);
);
Now p1 will contain the first element of the p.
Hopefully, it will help you.

betulkayisoglu
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Re: Any element from a set, urgent

Post by betulkayisoglu » 3 months ago

Hi Hussain,
Firstly thank you very much for your answer
In fact I am not trying to obtain the value of an element.
I am working on a network, assume there are 100 nodes.
Set P contains 3 of these nodes say 15,17,65
I need to send a flow from one of these nodes so in fact I need to create again a set i.e. P1 not a parameter
P1 will contain one of the elements of P
It does not matter if it is 15 or 17 or 65
and when I write my constraint I want to say that for example i$P1 so that flow will go out from the single node which is in the set P1

maybe there may be other ways to overcome this

Hussain_TUT
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Re: Any element from a set, urgent

Post by Hussain_TUT » 3 months ago

Hi,
As far as i know, set need to be defined in advanced. You can also use subset concept e.g, P can be a super set while P1 can a subset. The issue you raised may be overcomes suing $ operator in constraint.

Fred
Posts: 70
Joined: 1 year ago

Re: Any element from a set, urgent

Post by Fred » 3 months ago

Hi betulkayisoglu,

It seems that p(i) is already a subset of superset p. You may want to introduce another subset pp(i) that selects only a single element from this set p at each iteration. Given that, pp(i) can be declared as singleton set (https://www.gams.com/latest/docs/UG_Set ... gletonSets). Since you can pick any element of p to be the single element in pp, you can for example just pick the first one using the corresponding set attribute .first (https://www.gams.com/latest/docs/UG_Set ... Attributes) .

I hope the following example illustrates how to do this.

Code: Select all

set i     / 1*100 /
    p(i)
singleton set pp(i);

p('65') = yes;
p('75') = yes;
p('82') = yes;
pp(p)   = p.first;
display p,pp;

option clear=p;
p('22') = yes;
p('35') = yes;
p('95') = yes;
pp(p)   = p.first;
display p,pp;
Best,
Fred

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