Re: next question - find some element of a set

Yes, sorry, forgot to mention that limitation.

I may be wrong, but getting a Singleton set is doable. Getting its only 
element is the thing for which I don't see a way now.

So it is possible to generate such a set, Selected, and include {s in 
Selected} everywhere where needed.

One way to generate it with the mentioned index solution:

set S;
set Indices := 1..card(S);
param position{s in S}:=sum{ss in S: ss<s}1;
set Selected := setof{s in S:position[s]=0} s;

Irand can be put on the top of it, if you need it to be more random.



On 12/12/19 7:12 PM, Meketon, Marc wrote:
> Thank you.  That works in the case that the array cost is numeric (and would work for me in this case!).
> 
> The  max  function will not work if the array is symbolic.  Out of sheer curiosity, I wonder if there is a general solution for getting an element from a set and putting that into a (symbolic) parameter.
> 
> -----Original Message-----
> From: Mate Hegyhati <[email protected]>
> Sent: Thursday, December 12, 2019 12:35 PM
> To: Meketon, Marc <[email protected]>; [email protected]
> Subject: Re: next question - find some element of a set
> 
> Hi!
> 
> Thanks for the clarification. If I understand correctly, and ANY proper cost value is ok, and you don't need the element itself, just the cost value, then min/max could also  work.
> 
> param some_valid_cost := max{s in S} cost[s];
> 
> Another more "random" solution (more like a workaround):
> 
> param selector{s in S} := Irand224();
> param some_valid_cost := sum{s in S: selector[s] == max {ss in S} selector[ss]} cost[s];
> 
> (but it fails, if the max value is generated twice)
> 
> I hope this helps.
> 
> All the best!
> 
> Mate
> 
> 
> 
> On 12/12/19 4:19 PM, Meketon, Marc wrote:
>> Here is more clarification.  I have a set  S  and an array  cost[].
>> I wanted to get a cost -- any cost -- that is found in the array cost.
>> I didn't want to have to specify in the data section a sample element
>> of  S, I just wanted to use GMPL to find one.  In the example below, I
>> want to find  SOME_ELEMENT_IN_S  as an index to the array cost[].
>>
>> set S; param cost{S};
>>
>> param some_valid_cost := cost[SOME_ELEMENT_IN_S];
>>
>> data; param : S : cost := A 100 B 105 C 198 ;
>>
>> -----Original Message----- From: Help-glpk
>> <[email protected]> On Behalf Of
>> Mate Hegyhati Sent: Thursday, December 12, 2019 2:29 AM To:
>> [email protected] Subject: Re: next question - find some element of a
>> set
>>
>> Hi!
>>
>> I'm not exactly sure, what you would like to do, but you could just
>> say, but maybe this answers your question:
>>
>> set sports; param  favorite symbolic in sports;
>>
>> var train{sports} >=0;
>>
>> s.t. foobar: train[favorite] >=2;
>>
>> minimize work: sum{f in sports} train[f];
>>
>> data;
>>
>> set sports := running cycling swimming; param favorite := 'running';
>>
>> end;
>>
>>
>> So put ' around the name for a symbolic parameter, and also don't put
>> comma between set elements.
>>
>> I hope this helped. If not, please clarify your situation.
>>
>> All the best,
>>
>> Mate
>>
>>
>>> set := A, B, C;
>>>
>>> param some_element_in_set, symbolic := ?????;
>>>
>>> The closest I saw was in this old help-glpk email:
>>> https://lists.gnu.org/archive/html/help-glpk/2006-11/msg00059.html
>>>
>>> Which says:
>>>
>>> set S;
>>>
>>> param ref{i in 1..card(S)}, symbolic, in S;
>>>
>>> # ref[i] refers to i-th element of S;
>>>
>>> param a{S};
>>>
>>> ... a[ref[i]]   ...  # means a[i-th element of S]
>>>
>>> But I cannot get this to work.  If I could get it to work, then I
>>> would do something like: param some_element_in_set := ref[1];
>>>
>>> BTW, I’m still hoping that someone will help solve how to work the
>>> sudoku_excel.mod example in the ‘examples\sql’ directory of the GLPK
>>> directory under Windows 10 64bit.  I still cannot get that to work.
>>>
>>> -Marc
>>>
>>