Re: [Help-glpk] pick element from set?

[glpk xypron]

Hello Kevin,

there is no function that will return an element of a set (except for 
one-dimensional numeric sets).

You can create subsets.

e.g.

set X, dimen 2 := {(5, 7), (1, 10), (23, -3), ("y", "x"), ("a", "abc")};
set S := setof{(i,j) in X : forall{(k,l) in X} i <=k && j <= l} (i,j);

see
http://en.wikibooks.org/wiki/GLPK/GMPL_Workarounds#Sorted_output

Best regards

Xypron

-------- Original-Nachricht --------
> Datum: Sun, 01 Jul 2012 03:11:00 -0400
> Betreff: [Help-glpk] pick element from set?

> Hello GLPK List,
> 
> Is there a method to pick an arbitrary single item from a set?  I don't 
> care which one, but I need to have a single element from a set such that 
> I can uniquely and consistently identify it.
> 
> There is no order to the set, but /if/ there were an ordering to this 
> set, I might reference this element as "nought".  Put differently, say I 
> have a set X:
> 
> set X, dimen 2 := {(5, 7), (1, 10), (23, -3), ("y", "x"), ("a", "abc")};
> 
> Is there functionality -- say a function called "pick()" -- to pick an 
> arbitrary element from that set?
> 
> Put another way, /if/ I could put that set into a list:
> 
> list(X) == [(5, 7), (1, 10), (23, -3), ("y", "x"), ("a", "abc")]
> 
> then I could get an arbitrary element, perhaps like 'get(list(X), 0)'.
> 
> Is there a method to grab a single element from the set?
> 
> I'll point out that I have an ad-hoc method of this for a 
> single-dimensional set (using a nested setof structure and a comparison 
> operator), but can't do it for any higher-dimensionality sets.
> 
> Thanks,
> 
> Kevin
> 
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