[gecode-users] semantics for count(Home home, const IntVarArgs &x, IntVar y, IntRelType r, IntVar z)

[Kish Shen]

Hi,

I am still not sure I understand the semantics -- the constraint is 
posted with domain consistency (the default). My understanding of domain 
consistency is that any values remaining in the domain is consistent 
with the constraint, i.e. the constraint should not fail if the variable 
is instantiated to one of the values, as long as this is the only change 
made. For

Y :: [3,4], count([3,0,3,3,4], Y, =, 2)

Y is the only domain variable with more than one value, so if I 
understand domain consistency correctly, then instantiating it to either 
3 or 4 should succeed, but doing so fails for both values.

The documentation states that:

Supports domain consistent propagation (icl = ICL_DOM, default) and 
"almost" domain consistent propagation (all other values for icl), where 
y is not pruned.

(I assume by other values of icl, you exclude ICL_DEF, which behaves as 
ICL_DOM as that is the default)

It seems to me the behaviour is that y is not pruned in the above, even 
when the constraint is posted with ICL_DOM and ICL_DEF -- I have 
explicitly posted the constraint with ICL_DOM, to check if it makes any 
difference, and it does not, i.e. the example with Y::[3,4] succeeds.

Cheers,

Kish


On 22/11/2011 08:49, Christian Schulte wrote:
> This is a fundamental missunderstanding of what an integer set and an
> integer variable denotes.
>
> It might be that the current domain of an integer variable is a set, however
> the variable denotes a single integer (which you do not know yet). That's
> fundamental and very different from a set.
>
> With that in mind, count behaves exactly like one would expect.
>
> In your example, the constraint does not succeed (as in being entailed). It
> just does not fail yet. Let's take the more general example (2 is replaced
> by an integer variable Z):
>
> Y :: [3,4], count([3,0,3,3,4], Y, =, Z)
>
> Then it will propagate that Z has the domain [0..4] as there are four
> elements in the array that still could be equal to Y with domain 3 or 4 and
> one is already known to be different (the element 0) from 3 or 4.
>
> Christian
>
> --
> Christian Schulte, www.ict.kth.se/~cschulte/
>
>
> -----Original Message-----
> From: users-bounces at gecode.org [mailto:users-bounces at gecode.org] On Behalf
> Of Kish Shen
> Sent: Tuesday, November 22, 2011 8:09 AM
> To: users at gecode.org
> Subject: [gecode-users] semantics for count(Home home, const IntVarArgs&x,
> IntVar y, IntRelType r, IntVar z)
>
> Hi,
>
> I have been looking at the behaviour of
>
> count(Home home, const IntVarArgs&x, IntVar y, IntRelType r, IntVar z)
>
> and cannot understand what is happening when IntVar y has more than one
> value in its domain. Until now, I have only tested this constraint with y
> having a singleton value in its domain, and in this case, the constraint
> behaves the same as the case where y is Int, i.e. the number of occurrences
> of the value y in x is counted.
>
> However, one of my user wanted to use the constraint where y has multiple
> values (i.e. the counts constraint in the global constraint catalog). I know
> this constraint is provided with y being an IntSet, but my user did not know
> about this constraint, so he used the version where y is a IntVar instead.
>
> I had expected that mulitple domain values in y would behave like multiple
> values in the IntSet for this constraint, but this does not seem to be the
> case. For example, the constraint succeeds with the following (in ECLiPSe
> syntax, but the argument order of Gecode's
> constraint):
>
> Y :: [3,4], count([3,0,3,3,4], Y, =, 2)
>
> where [3,0,3,3,4] is x, IntRelType is IRT_EQ.
>
> I don't understand why this succeeds -- the count for 3 is 3, and for 4 is
> 1, and for both 3 and 4 is 4, none of which is equal to 2. So what is the
> semantics?
>
> The equivalent call using y as an IntSet [3,4] fails as expected. Also, for
> the same count constraint, if y has a single domain value (e.g. 3 or 4),
> then the constraint behave as I expected, i.e. z = 3 for y = 3, and z = 1
> for y = 1.
>
> Thanks and cheers,
>
> Kish
>
>
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Cisco Systems Limited (Company Number: 02558939), is registered in
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