[Scip] have binary variable represent equality of two other vars?

[Ambros Gleixner]

Hi George,

mathematically, you want c = 1 - |a-b|, and James' answer is the 
standard linearization for this.  You can let Zimpl do this 
reformulation for you automatically by writing

    c == vabs( a - b )

or using vabs( a - b ) directly in your constraints.

Happy zimpling,

Ambros




Am 20.09.2013 03:25, schrieb George Nychis:
> Haha, I had thought of that solution James.  It seemed a little overkill
> to me, but maybe it is the only good way to do this.  Let's see if
> someone can turn up with something better, and if all fails I'll settle
> for it.  Thanks for pointing it out!
>
>
> On Thu, Sep 19, 2013 at 9:21 PM, James Gunning <James.Gunning at csiro.au
> <mailto:James.Gunning at csiro.au>> wrote:
>
>     George, this may be completely dopey, but if one added a slack z to
>     the objective, and equations
>     z<=1, z>=a-b, z>= b-a, z=1-c   (or some suitable simplification),
>     that might work..
>
>     George Nychis wrote:
>>     This might be slightly off-topic, but I don't know where better to
>>     ask.  I'm having a hard time expressing something in ZIMPL
>>     linearly. I have two variables a and b. I want the variable c to
>>     be strictly binary and take on the value of 1 when a and b are
>>     equal, and 0 otherwise. Does anyone know of a good way to express
>>     this?
>>
>>     Thanks!
>>     George
>>
>>
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>
>
>
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