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

[George Nychis]

Hi Ambros,

Thanks a bunch for that trick, I appreciate it.  That does work for me
also.  Learning more of these hidden gems day by day in ZIMPL and SCIP...

- George


On Mon, Sep 23, 2013 at 1:19 AM, Ambros Gleixner <gleixner at zib.de> wrote:

> Of course it should read c == 1 - vabs( a - b ).
>
> Am 23.09.2013 07:13, schrieb 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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