Re: [Help-glpk] How to get a variable's absolute value with GNU mathprog

[hhb83]

It works, thank you very much!
Haibin


Andrew Makhorin wrote:
> 
>> Would anyone tell me how to get a variable's absolute value with GNU
>> mathprog? 
>> var X;
>> var Xabs;
>> s.t. absolte: Xabs = abs(X);
>> It always says: argument for abs has invalid type.
> 
> You cannot use variables in abs(), because this leads to a non-linear
> constraint not allowed in glpk.
> 
>> If I write this:
>> s.t absolte: Xabs = if X<=0 then (-1)*X else X;
>> It says: operand proceding <= has invalid type.
> 
> Analogously.
> 
>> Then what should I do?
> 
> It depends on the context where you need to use absolute value.
> In general case you can model abs with binary variables. However,
> sometimes this is not needed, for example, if abs is part of the
> objective function which should be minimized; see:
> http://lists.gnu.org/archive/html/help-glpk/2007-08/msg00036.html
> 
> 
> 
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> 
> 

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