[FlopCpp] basic question about constraint from GNU Math Prog to Flopcpp

[Tim Hultberg]

I think the cleanest way to expres this constraint would be to have
three constraints indexed over three different subsets of N*K.

But if you want to translate your constraint directly to flopc++ you
must first add the following:

in MP_constant.hpp:

  class MP_boolean;
  Constant mpif(const MP_boolean& c, const Constant& a, const Constant& b);

in MP_constant.cpp:

  class Constant_if : public Constant_exp {
    friend Constant mpif(const MP_boolean& c, const Constant& a, const Constant& b); 
  private:
    Constant_if(const MP_boolean b, const Constant& i, const Constant& j) : Constant_exp(i,j), B(b) {};
    double evaluate() const {
      if (B->evaluate()==true) {
	return left->evaluate();
      } else {
	return right->evaluate(); 
      }
    }
    
    MP_boolean B;
  };
  
  Constant mpif(const MP_boolean& c, const Constant& a, const Constant& b) {
    return new Constant_if(c,a,b);
  }


Now you can write your constraint as:  

  r1(i,ka) = 
    sum(N(j), f(i,j,ka)) - sum(N(l), f(l,i,ka)) ==	
    mpif(i==MP_index_exp(O(ka)), R(ka), mpif(i==MP_index_exp(D(ka)), Constant(0)-R(ka), 0 )) ;
			 

(off course you are free to choose another name instead of "mpif")

Note: 
a) the explicit conversion to MP_index_exp 
b) that unary minus is not (yet) implemented for Constant. 
This can (and will) be implemented similarly to the mpif above.

As mentioned above, I do not particularly like this mpif construct,
but if there is a "popular demand", I will consider to include it in
"standard" flopc++.

Cheers, Tim




>>> "marcos.roberto.silva" <marcos.roberto.silva at uol.com.br> 14/12/2006 21:28 >>>
Hi all,
I have a model written in GNU MathProg (GLPK) for the
Network Design Problem with the following constraint:

s.t. r1 {i in N, ka in K} : sum{j in N} f[i,j,ka] - sum{l in
N} f[l,i,ka] = (if i==O[ka] then R[ka] else (if i==D[ka]
then -R[ka] else 0));

How can I represent this constraint in Flopcpp?

Thank you.
Best Regards.

Marcos Roberto Silva
Brazil


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