Re: Solver delivers wrong answer when 2 constraints are close

Testing this problem I discover that if we change the order of 
constraint declarations it seems to give the expected answer as stated 
by Thiago (what I think could be another bug).

====

/param min_bound default 0;/
/var x >= 0;

minimize y: x;/
*
*/*s.t. PART_MIN_X: x >= 1 + min_bound;*
/
/*s.t. LIM_INF_X: x >= 1;
*
/
/solve;
display min_bound;
display x; # EXPECTED RESULT: X ==  1.0001

data;
param min_bound := 1e-4;
end;/

====

Output:

====

x.val = 1.0001

====

On 3/3/21 19:19, Thiago Neves wrote:
> Hi.
> I've found a strange behaviour in glpk which I don't know how to fix 
> nor how to contour it. It seems like GLPK can't distinguish 
> constraints that differs from about 1e-4.
>
> Follows simple examples that explain and reproduce the problem.**
> *
> *
> *The first model gives the desired answer (x = 1.0001):*
> /
> param min_bound default 0;/
> /var x >= 0;
>
> minimize y: x;/
> /*
> s.t. PART_MIN_X: x >= 1 + min_bound;*
>
> solve;
> display min_bound;
> display x; # EXPECTED RESULT: X ==  1.0001
>
> data;
> param min_bound := 1e-4;
> end;
> /
> /_____________________________________/
> /OUTPUT:/
> /x.val = 1.0001/
> /_____________________________________ /
>
> *Now, if I add a second constraint "close" to the first one, the 
> solver will deliver an answer that is actually infeasible:*
>
> /param min_bound default 0;/
> /var x >= 0;
>
> minimize y: x;/
>
> *s.t. LIM_INF_X: x >= 1;
>
> */*s.t. PART_MIN_X: x >= 1 + min_bound;*
>
> solve;
> display min_bound;
> display x; # EXPECTED RESULT: X ==  1.0001
>
> data;
> param min_bound := 1e-4;
> end;/
> /_____________________________________/
> /OUTPUT:/
> x.val = 1
> /_____________________________________ /
>
> *If I change the "min_bound" parameter to 1e-2, the second model works 
> as expected (x = 1.01):*
>
> /param min_bound default 0;/
> /
> /
> /var x >= 0;
>
> minimize y: x;/
> *
> *
> *s.t. LIM_INF_X: x >= 1;
>
> */*s.t. PART_MIN_X: x >= 1 + min_bound;*
>
> solve;/
> /
> display x; # EXPECTED RESULT: X ==  1.01
>
> data;
> param min_bound := 1e-2;
> end;/
> /_____________________________________/
> /OUTPUT:/
> x.val = 1.01
> /_____________________________________ /
> /
> /
>
> Att,
>
> *Thiago H. Neves*
> (31) 98608-0666
>