Optimization with Minizinc - Only print optimal solutions

Question

at the moment I'm having a closer look on Minizinc. Minizinc is showing all valid solutions of my model in the output window when solving the model. I'm a bit confused because I did not ask minizinc to solve the model as a satisfaction problem. Is there a possibility that only optimal solutions are shown? Thanks for your answers.

best regards

score 1 · Accepted Answer · answered Jan 20 '18 at 06:59

1

What is your solve statement? If it's solve satisfy then you ask for all solutions. If it's solve minimize x or solve maximize x then the solver will show the progression of optimal solutions, with the optimal solution is shown last.

answered Jan 20 '18 at 06:59

hakank

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Hi hakank, thanks for your answer. My sovle statement is "solve minimize". Is there a possibilty to configure minizinc that only the optimal solution(s) are shown when the solving process is finished? – Westfale Jan 21 '18 at 13:13
There's no option to get only the last, optimal, solution. Some solvers handle the "-n " option differently for optimal problems, but they either give the first solution (not the last) or they simply ignore that flag. – hakank Jan 21 '18 at 17:35
@hakank In later versions of the MiniZinc IDE, you can change the solver configuration to only output the optimal solution. Click on the "Show configuration editor" button in the top right corner of the IDE. In the pane that opens, find the radio button "User-defined behavior" and click on it. Lastly, uncheck the box (if checked) that says "Print intermediate solutions ...". This will make the solver have the behaviour you want, as long as the solver MiniZinc uses has this option... – Lluís Alemany-Puig Oct 20 '20 at 13:27

Patrick Trentin · Answer 2 · 2018-03-01T16:59:57.660

With OptiMathSAT version 1.5.1, one can now print all same-cost optimal solutions of a given FlatZinc formula using option

-opt.fzn.all_solutions=[BOOL]

e.g. take the following test.fzn file

var 0..3: x ::output_var;
var 0..3: y ::output_var;
var bool: r1 ::output_var;
var bool: r2 ::output_var;

constraint int_lin_le_reif([1, 1, -1], [x, y, 4], 0, r1);
constraint int_lin_le_reif([1, 1, -1], [x, y, 2], 0, r2);
constraint bool_or(r1, r2, true);

solve maximize x;

and solve it as follows:

~$ ../bin/optimathsat -input=fzn -opt.fzn.all_solutions=True < test.fzn
% allsat model
x = 3;
y = 0;
r1 = true;
r2 = false;
----------
% allsat model
x = 3;
y = 1;
r1 = true;
r2 = false;
----------
=========

The maximum value for x is 3, hence the solver prints all-and-only those models of test.fzn for which x is equal to 3.

Naturally, as mentioned by @hakank in his answer, one might be interested in the progression of solutions towards the optimal one. This can also be done in OptiMathSAT using the option

-opt.fzn.partial_solutions=[BOOL]

On the example above, that would yield

~$ ../bin/optimathsat -input=fzn -opt.fzn.partial_solutions=True < test.fzn
% objective: x (model)
x = 2;
y = 0;
r1 = true;
r2 = true;
----------
% objective: x (model)
x = 3;
y = 0;
r1 = true;
r2 = false;
----------
% objective: x (optimal model)
x = 3;
y = 0;
r1 = true;
r2 = false;
----------
=========

Here the optimization search finds two different models: an initial one in which x is equal to 2, and an optimal one in which x is equal to 3. The latter model gets printed twice: the first time as soon as it is found, the second time when the solver is able to prove that it is actually the optimal solution of the problem.

Optimization with Minizinc - Only print optimal solutions

2 Answers2