Unexpected result using float variables #159

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1. a1880 commented on Apr 10, 2017

   

   a1880

   on Apr 10, 2017

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   I tried to uncover some rounding problems and modified the example:

   float: k = 1e300;
   var 0.0..100.0: a;
   var 0.0..100.0: b;
   constraint (a > 94.0 /\ b > 94.0) /\ (a > b);
   solve minimize b;
   output ["\na = " ++ show(k * a) 
        ++ "\nb = " ++ show(k * b) 
        ++ "\na - b = " ++ show(k * (a-b))];
   
   

   The output is:

   a = 9.500000000000001e+301
   b = 9.500000000000001e+301
   a - b = 0.0
   ----------
   ==========
   
2. Dekker1 commented on Oct 4, 2017

   

   Dekker1

   on Oct 4, 2017

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   This bug seems to be caused by a faulty internal transformation. The resulting FlatZinc for this model is:

   array [1..2] of float: X_INTRODUCED_0_ = [-1.0,1.0];
   var 95.0..100.0: a:: output_var;
   var 95.0..100.0: b:: output_var;
   constraint float_lin_lt(X_INTRODUCED_0_,[a,b],0.0);
   solve  minimize b;
   
3. Dekker1

   closed this as completedon Oct 4, 2017
4. Dekker1

   reopened this on Oct 4, 2017
5. Dekker1

   added

   bug

   on Oct 6, 2017
6. guidotack

   added a commit that references this issue on Oct 6, 2017

   Fix incorrect simplification of float domain constraints. Fixes #159.

   This issue will close once commit a7b4034 is merged into the 'develop' branch.

   a7b4034
7. guidotack commented on Oct 6, 2017

   

   guidotack

   on Oct 6, 2017

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   The incorrect simplification (changing the bound to 95.0) is now fixed in the compiler. The fact that a>b is violated is a bug in Gecode, not in MiniZinc (I will close this bug here and report it for Gecode).
8. feserafim commented on Oct 20, 2017

   

   feserafim

   on Oct 20, 2017

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   I have just updated MiniZinc IDE (bundled) newest version (2.1.6) and executed

   $ path/to/mzn2fzn.exe my-example.mzn
   

   but I am still getting the same result as @Dekker1.

   Am I doing anything wrong?
9. Dekker1 commented on Oct 20, 2017

   

   Dekker1

   on Oct 20, 2017

   Last edited by Dekker1

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   Hello @feserafim,

   Although Guido managed to fix the flattening bug, the fix has only been implemented on the development branch. (We haven't released any new versions since the bug was fixed). This is also the reason that the issue is still open.

   If you are still experiencing problems and you can't wait until the next release, then you might want to try to compile the develop branch and use that mzn2fzn binary. The problem should be fully fixed on that branch. (Although the problem in Gecode is a different issue as Guido mentioned)
10. guidotack

    closed this as completedin 9aa1173on Jan 10, 2018
11. cyderize

    added the

    Bug

    issue type on Jan 22, 2026