[GiNaC-devel] A smarter relational to Boolean conversion

[Vladimir V. Kisil]

>>>>> On Sat, 5 Jun 2021 21:55:53 +0000, "Vladimir V. Kisil" <V.Kisil at leeds.ac.uk> said:

    VVK> 	You are right, normal() shall be called by
    VVK> the user if needed. I am attaching the amended patch.

    Yet, if some simplification to a relational appears from
  cancellations between the left- and the right-hand sides, then
  application of normal() before the conversion to bool will miss it.

  Thus, I propose an additional method relational::canonical(), which
  returns an equivalent relational with the zero right-hand side. With
  this method the following is possible:

	possymbol p("p");
	relational rel =(p >= (p*p-1)/p);
	if (ex_to<relational>(rel.canonical().normal()))
		cout << "correct inequality" << endl;

  The respective patch (including the tutorial) is attached.

  Best wishes,
  Vladimir
-- 
Vladimir V. Kisil                 http://www.maths.leeds.ac.uk/~kisilv/
  Book:      Geometry of Mobius Maps       https://doi.org/10.1142/p835
  Soft:      Geometry of cycles         http://moebinv.sourceforge.net/
  Jupyter notebooks:       https://github.com/vvkisil/MoebInv-notebooks

>>>>> On Sat, 5 Jun 2021 22:14:33 +0200, "Richard B. Kreckel"
    VVK> <kreckel at in.terlu.de> said:

    RK> Dear Vladimir,

    RK> On 04.06.21 10:31, Vladimir V. Kisil wrote:
    >>> I am attaching a small patch. With it the following conversions
    >>> from relational to Boolean will become "true" from the current
    >>> "false":
    >>> 
    >>> realsymbol x("x"); possymbol p("p"); cout << bool(p>0) << endl;
    >>> cout << bool(abs(x)>=0) << endl;

    RK> Thank you for submitting this patch!

    RK> What is the reason to call .normal() in the symbolic case?
    RK> Generally, we never do automatic evaluation of expressions with
    RK> complexity worse than O(n log n). See section 4.2 of the
    RK> tutorial.

    RK> The reason we don't do this is to not surprise the user with
    RK> skyrocketing run-times, especially when expressions get longer
    RK> or nested. It is a different story when the user calls a special
    RK> transformation explicitly.

    RK> Admittedly, this case here is not exactly automatic
    RK> evaluation. But still, it is a normal() hidden deep inside a
    RK> seemingly innocent coercion to Boolean. I think it would be more
    RK> appropriate to let the user call .normal() explicitly, it that
    RK> is desired.

    RK> May I suggest to remove this one line?

    RK> All my best, -richy.  --

    RK> Richard B. Kreckel

    RK> <https://in.terlu.de/~kreckel/>

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