Yes, you are right that the email is about a different topic. However, it can be worth noting that evaluating floating-point expressions in different order, which is basically what you do using cse:s, can give different results due to the floating-point quantization/rounding happening at different places. Hence, it is not always obvious to tell if the results are "identical". If you are unlucky, the error propagation can be quite disastrous at times, but you are probably correct in that there is something else that is the primary issue here.
BR Oscar Den tors 22 okt. 2020 kl 18:57 skrev Jason Moore <moorepa...@gmail.com>: > As far as I understand the results should match to machine precision. > > We are able to match these kinds of models executed on different platforms > using different formulation techniques to machine precision (e-14 or more). > In fact, we use this to be able to verify that two independent modelers > have created the same model. > > The fact that my model can't get the same results with two numerical > evaluations of sympy code is concerning. > > My question in this email though, is how to improve the comparison code so > that it doesn't crash for large expressions. > > Jason > moorepants.info > +01 530-601-9791 > > > On Thu, Oct 22, 2020 at 6:49 PM Oscar Gustafsson < > oscar.gustafs...@gmail.com> wrote: > >> How much do they differ? I checked the issue and the latest result wasn't >> that bad, although probably too much to blame floating-point errors. >> However, if you push the numerical range into e.g. denormalized numbers I >> guess it can simply be that. Not very likely though. Is it still that type >> of numerical errors? >> >> BR Oscar >> >> Den tors 22 okt. 2020 09:37Jason Moore <moorepa...@gmail.com> skrev: >> >>> Howdy, >>> >>> I'm trying to track down a bug in this PR: >>> >>> https://github.com/pydy/pydy/pull/122 >>> >>> I have quite large SymPy expressions which I evaluate with floating >>> point numbers. I also cse those expressions and evaluate those with the >>> same floating point numbers. I'm getting discrepancies in the results. I'd >>> like to eliminate the possibility that the cse'd expressions are different >>> from the original expression. I wrote this naive function to make the >>> comparison: >>> >>> def compare_cse(expr): >>> >>> args = list(expr.free_symbols) >>> args.remove(TIME) >>> args += me.find_dynamicsymbols(expr) >>> args = list(sm.ordered(args)) >>> vals = list(np.random.random(len(args))) >>> >>> eval_expr = sm.lambdify(args, expr) >>> res = eval_expr(*vals) >>> >>> replacements, reduced_expr = sm.cse(expr) >>> for repl in replacements: >>> var = repl[0] >>> sub_expr = repl[1] >>> eval_sub_expr = sm.lambdify(args, sub_expr) >>> vals.append(eval_sub_expr(*vals)) >>> args.append(var) >>> eval_reduced_expr = sm.lambdify(args, reduced_expr[0]) >>> cse_res = eval_reduced_expr(*vals) >>> >>> np.testing.assert_allclose(res, cse_res) >>> >>> This seems to work for small expressions, but for the large expressions >>> run this actually kills my Python interpreter after some minutes. >>> >>> Is there a better way to write this? I need some way to verify that >>> numerical evaluation of a cse'd expression gives the same result as >>> numerical evaluation of the expression that can actually complete in a >>> reasonable amount of time. >>> >>> Jason >>> moorepants.info >>> +01 530-601-9791 >>> >>> -- >>> You received this message because you are subscribed to the Google >>> Groups "sympy" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to sympy+unsubscr...@googlegroups.com. >>> To view this discussion on the web visit >>> https://groups.google.com/d/msgid/sympy/CAP7f1Aivs53AcwyaoyO5%2Bzkn%2BPrX3TfoJJ5X85qQNZ6JRbo5JQ%40mail.gmail.com >>> <https://groups.google.com/d/msgid/sympy/CAP7f1Aivs53AcwyaoyO5%2Bzkn%2BPrX3TfoJJ5X85qQNZ6JRbo5JQ%40mail.gmail.com?utm_medium=email&utm_source=footer> >>> . >>> >> -- >> You received this message because you are subscribed to the Google Groups >> "sympy" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to sympy+unsubscr...@googlegroups.com. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/sympy/CAFjzj-LQNBFoF7Q4Rb%2BZG3J35-Et9b__SED-DMJvwUYqyKYADQ%40mail.gmail.com >> <https://groups.google.com/d/msgid/sympy/CAFjzj-LQNBFoF7Q4Rb%2BZG3J35-Et9b__SED-DMJvwUYqyKYADQ%40mail.gmail.com?utm_medium=email&utm_source=footer> >> . >> > -- > You received this message because you are subscribed to the Google Groups > "sympy" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to sympy+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/sympy/CAP7f1AgdkaC0D69s_ohka_AjDFvKCD%2BMoVKyFhs0Vn2b_DtFkw%40mail.gmail.com > <https://groups.google.com/d/msgid/sympy/CAP7f1AgdkaC0D69s_ohka_AjDFvKCD%2BMoVKyFhs0Vn2b_DtFkw%40mail.gmail.com?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "sympy" group. To unsubscribe from this group and stop receiving emails from it, send an email to sympy+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/sympy/CAFjzj-LFUSgmQ90Uok2LxKSTZ-bJKysCy%2BGqvs1wYtDErUAZNA%40mail.gmail.com.
