On Fri, Feb 23, 2018 at 5:53 AM, Matthias Geier <matthias.ge...@gmail.com> wrote: > Thanks Leonid for this quick and helpful answer! > > I just have a few follow-up questions: > > On Thu, Feb 22, 2018 at 11:38 PM, Leonid Kovalev wrote: >> In SymPy, polynomials have extra structure that distinguishes them from >> generic expressions. a3 * t**3 + a2 * t**2 + a1 * t + a0 is an expression. >> If you create a polynomial in t, it will print with the order of terms being >> from highest to lowest. >> >> >>> p = sp.Poly([a3, a2, a1, a0], t) >> >>> print(p) >> Poly(a3*t**3 + a2*t**2 + a1*t + a0, t, domain='ZZ[a0,a1,a2,a3]') > > Ah, that's interesting! > > I was actually using a Jupyter notebook with MathJax output most of > the time, and there this is not true! > > Same for the raw LaTeX output: > > >>> sp.latex(p) > '\\operatorname{Poly}{\\left( a_{0} + a_{1} t + a_{2} t^{2} + > a_{3} t^{3}, t, domain=\\mathbb{Z}\\left[a_{0}, a_{1}, a_{2}, > a_{3}\\right] \\right)}' > > Is this a bug? > >> Also, the order can be specified in the print command >> >> >>> pprint(a3 * t**3 + a2 * t**2 + a1 * t + a0, order='grevlex') >> 3 2 >> a₃⋅t + a₂⋅t + a₁⋅t + a₀ >> >> >> or, staying with str format, >> >> >> >>> sstrrepr(a3 * t**3 + a2 * t**2 + a1 * t + a0, order='grevlex') >> 'a3*t**3 + a2*t**2 + a1*t + a0' > > That's great, I think 'grevlex' is what I want! > > I was actually already playing around with 'lex', 'revlex' and > 'grevlex', but I got confused at some point. > It looks like this doesn't work if the highest power doesn't have a > symbolic coefficient, e.g.: > > >>> sp.sstrrepr(t**2 + a1 * t, order='grevlex') > 'a1*t + t**2' > > I assume there is a perfectly reasonable explanation for that, and in > my case such expressions didn't actually appear yet, so that's fine > for me. > > I think I will mainly use 'grevlex' with: > > sp.init_printing(order='grevlex') > > But when I'm dealing with expressions that have only the coefficients > and no powers of t in them, e.g.: > > 3*a3 + 2*a2 + a1 > > ... then I'll temporarily switch: > > sp.init_printing(order='rev-lex') > > Or is this a bad idea? > Or is there a better way to temporarily change the order? > > Is there a way to specify the order for a single Jupyter output cell?

It is possible to create a custom order, however the documentation is pretty sparse. The place to start is here if you want to look into it https://github.com/sympy/sympy/blob/fb536869fb7aa28b2695ad7a3b70949926b291c4/sympy/polys/orderings.py#L199. The printers (including the latex printer) have an order setting which ends up getting passed to that function. Aaron Meurer > >> The printing module has a number of printers which support a number of >> settings. > > Thanks for the reference to > http://docs.sympy.org/latest/modules/printing.html, that's a very > helpful page. > > cheers, > Matthias > >> On Thursday, February 22, 2018 at 2:02:20 PM UTC-5, Matthias Geier wrote: >>> >>> Dear SymPy list. >>> >>> I'm playing around with polynomials in the context of spline curves. >>> >>> I want to use a cubic polynomial with yet unknown coefficients like this: >>> >>> >>> import sympy as sp >>> >>> t, a0, a1, a2, a3 = sp.symbols('t, a:4', real=True) >>> >>> a3 * t**3 + a2 * t**2 + a1 * t + a0 >>> a0 + a1*t + a2*t**2 + a3*t**3 >>> >>> The problem here is that the displayed order of terms is reversed, >>> normally the highest power of t should come first. >>> I guess this is because SymPy doesn't know that the coefficients a0 >>> etc. are constants and shouldn't be treated like variables. >>> So in fact this polynomial isn't sorted by powers of t but instead by >>> the coefficients. >>> >>> Is there a way to get around this? >>> >>> At some later point, I have expressions like this (without t): >>> >>> a1 + 2*a2 + 3*a3 >>> >>> It would make sense in my case to also display them reversed like this: >>> >>> 3*a3 + 2*a2 + a1 >>> >>> Is it possible to create a new type of symbol with non-default ordering? >>> Is it possible to define that this order is "ascending": a3, a2, a1, a0? >>> It doesn't have to be a generic solution, I'm OK with having those 4 >>> special symbols. >>> >>> Or is there an entirely different and much better way to do this? >>> >>> I know that I could just use a, b, c, d instead of a3, a2, a1, a0 and >>> it would work, but I would really like to see the connection between a >>> coefficient and its power of t. >>> >>> For the record, I also quickly tried to use IndexedBase to get a3, a2, >>> a1 and a0, and it turns out that although the LaTeX display of the >>> symbols looks the same (in text mode it's different), they are sorted >>> differently. >>> >>> >>> b = sp.IndexedBase('b') >>> >>> b[3] * t**3 + b[2] * t**2 + b[1] * t + b[0] >>> t**3*b[3] + t**2*b[2] + t*b[1] + b[0] >>> >>> They are sorted after the powers of t, which isn't what I want, either. >>> >>> cheers, >>> Matthias > > -- > 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 post to this group, send email to sympy@googlegroups.com. > Visit this group at https://groups.google.com/group/sympy. > To view this discussion on the web visit > https://groups.google.com/d/msgid/sympy/CAFesC-cN3_h-dVm854PeU%3Dkr3J%2Bhywyvkk0JBMbDrqQiRcbinA%40mail.gmail.com. > For more options, visit https://groups.google.com/d/optout. -- 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 post to this group, send email to sympy@googlegroups.com. Visit this group at https://groups.google.com/group/sympy. To view this discussion on the web visit https://groups.google.com/d/msgid/sympy/CAKgW%3D6%2BraBmXYn8_hdxGeyaqzLXk9yovpwoeOtPnYjVuEzXUGQ%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.