[GiNaC-list] ldegree changes after expand

[Matthias Naaf]

Dear Alexey,

thanks for your reply, that answers my question.

Since I mostly relied on the (otherwise quite helpful) tutorial when
learning about GiNaC, I would appreciate it if the wording in the
tutorial could be clarified.

Best regards,
Matthias


Am 04.06.2018 um 10:31 schrieb Alexey Sheplyakov:
> Hi,
>  
> 02.06.2018, 01:50, "Matthias Naaf via GiNaC-list" <ginac-list at ginac.de>:
>  
>> I've just hit an unexpected behavior of the ex.ldegree() method in my
>> application. The problem is that ldegree() gives 0 before and 1 (the
>> expected value) after calling expand() on the expression. I did not
>> expect this, as the tutorial says that degree() and ldegree() "work
>> reliably on non-expanded input polynomials".
>  
> The tutorial is misleading. The only reason why one could claim that
> [l]degree()
> "works reliably" with non-expanded inputs is that the program won't crash.
> Proof: try this in ginsh:
>  
> ldegree(x*(x^2-1) - x^2*(x - 1), x)
>  
> The code [1] does not expect different terms to cancel each other, hence
> the problem
>  
> [1]
> https://www.ginac.de/ginac.git/?p=ginac.git;a=blob;f=ginac/add.cpp;h=25de6cd2e889555f52aeccae2b263c4ccc2f74a5;hb=refs/heads/master#l279
>  
>> A small running example and its output are attached. The expression is
>> the one I encountered in my application. This looks like a bug to me, or
>> is it necessary to call expand() before using ldegree()?
>  
> One should expand the expression before using degree(), ldegree(), coeff()
>  
> Best regards,
> Alexey