Re: [eigen] Vector's -= operator does not seem to work

["Janek Kozicki (yade)" <jkozicki-yade@xxxxxxxxx>]

Hung Dang said:     (by the date of Wed, 18 Mar 2020 09:06:02 -0400)

> [...] I only look from the implement perspective (\=, *=, -= ,
> and += operators are the same) and do not pay attention to the mathematical
> meaning of the expression.

Whoa! That's surprising. I am really glad that Eigen library through
the compiler errors prevents people from doing simple mathematical
mistakes.

best regards
Janek




> 
> Best,
> Hung
> 
> On Tue, Mar 17, 2020 at 10:53 PM Adrien Escande <adrien.escande@xxxxxxxxx>
> wrote:
> 
> > Hello,
> >
> > this is not working, because you are trying to subtract two elements that
> > are different in nature: a vector and a scalar. From a mathematical point
> > of view, it doesn't make sense.
> > By doing v -=2 I assume you are trying to subtract 2 from every component
> > of v. Some software like matlab offer a *shorcut* in the form v - 2, but
> > Eigen doesn't (at least not directly).
> > There are at least two ways to achieve what you want:
> >  * v -= Vector3d::Constant(2), that is you express in a proper
> > mathematical way what you want
> >  * going into array world as Janos suggested: v.array() -= 2;
> >
> > Note that v *= 2 works because it is mathematically valid to multiply a
> > vector or matrix by a scalar.
> >
> > Best regards,
> > Adrien Escande
> >
> > On Wed, Mar 18, 2020 at 10:04 AM Everton Rufino Constantino1 <
> > everton.constantino@xxxxxxx> wrote:
> >
> >> Looking at VectorwiseOp.h there is an -= operator implemented...
> >> What kind of error are you seeing exactly?
> >>
> >> Everton Constantino
> >> Software Developer
> >> Application Libraries Optimisation
> >> Linux Technology Center, IBM Systems
> >> e-mail: everton.constantino@xxxxxxx
> >>
> >>
> >>
> >>
> >> ----- Original message -----
> >> From: Janos Meny <janos.meny@xxxxxxxxxxxxxx>
> >> To: eigen@xxxxxxxxxxxxxxxxxxx
> >> Cc:
> >> Subject: [EXTERNAL] Re: [eigen] Vector's -= operator does not seem to work
> >> Date: Tue, Mar 17, 2020 21:56
> >>
> >> How about using v.array() -= 2; ?
> >>
> >> On Wed 18. Mar 2020 at 01:53, Hung Dang <hungptit@xxxxxxxxx> wrote:
> >>
> >> Hi,
> >>
> >> I try some Eigen examples (see below) and the -= operator is not
> >> supported. Is there any way to make it work without creating a constant
> >> vector?
> >>
> >> Thank a lot,
> >>
> >> Hung
> >>
> >>      Matrix2d a;
> >>      a << 1, 2, 3, 4;
> >>      Vector3d v(1, 2, 3);
> >>      std::cout << "a * 2.5 =\n" << a * 2.5 << std::endl;
> >>      std::cout << "0.1 * v =\n" << 0.1 * v << std::endl;
> >>      std::cout << "Doing v *= 2;" << std::endl;
> >>
> >>      v *= 2;
> >>      std::cout << "Now v =\n" << v << std::endl;
> >>
> >>      // Does not work
> >>      // v -= 2;
> >>      // std::cout << "Now v =\n" << v << std::endl;
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>


-- 
--
Janek Kozicki, PhD. DSc. Arch. Assoc. Prof.
Gdańsk University of Technology
Faculty of Applied Physics and Mathematics
Department of Theoretical Physics and Quantum Information
--
http://yade-dem.org/
http://pg.edu.pl/jkozicki (click English flag on top right)