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

[Hung Dang <hungptit@xxxxxxxxx>]

Hi Janos, Everton, and Adrien,

Thank you very much for your suggestions especially Adrien's explanations are very helpful. I only look from the implement perspective (\=, *=, -= , and += operators are the same) and do not pay attention to the mathematical meaning of the _expression_. 

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;