Personally, I like Mathew's idea of using MatrixExprs to represent vectors.
This will not just ensure smooth functioning with the rest of sympy, but
will also help in easing the 'garbage detection' process. Plus,
representation (as well as stuff like pretty printing) would get easier.
The only 'oddity', once again, would be vectors that are an addition of
bases from two different frames. Maybe use something on the lines of MatAdd?
On Thu, Jun 13, 2013 at 6:55 PM, Matthew Rocklin <mrock...@gmail.com> wrote:
> I don't remember enough of the details to give you a comprehensive answer.
> Here are a few thoughts that come to mind.
>
> You won't be able to change much of the Add and Mul code to handle
> Vectors. For example you won't be able to add shape checking at
> construction time. This forces you to follow the "Garbage in, Garbage out"
> philosophy (which isn't necessarily bad)
>
> In [18]: A = MatrixSymbol('A', n, 1)
> In [19]: B = MatrixSymbol('B', m, 1)
> In [20]: x = Symbol('x')
> In [21]: Add(x, A, B)
> Out[21]: x + A + B
>
> In [22]: MatAdd(x, A, B)
> TypeError: Mix of Matrix and Scalar symbols
>
> In [23]: MatAdd(A, B)
> ShapeError: Matrices A and B are not aligned
>
> Add and Mul inherit from Expr. The Expr codebase in general assumes that
> its elements are scalars. This pops up in many of the methods on Expr
> classes.
>
> In [26]: Mul(A, A, B)
> AttributeError: 'MatrixSymbol' object has no attribute 'as_base_exp'
>
> I really don't think I should have to implement as_base_exp for matrices
> but I do if I want to use Mul. Actually this sort of thing usually isn't
> so bad, the defaults you get from inheriting from Expr are reasonable. I
> remember having lots of trouble with some of the more aggressive scalar
> functions. People kept using expand and factor on matrix expressions.
> They sometimes got errors and filed bug reports that were very difficult
> to resolve without completely rewriting these functions to be more general.
>
> Using Mul and Add is fine. It'll work for a while. Every once in a while
> you'll come across a wart. I'm personally biased against the Expr
> codebase; I think it's the result of a lot of glomming on of special cased
> code. By my aesthetic MatrixExprs are a lot cleaner and more extensible.
> I'm personally happy with the choice *not* to inherit from Expr. Other
> groups have gone other ways though. You could look at physics.quantum if
> you wanted to see how Exprs work on non-scalars.
>
> Also, just a thought, but if you're just using vectors you could
> presumably build off of MatrixExprs. The container classes (MatAdd,
> MatMul) make very few assumptions about the things they hold other than
> that they are Basics and have a shape.
>
>
> On Thu, Jun 13, 2013 at 1:52 AM, Prasoon Shukla
> <prasoon92.i...@gmail.com>wrote:
>
>> @Matthew Rocklin:
>>
>> Can you please tell what exactly would be the advantage of having a
>> separate VectAdd class? I mean I don't see the problems with using the
>> current Add and Mul classes.
>>
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