Matrix constructor does not return over padic field

[saraedum]

The following code does not return in sage 5.1.rc1.

sage: MatrixSpace(Qp(3),1,1)([Qp(3).zero()]) 

The problem is that the matrix constructor tries to unpack a padic zero which is an infinite list of zeros.

Another example where the unpacking does not work is the following:

sage: MatrixSpace(Qp(3),1,1)([Qp(3)(4/3)])
[1 + O(3^20)]

The problem here is that the iterator returned for a padic (implicitly defined through __getitem__) starts at the term with valuation 0.

[saraedum]

[not relevant anymore]

[saraedum]

[moved to description]

[saraedum]

I think that padics should probably not return iterators at all.

What should the iterator for Qp(3).zero() be — should it be infinite or empty? Should the iterator for Zp(3)(3) and Qp(3)(3) iterate over [0,1] or over [1]?

I believe that if one wants to iterate over the terms, then one should have to call .list() or similar explicitly.

[mderickx]

I think the real problem is that your example is missing some parentheses

sage: MatrixSpace(Qp(3),1)([[Qp(3).zero()]])
[0]
sage: MatrixSpace(Qp(3),1,1)([[Qp(3)(4/3)]])
[3^-1 + 1 + O(3^19)]

This causes the matrix constructor to look if it can unpack the element Qp(3).zero()] in the list [Qp(3).zero()] .

If you want it also to work with fewer parentheses I'd say that it's the fault of MatrixSpace?.call not checking if Qp(3).zero() is a ring element before just trying to unpack it.

Note that it's not only the p-adics that give rise to strange behaviour. You can also use polynomials:

sage: K.<x>=QQ[]
sage: MatrixSpace(K,2)([x+2,3*x+4])
[2 1]
[4 3]

[saraedum]

Thanks for pointing this out Maarten.

I'll have a look if the check you propose can be done easily. Btw, the original problem I had was the following:

sage: K=Qp(3)
sage: V=K^1
sage: V.hom([V.zero()])

This hangs and there don't seem to be any parentheses missing, right?

So apparently the hom does too much unpacking along the way.

Anyway, I'll look into this and most probably split this ticket into two tickets, since I still think that padics should not be iterable.

[mderickx]

hom is broken in even more ways. see the discussion (maybe monologue is more adequate)  https://groups.google.com/forum/?fromgroups#!topic/sage-devel/UdHakJf5A5Q

I created #13321 for this.

[saraedum]

I created #13592 for the __iter__ issue. This fixes the issue described here and #13321 takes care of hom(), so I believe this ticket can be closed now.