[with patch; positive review] show subdivisions for matrices over GF(2)

[jhpalmieri]

Since matrix_mod2_dense defines its own str function, it overrides the function for general matrices, so for dense matrices over GF(2), subdivisions are not printed. (See ​this discussion.)

This patch deletes the str method for dense matrices over GF(2).

[jhpalmieri]

This is a duplicate of #5714.

[was]

Needs work. There is a *very good* reason that there is a str method for GF(2) -- it's for speed! Check this out:

BEFORE YOUR PATCH:
sage: a = random_matrix(GF(2),1000)
sage: time b = a.str()
CPU times: user 0.41 s, sys: 0.01 s, total: 0.42 s
Wall time: 0.42 s

AFTER YOUR PATCH:
sage: a = random_matrix(GF(2),1000)
sage: time b = a.str()
CPU times: user 5.02 s, sys: 0.86 s, total: 5.88 s
Wall time: 5.89 s

[robertwb]

apply only this patch

[robertwb]

I posted a new patch that handles subdivisions. I went ahead and sped up the str method while I was there too.

Before

sage: a = random_matrix(GF(2),1000)
sage: time b = a.str()
CPU times: user 0.48 s, sys: 0.01 s, total: 0.49 s
Wall time: 0.50 s

After

sage: a = random_matrix(GF(2),1000)
sage: time b = a.str()
CPU times: user 0.01 s, sys: 0.01 s, total: 0.02 s
Wall time: 0.02 s

[mabshoff]

Hmm, shouldn't this patch be reopened or maybe go to another ticket? We need to do one thing since this patch is currently closed as a dupe.

Cheers,

Michael

[robertwb]

I just reopened this ticket, as it seems to have the most activity.

[jhpalmieri]

Looks good, passes all tests when applied to 3.4.1.rc2. Also fast, as robertwb pointed out above.

A question: how do you tell if a sparse matrix is too big to be converted to a dense one? If we have a sparse mod 2 matrix, should we consider printing it by converting to a dense one and using this routine? For a 1000x1000 random matrix, converting and using this one was about 5 times faster than just printing it. (If this is a good idea, then it belongs in a different ticket, but I don't know if this is a good idea...)

[mabshoff]

Merged 5715-mat2-subdiv.patch in Sage 3.4.1.rc3.

Cheers,

Michael

[robertwb]

How big is too big is an interesting question, but too big to convert > to big to print. Probably should be a separate ticket, but the optimizer in me sees an even easier way: create the zero matrix doing "[0 0 ... 0]" * nrows, then set the 1 entries. This wouldn't of course handle subdivisons though, but could be easily adapted to do so the same way as this code.