Graph.is_distance_regular is awfully wrong

[Nathann Cohen]

As reported in [1] is_distance_regular is SOooooooo awfully wrong that the definition it uses is ... just wrong. I'm sorry for that :-/

The current patch fixes it.

Nathann

[1] ​http://trac.sagemath.org/ticket/14619#comment:27

[Nathann Cohen]

New commits:

​f654144

trac #15864: Graph.is_distance_regular is awfully wrong

[Dima Pasechnik]

IMHO

sage: g.is_distance_regular(parameters = True)

is terribly confusing to the user. Indeed, is_distance_regular is meant to return True or False, just like any other is_blahbla member functions are. There should be a member function named something like distance_regularity_parameters which would return the parameters (what it returns for a non-distance-regular graph, is open to discussion - e.g. it can even return lists of all the different values, or empty entries for the values which are not-constant.

[Nathann Cohen]

Yooooooooo !

is terribly confusing to the user. Indeed, is_distance_regular is meant to return True or False, just like any other is_blahbla member functions are.

Well. Not exactly "all" :-P

I did this quite quite often in the graph code, in order to avoid having to create three functions for every feature : one which returns True/False?, one which returns a certificate, and a hidden one which returns both. Besides it is weird to call a function which is meant to return a certificate when you do not even know if the graph is distance-regular or not, and there is a risk that users would call BOTH functions (first boolean check THEN certificate) and so compute everythin twice.

There should be a member function named something like distance_regularity_parameters which would return the parameters (what it returns for a non-distance-regular graph, is open to discussion - e.g. it can even return lists of all the different values, or empty entries for the values which are not-constant.

Well... It's the usual way to do stuff, but with Python we do not really have to follow this limitation as there is no return type... is_perfect can ive you an odd hole/antihole, is_forest returns cycles, is_planar gives you a K5/K33 minor. Really, it's cool :-P

Nathann

[Dima Pasechnik]

in the line A regular graph G is distance-regular if for any integers j,k the value... the word regular is not needed, as you can take j=k=0 and u=v.

[Dima Pasechnik]

in the literature the order of these lists:

sage: g.is_distance_regular(parameters = True)
        ([0, 1, 1], [3, 2, 0])

is normally the opposite, and the last entry, which is always 0, 0 in the list beginning with the valence is omitted, and the 1st entry, always 0, in the other list is omitted too. I suggest we follow this convention. I.e. here it should be ([3,2],[1,1])

[Dima Pasechnik]

I'm doing a reviewer patch.

[Dima Pasechnik]

Replying to dimpase: Nathann, if you're happy with my changes, I'll make it positive review. Let's postpone foo.is_bla... cleanup to another ticket.

---

New commits:

​e5a990d

fixed typos, corrected the definitions of b, c, chnaged the way they are returned, one more example

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​0b540be

docs fix: returning (b,c), not (c,b)

[Nathann Cohen]

Yooooooooooooo Dima !

I am back to work, in Brussels. That's a good news :-P

Aren't you afraid of the fact that c[i] is not c_i but c_{i+1} ? Don't you think it's dangerous ?

Nathann

[Dima Pasechnik]

Well, this is the standard notation. Might be a bit awkward, indeed. These numbers are best understood as entries of the ​intersection matrix, which in this case happens to be 3-diagonal.

[Nathann Cohen]

Why don't we set the missing entries to None ? Like c_0 and b_d ?

Nathann

[Dima Pasechnik]

Replying to ncohen:

Why don't we set the missing entries to None ? Like c_0 and b_d ?

c[i] is more of a function of i than anything else (same applied to b). You would not insist on 1/x returning None for x=0, would you?

[Nathann Cohen]

Come on Dima... You know people will make mistake because of that. We would just avoid them this mistake.

Nathann

[Dima Pasechnik]

Replying to ncohen:

Come on Dima... You know people will make mistake because of that. We would just avoid them this mistake.

OK, fine, if you want to pad with None entries, go for it. Let me know if you want me to update the patches, or do it yourself.

[Nathann Cohen]

Updated ! I also changed the "return" line to return b,c, as you fixed the docstring. Is that right ? O_o

Nathann

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​e5a990d	fixed typos, corrected the definitions of b, c, chnaged the way they are returned, one more example
​0b540be	docs fix: returning (b,c), not (c,b)
​20c5a2a	trac #15864: bugfix et None dans bi et ci

[Dima Pasechnik]

Replying to ncohen:

Updated ! I also changed the "return" line to return b,c, as you fixed the docstring. Is that right ? O_o

What? Why don't you just start from my latest commit, rather than redoing all my fixes?!

[Dima Pasechnik]

Replying to dimpase:

Replying to ncohen:

Updated ! I also changed the "return" line to return b,c, as you fixed the docstring. Is that right ? O_o

What? Why don't you just start from my latest commit, rather than redoing all my fixes?!

I had the order of b and c as it must be, why are you swapping it back?

[Dima Pasechnik]

Replying to dimpase:

Replying to dimpase:

Replying to ncohen:

Updated ! I also changed the "return" line to return b,c, as you fixed the docstring. Is that right ? O_o

What? Why don't you just start from my latest commit, rather than redoing all my fixes?!

I had the order of b and c as it must be, why are you swapping it back?

I guess your code names the stuff inconsistently, i.e. the meanings of b and c are swapped, compared with your docstrings.

[Nathann Cohen]

In the first of your two commits, you make the function return ci, bi. In the second of your two commits you change the documentation to say that the function should return bi,ci. But the code did not change ! I just changed the code to return bi,ci which follows the doc as you changed it !

Nathann

[Nathann Cohen]

The definitions are inconsistant since you changed it in your first commit :

​http://git.sagemath.org/sage.git/commit/?h=20c5a2ad3ec2074eb2c89ca27622907a2e115557&id=e5a990dd8f2b0ff38dcee683e9d59182d2614eb1

Could you fix that to make the doc and code consistent ?

Nathann

[Dima Pasechnik]

Replying to ncohen:

The definitions are inconsistant since you changed it in your first commit :

​http://git.sagemath.org/sage.git/commit/?h=20c5a2ad3ec2074eb2c89ca27622907a2e115557&id=e5a990dd8f2b0ff38dcee683e9d59182d2614eb1

Could you fix that to make the doc and code consistent ?

OK, I see, that's what happened: indeed, I changed docs (except the docs for the return value, just forgot these - that's why there was one more commit) in the first commit, as well as the order of the things returned in return. For the sake of code readability it would be great to change the code too, indeed. Do you want me to do this, or you prefer to do this yourself?

[Nathann Cohen]

What about this ?....

Nathann

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​f654144	trac #15864: Graph.is_distance_regular is awfully wrong
​e5a990d	fixed typos, corrected the definitions of b, c, chnaged the way they are returned, one more example
​0b540be	docs fix: returning (b,c), not (c,b)
​1555dc7	trac #15854: consistency between code and doc, and None in bi and ci

[Dima Pasechnik]

Replying to ncohen:

What about this ?....

the diff looks good. How do I update the installation? sage --dev checkout --ticket=15864 does not get it right.

[Dima Pasechnik]

Replying to dimpase:

Replying to ncohen:

What about this ?....

the diff looks good. How do I update the installation? sage --dev checkout --ticket=15864 does not get it right.

you've mixed up the ticket number: look, the last commit line above says

​1555dc7	trac #15854: consistency between code and doc, and None in bi and ci

but we are doing #15864 ! No wonder I can't check out a sh*t, no matter what I try...

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​f654144	trac #15864: Graph.is_distance_regular is awfully wrong
​e5a990d	fixed typos, corrected the definitions of b, c, chnaged the way they are returned, one more example
​0b540be	docs fix: returning (b,c), not (c,b)
​faffd86	trac #15864: consistency between code and doc, and None in bi and ci

[Nathann Cohen]

Ahahahah. Well, I don't think that the dev scripts are based on the commit messages, but I fixed that, just in case :-P

I think that the dev script is mostly worried over the fact that I rewrote history. That's why you should probably find a way to delete that branch from your local repository again before running your checkout command.

By the way, how is it going in Oxford ? Do you finally have an office and everything ?

Cheers from Brussels,

Nathann

---

New commits:

​faffd86

trac #15864: consistency between code and doc, and None in bi and ci

[Dima Pasechnik]

OK, this works to get the right branch:

git fetch origin u/ncohen/15864:15864
git checkout 15864

looks good.

[Volker Braun]

Please fill in reviewer names