Opened 6 years ago

Closed 4 years ago

# Pass number of variables to polygens

Reported by: Owned by: mforets major sage-8.0 commutative algebra polynomial vector nbruin, tscrim, vdelecroix Marcelo Forets Simon King N/A 5f1a4da 5f1a4da2392e198878f33f3bc8fc118178dd6218

### Description

The number of generators of a polynomial can be specified as a number:

sage: PolynomialRing(QQ, 'x', 4).gens()
(x0, x1, x2, x3)


This ticket should allow doing:

sage: polygens(QQ, 'x', 4)
(x0, x1, x2, x3)


Currently it returns TypeError: polygens() takes at most 2 arguments (3 given).

### comment:1 Changed 6 years ago by mforets

Branch: → u/mforets/pass_number_of_variables_to_polygens

### comment:2 Changed 6 years ago by mforets

Commit: → df2e39f6096e63e18a7014717e44a2013450cb36

apart from that, i'm tempted to propose making this possible too:

sage: polygens(QQ, 'x', 4, start_at=1)
(x1, x2, x3, x4)


New commits:

 ​df2e39f add optional argrs to polygens

### comment:3 follow-up:  5 Changed 6 years ago by kcrisman

Hmm, I wasn't aware of this other functionality. I have no opinion about it per se, but if you do it, I wonder if you could experiment with what might happen if one did

L = polygens(QQ, 'x', 4)
for l in L:
SR(l).inject_variable()


or whatever the right commands would be for that. Or even

polygens(SR, 'x', 4)


though I assume that wouldn't work right. See #11576 for motivation. (Or forget this comment and just be aware of #11576, which had a flurry of activity four years ago but hasn't been looked at since.)

### comment:4 Changed 6 years ago by git

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

 ​6d2082c allow inject_variables and start_at options

### comment:5 in reply to:  3 Changed 6 years ago by mforets

Hmm, I wasn't aware of this other functionality. I have no opinion about it per se, but if you do it, I wonder if you could experiment with what might happen if one did

L = polygens(QQ, 'x', 4)
for l in L:
SR(l).inject_variable()


or whatever the right commands would be for that. Or even

polygens(SR, 'x', 4)


though I assume that wouldn't work right.

great idea! it is very useful that these variables 'exist' in the calling method. as you can see in revised patch i used get_main_globals to get around the issue.

how does it look? does someone suggest other examples?

See #11576 for motivation. (Or forget this comment and just be aware of #11576, which had a flurry of activity four years ago but hasn't been looked at since.)

ok, thanks for pointing that out! although it seems to me that #11576 is an advanced version of the simpler (but yet lacking) uses var('x', 4) and var('x', 4, 4) for vectors and matrices respectively. or i'm missing some basic reason why they don't exist already?

### comment:6 Changed 6 years ago by nbruin

Don't both return a result and inject bindings (and certainly don't go grabbing the global scope!). It's pretty clear that the behaviour of var to both inject and return variables is a mistake once you see the confusion it causes.

Injection of polynomial variables is quite efficiently done with PolynomialRing(R,'x',4).inject_variables(), which prints a message and returns None on purpose.

polygen(*args) is just a short form of PolynomialRing(*args).gens() which helps people who know what a polynomial variable is but not what a polynomial ring is. I think it's a bad design decision to muddle this with other features.

I'm also against providing options to modify the numbering. There's a good reason why RX=PolynomialRing(R,'x',n) is allowed: it's clearly desirable to make "a polynomial ring in n variables" in various settings. Sage requires names for these variables, but specifying n names is bothersome if you don't care about the names. Hence the indexing. which agrees with what RX.gen(i) does. If you do care what your variable names are called, it's easy to specify a list of names. All kinds of possibilities exist: ["x_%s"%s for i in range(n)], ["x^(%s)"%s for i in range(1,n+1)] etc. It's much better to teach people how to get full customizibility than to give them one more option to discover, which will probably not meet their needs if they develop further.

Last edited 6 years ago by nbruin (previous) (diff)

### comment:7 follow-up:  8 Changed 6 years ago by mforets

@nbruin : thanks for the prompt feedback :) but are you ok with df2e39f or not? notice that *args was missing there on the first place.

Don't both return a result and inject bindings (and certainly don't go grabbing the global scope!).

is there an alternative to global_scope()? i tried with just R.inject_variables() and it didn't work. i suppose it works via return R.injected_variables(), but then you don't have access to your vector of variables, right? for multiplication and so on, it is useful to have it.

It's pretty clear that the behaviour of var to both inject and return variables is a mistake once you see the confusion it causes.

could you point me to some threads on this issue, so that i can catch up? for me there are no surprises with var. moreover, i find it practical that the function returns you the variables and/or allows you to use them in the console.

Injection of polynomial variables is quite efficiently done with PolynomialRing(R,'x',4).inject_variables(), which prints a message and returns None on purpose.

ok, but let's agree it is a rather long construction for people with bad memory (i'm in that group).

polygen(*args) is just a short form of PolynomialRing(*args).gens() which helps people who know what a polynomial variable is but not what a polynomial ring is. I think it's a bad design decision to muddle this with other features.

yes (essentially i'm on that group, too). that's arguable: why should a feature bother at all? so long as it is consistent with the rest & well documented.. (more on this below)

If you do care what your variable names are called, it's easy to specify a list of names. All kinds of possibilities exist: ["x_%s"%s for i in range(n)], ["x^(%s)"%s for i in range(1,n+1)] etc.

that's definitely not easy.

It's much better to teach people how to get full customizibility than to give them one more option to discover, which will probably not meet their needs if they develop further.

i get the point about "teaching a man to fish", 100%. at the same time, i'm with the lazy people here: if someone thought the solution and shows you a little example in command help, then i prefer using it rather than working it out myself (saves time, and is less error-prone).

for the point on features: what about LaTeX? in general people are happy with lots and lots of customizations ready to be discovered.

### comment:8 in reply to:  7 ; follow-up:  10 Changed 6 years ago by nbruin

@nbruin : thanks for the prompt feedback :) but are you ok with df2e39f or not? notice that *args was missing there on the first place.

Yes, that looks pretty straightforward. It might be that passing *args on wholesale is problematic (it causes erroneous parameters to be reported with a deeper traceback), but I don't see a big problem immediately. That's also something that is easy to fix later.

• in python it is usual that routines that are called because of their side-effects don't return anything. Compare L.sort() versus sorted(L)
• just google or search the documentation for the number of examples of y=var('y') etc.
• I don't think you will find many people who would give LaTeX as an example of a well-designed interface.
• implementing extra features adds bloat and other maintenance burden, and makes it more confusing for new people, because now they will be confronted with multiple ways of doing things. See "The zen of python". Not all of it applies to math software, but generally the ideas there have worked well to keep python a pleasant and easy to learn language.

That said, nothing prevents you from making your own convenience library functions and maintaining them in your own installation. Once they have been sitting around for a year or two and you're finding that you use them often, you'll have a nice body of code to demonstrate in exactly what way they are more convenient. Practice learns that what seems convenient when you're writing/designing something is often not used much in actual use.

Sage is getting large and mature enough that if there is a reasonable way of doing something, we should be hesitant about adding alternative constructions.

### comment:9 Changed 6 years ago by kcrisman

ok, thanks for pointing that out! although it seems to me that #11576 is an advanced version of the simpler (but yet lacking) uses var('x', 4) and var('x', 4, 4) for vectors and matrices respectively. or i'm missing some basic reason why they don't exist already?

Because nobody has both implemented and reviewed them. Lots of bikeshedding around exactly what the defaults and syntax should be.

### comment:10 in reply to:  8 ; follow-up:  11 Changed 6 years ago by mforets

@nbruin : thanks for the prompt feedback :) but are you ok with df2e39f or not? notice that *args was missing there on the first place.

Yes, that looks pretty straightforward. It might be that passing *args on wholesale is problematic (it causes erroneous parameters to be reported with a deeper traceback), but I don't see a big problem immediately. That's also something that is easy to fix later.

got it. df2e39f does solve the ticket's requirement. yet i suggest to leave it in the current status for 1 week in case someone else want to chime in.

BTW I'm willing to learn and apply the more robust solution if you have time now (or in the future).

That said, nothing prevents you from making your own convenience library functions and maintaining them in your own installation. Once they have been sitting around for a year or two and you're finding that you use them often, you'll have a nice body of code to demonstrate in exactly what way they are more convenient. Practice learns that what seems convenient when you're writing/designing something is often not used much in actual use.

Yes, I need a year or two (more than just time, certainly!) to just grasp what is convenient or not in terms of good design from an expert Python / Sage developer's standpoint.

No, I don't need a year or two to know that any of the possibilities that you suggest ["x_%s"%s for i in range(n)], ["x^(%s)"%s for i in range(1,n+1)] are not easy. Just because of this:

• Maple (born 1982) -- Vector(n, symbol = x)
• Matlab (born 1984) -- sym('x', [n 1])
• Mathematica (born 1988) -- Array[x, n]
• SymPy? (born 2007) -- symbols('x:n').

Matrices are constructed in a similar way.

### comment:11 in reply to:  10 ; follow-up:  12 Changed 6 years ago by nbruin

• Maple (born 1982) -- Vector(n, symbol = x)
• Matlab (born 1984) -- sym('x', [n 1])
• Mathematica (born 1988) -- Array[x, n]
• SymPy? (born 2007) -- symbols('x:n').

Matrices are constructed in a similar way.

Doesn't vector(polygen(QQ,'x',n)) and matrix(n,m,polygen(QQ,'x',n*m)) fit the bill for that? We need a little more symbols, but that's because sage is more explicit about the ring over which one works.

### comment:12 in reply to:  11 ; follow-up:  13 Changed 6 years ago by mforets

• Maple (born 1982) -- Vector(n, symbol = x)
• Matlab (born 1984) -- sym('x', [n 1])
• Mathematica (born 1988) -- Array[x, n]
• SymPy? (born 2007) -- symbols('x:n').

Matrices are constructed in a similar way.

Doesn't vector(polygen(QQ,'x',n)) and matrix(n,m,polygen(QQ,'x',n*m)) fit the bill for that? We need a little more symbols, but that's because sage is more explicit about the ring over which one works.

Oh yes, that's a nice construction for \mathcal{M}_{n\times m}(\mathbb{K}[x]) (minor: it is the plural, polygens).

Another thing that the commercial software have in common is that they follow mainstream Linear Algebra convention about variable names. That construction doesn't, especially:

sage: matrix(2, 4, polygens(QQ, 'x', 8))

[x0 x1 x2 x3]
[x4 x5 x6 x7]


What do you people think about the following. There is an open ticket #13703 which targets common matrix constructors. In the same spirit as the polytope library (type polytopes.?), with some work we can have a comprehensive matrix library, matrices.?, that features in particular:

sage: matrices.symbolic(2)

[a11 a12]
[a21 a22]


### comment:13 in reply to:  12 ; follow-up:  14 Changed 6 years ago by nbruin

What do you people think about the following. There is an open ticket #13703 which targets common matrix constructors. In the same spirit as the polytope library (type polytopes.?), with some work we can have a comprehensive matrix library, matrices.?, that features in particular:

sage: matrices.symbolic(2)

[a11 a12]
[a21 a22]


What would it be used for? If the matrix is of any significant size, there is not much computation you can do with these things. And if the matrix is small you might as well construct it manually. In any case:

• you'd want to be able to specify what the base ring is (or specify you want elements of SR as entries)
• once you're making these "universal" matrices, you should probably be able to specify the form (symmetric, antisymmetric, upper triangular, etc.)
• The labelling should be 0-based. You'd want M[1,2] to be a12.

### comment:14 in reply to:  13 Changed 6 years ago by mforets

What do you people think about the following. There is an open ticket #13703 which targets common matrix constructors. In the same spirit as the polytope library (type polytopes.?), with some work we can have a comprehensive matrix library, matrices.?, that features in particular:

sage: matrices.symbolic(2)

[a11 a12]
[a21 a22]


What would it be used for?

• Teaching : undergraduate level or before using SageMath.
• Mathematical Modeling : to represent an optimization variable in a semidefinite program.

I'd be delighted to go deeper into any of these use cases, or to further exchange our ideas and ideals about less concrete, open-ended issues that innocent #22809 has raised, such as why this very basic construct exists in all the software cited before, while we shouldn't expect a useful answer for the inverse of a big matrix whose coefficients are variables.

But, is this relevant at all to #22809? Maybe in Sage-devel?

### comment:15 Changed 6 years ago by git

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

 ​7a09c12 Revert "allow inject_variables and start_at options"

### comment:16 follow-up:  18 Changed 6 years ago by mforets

fair enough about naming conventions or handy constructions. as far as i'm concerned, this ticket is ready for peer-review!

side note: i've used git revert as suggested here trying to avoid messing up history (*). please let me know if this is not the correct way in this context.

(*) for the record: checked chapter 1 of the Guide, but didn't find this use case.

### comment:17 Changed 6 years ago by mforets

Status: new → needs_review

ps: sorry, next time i'll write the updates message in this place so that you don't receive 2 emails..

### comment:18 in reply to:  16 ; follow-up:  19 Changed 6 years ago by nbruin

side note: i've used git revert as suggested here trying to avoid messing up history (*). please let me know if this is not the correct way in this context.

You can do that if you care about preserving in the history that a change was made and rolled back. In a case like this I would just go with the branch that does NOT have the changes that were rolled back (rebase, or otherwise rebuild your branch into the state you want directly). Rebasing becomes a real problem if other people have branches based on your branch. I don't think that's an issue here.

### comment:19 in reply to:  18 ; follow-up:  20 Changed 6 years ago by mforets

side note: i've used git revert as suggested here trying to avoid messing up history (*). please let me know if this is not the correct way in this context.

You can do that if you care about preserving in the history that a change was made and rolled back. In a case like this I would just go with the branch that does NOT have the changes that were rolled back (rebase, or otherwise rebuild your branch into the state you want directly). Rebasing becomes a real problem if other people have branches based on your branch. I don't think that's an issue here.

ok. may i ask for some extra help here please, like an example? the one in page 19 of the Guide doesn't seem to treat this case. (i did some wrong stuff with git in days84, so i pretend to be more cautious!.)

### comment:20 in reply to:  19 Changed 6 years ago by nbruin

side note: i've used git revert as suggested here trying to avoid messing up history (*). please let me know if this is not the correct way in this context.

You can do that if you care about preserving in the history that a change was made and rolled back. In a case like this I would just go with the branch that does NOT have the changes that were rolled back (rebase, or otherwise rebuild your branch into the state you want directly). Rebasing becomes a real problem if other people have branches based on your branch. I don't think that's an issue here.

ok. may i ask for some extra help here please, like an example? the one in page 19 of the Guide doesn't seem to treat this case. (i did some wrong stuff with git in days84, so i pretend to be more cautious!.)

In this case it would seem to be that a hard reset (as described on the same page) and a forced push to the branch on this ticket would be fine. Anyone who has work based on your branch will be seriously inconvenienced, but I expect there to be no-one in that group here. Note that your actual change is (-2/+7) lines and that with the revert, you'd be archiving the adding and removal of an additional 42 lines.

As is described in many places, as soon as you publish a git branch you should be careful rewriting its history, because you could affect people quite badly. However, in this case, with changes so small and localized, any rebase merge would be really simple (and probably handled automatically), so I'd opt for publishing a cleaner history. People shouldn't really be basing work off branches for tickets that are still developing, unless there's a good reason for it.

### comment:21 Changed 6 years ago by git

Commit: 7a09c12b7f8b1ff03fdf2093d1f57d41bd622974 → 5f1a4da2392e198878f33f3bc8fc118178dd6218

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

 ​5f1a4da add optional args to polygens

### comment:22 Changed 6 years ago by mforets

Authors: → Marcelo Forets

### comment:23 Changed 6 years ago by mforets

@nbruin : thanks for your detailed answer concerning git reset. i've done what you suggested.

All: do you have any further inputs with respect to this ticket? thanks.

### comment:24 Changed 6 years ago by rws

Component: calculus → commutative algebra

### comment:25 Changed 6 years ago by mforets

friendly reminder for ticket review -- seemingly green light from Nils five weeks ago.

thanks

### comment:26 Changed 4 years ago by SimonKing

Reviewers: → Simon King needs_review → positive_review

Although I have never used polygens, I'd say it is an obviously reasonable idea to allow the same optional arguments that are available for the polynomial ring constructor. It doesn't break anything, the patchbot is happy, and it seems to me that 2 years ago people just forgot to set it to positive review. Doing it now. If Nils want to have his name as a reviewer added, please go ahead.

### comment:27 Changed 4 years ago by vbraun

Branch: u/mforets/pass_number_of_variables_to_polygens → 5f1a4da2392e198878f33f3bc8fc118178dd6218 → fixed positive_review → closed
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