Difference between var('x') and x=var('x')?

[Jacare Omoplata]

In some tutorials, when a variable is declared, it is done like,

var('x')

In some others, it is done like,

x = var('x')

What is the difference between the two, if any?

Also, is it better to ask this kind of basic question in the Asksage
forum or the sage-support mailing list?

Thanks for your patience.

[Harald Schilly]

the difference is, that var(..) itself already injects a variable with that name into the global namespace, where the other one is an assignment.

i.e.

z = var('y')

gives you a variable y (named "y") and a variable z with the name "y"

sage: z = var('y')

sage: z

y

sage: y

y

you can also create invalid variable names, i.e. the ones starting with a number:

sage: var('2x')

2x

sage: 2x

------------------------------------------------------------

   File "<ipython console>", line 1

     2x

      ^

SyntaxError: invalid syntax

sage: globals()['2x']

2x

sage: type(globals()['2x'])

<type 'sage.symbolic.expression.Expression'>

H

[kcrisman]

Harald explained this :)

> Also, is it better to ask this kind of basic question in the Asksage
> forum or the sage-support mailing list?

Either one is fine.

[Burcin Erocal]

On Fri, 17 Jun 2011 08:10:26 -0700 (PDT)
Harald Schilly <harald....@gmail.com> wrote:

> you can also create invalid variable names, i.e. the ones starting
> with a number:
>
> sage: var('2x')
> 2x
>
> sage: 2x
> ------------------------------------------------------------
> File "<ipython console>", line 1
> 2x
> ^
> SyntaxError: invalid syntax
>
> sage: globals()['2x']
> 2x
>
> sage: type(globals()['2x'])
> <type 'sage.symbolic.expression.Expression'>

Note that this behavior is not going to last. Only valid identifiers
will be allowed in variable names. We have 2 tickets on this:

#7496 symbolic variable names should be valid identifiers
#9724 Sage allows creation of variables with empty name

The second should be closed as a duplicate. :)

Cheers,
Burcin

[William Stein]

On Fri, Jun 17, 2011 at 7:54 AM, Jacare Omoplata
<walkey...@gmail.com> wrote:
> In some tutorials, when a variable is declared, it is done like,
>
> var('x')
>
> In some others, it is done like,
>
> x = var('x')
>
> What is the difference between the two, if any?

If you are writing code to include in .py files, then x=var('x') will
work, but var('x') will totally fail.
For purely interactive use in the notebook or command line, var('x')
is fine. But for writing programs, you should always use

x = var('x')

-- William

>
> Also, is it better to ask this kind of basic question in the Asksage
> forum or the sage-support mailing list?
>
> Thanks for your patience.
>

> --
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--
William Stein
Professor of Mathematics
University of Washington
http://wstein.org

[Volker Braun]

I just fixed it in the patch on #7496:

sage: var('2x')

---------------------------------------------------------------------------

ValueError                                Traceback (most recent call last)

/home/vbraun/opt/sage-4.7.1.alpha2/devel/sage-main/<ipython console> in <module>()

/home/vbraun/Sage/sage/local/lib/python2.6/site-packages/sage/calculus/var.so in sage.calculus.var.var (sage/calculus/var.c:687)()

/home/vbraun/Sage/sage/local/lib/python2.6/site-packages/sage/symbolic/ring.so in sage.symbolic.ring.SymbolicRing.var (sage/symbolic/ring.cpp:6276)()

/home/vbraun/Sage/sage/local/lib/python2.6/site-packages/sage/symbolic/ring.so in sage.symbolic.ring.SymbolicRing.var (sage/symbolic/ring.cpp:6048)()

ValueError: The name "2x" is not a valid Python identifier.

sage: 

[Jacare Omoplata]

Thanks for the replies.