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Is there any difference between the two different ways of declaring variables?

asked 2020-10-21 21:11:30 -0600

cybervigilante gravatar image

I sometimes see variables declared as y,z,t = var("y z t") and others simply as var("y z t") Both seem to work. Is there any actual difference between them?

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answered 2020-10-22 14:59:23 -0600

nbruin gravatar image

The top-level var has this "binding injection behaviour" to avoid having to write the print name of the symbolic variable "x" and the python symbol you bind it to separately, since generally you'd want them to agree. The non-toplevel SR.var does not have that behaviour, so SR.var("x,y,z") returns the right symbols, but doesn't bind them. You have to assign them, giving you the option of naming them differently, like

symbol_x, symbol_y = SR.var("x,y")

(with the top-level var this would result in symbol_x as well as x being bound to the same symbol x). Explicit assignment can sometimes be useful to avoid name clashes.

You can invoke darker magic to get the result of the top-level var. For instance, once you've done var('symbols') you can use

_(x,y,z)=symbols

which works because of sage's preparser:

sage: preparse("_(x,y,z)=symbols")
'__tmp__=var("x,y,z"); _ = symbolic_expression(symbols).function(x,y,z)'

so it calls var for you.

Alternatively, with the following definition:

class symbols:
    def __init__(self, names):
        self._symbols = map(SR.symbol, names)
    def _first_ngens(self,n):
        return self._symbols[:n]

you'd be able to leverage another preparser feature to write

_.<x,y,z>=symbols()

because it preparses as:

sage: preparse("_.<x,y,z>=symbols()")
"_ = symbols(names=('x', 'y', 'z',)); (x, y, z,) = _._first_ngens(3)"
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answered 2020-10-22 09:31:23 -0600

Emmanuel Charpentier gravatar image

updated 2020-10-22 09:39:54 -0600

var("x,y,z") :

  • creates the Sage objects representing the symbolic variables called $x$, $y$ an,d $z$,

  • attachs them to the Python variables x, y and z, and

  • returns them (as a tuple).

The difference is that x, y, z = var("x, y, z") re-affects the elements of the tuple to the Python variables x, y and z, which is pretty unuseful but forbids the Python interpreter to print them.

The homonymy between symbolic variable names and the names pf the Python variables pointing to them is often quite useful. A great way to confuse oneself (read : to shoot oneself in the foot) is to break it ; for example, it would be legal to write y, z, x=var("x, y, z") ; this can happen in successive "improvements" to a portion of code (don't ask me how I know it... ;-).

A potentially more useful use would be to affect the tuple to another Python variable representing them collectively. See my answer to this question for an (useful) example.

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Asked: 2020-10-21 21:11:30 -0600

Seen: 85 times

Last updated: Oct 22