#12834: Modify subs so that it can accept multiple equations just like subs_expr
-------------------------------------+-------------------------------------
Reporter: JoalHeagney | Owner: AlexGhitza
Type: enhancement | Status: needs_info
Priority: minor | Milestone: sage-6.4
Component: algebra | Resolution:
Keywords: subs algebra | Merged in:
solving | Reviewers:
Authors: Michael Orlitzky | Work issues:
Report Upstream: N/A | Commit:
Branch: public/12384 | 045eebfd67858405c6665acfa1eea5f22ca7fc6f
Dependencies: | Stopgaps:
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Comment (by mjo):
@Vincent: Most of your changes look fine to me, thanks for picking this up
again. I have a few comments:
1. When making multiple duplicate substitutions, all of the errors are no
longer reported. This used to work like,
{{{
sage: d1 = {'a': 1, 'b': 2}
sage: d2 = {'b': 1, 'a': 2}
sage: _subs_safe_merge(d1, d2)
ValueError: Duplicate substitutions given: a, b
}}}
showing that both `a` and `b` had duplicate substitituons. Now it only
shows the error for `a`:
{{{
ValueError: duplicate substitutions a->1 and a->2
}}}
The test "We should report all such conflicts..." was intended to show
that =)
2. I like the fact that you show the whole substitution in the error, e.g.
`ValueError: duplicate substitutions a->b and a->c`, but I think the arrow
notation is confusing. I can't tell at a glance whether `a` or `b` is the
variable. In some programming languages, the arrow is used to mean "take
the value of `a` and put that into `b`." In other contexts, you could
think of it as "take `a` and make it become `b`", which is actually the
reverse substitution.[[BR]][[BR]]Above, you can deduce which is which (if
you think for a minute), but you can also wind up with `ValueError:
duplicate substitutions x->y and x->y` where it's totally
ambiguous.[[BR]][[BR]]Is there some other notation that people generally
agree on? Maybe the colon-equals, which defines the thing on the left to
be equal to the thing on the right? For example, `x := 3` would mean that
`x` is being defined to be `3`; i.e. `3` is substituted for `x`. It's a
slight abuse of notation, but we can think of `:=` as "make the thing on
the left equal the thing on the right from now on." That's basically what
a substitution is.
3. My "And finally, a list containing one of everything" test was meant to
show that we can handle all three types (symbolic equality, a dict, and
another list) at the same time. You noticed that I forgot tuples
(thanks!), but can we do them all at the same time? For example,[[BR]]
{{{
sage: w, x, y, z = SR.var('w, x, y, z')
sage: _subs_make_dict([w == 1, {x: 1}, [y == 1], (z == 1,)])
{w: 1, y: 1, x: 1, z: 1}
}}}
4. I agree that we can deprecate the other substitution methods
`subs_expr` and `substitute_expression`.
That's it, thanks again for taking the time to work on this.
--
Ticket URL: <http://trac.sagemath.org/ticket/12834#comment:16>
Sage <http://www.sagemath.org>
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