Re: Re: Replace and ReplaceAll -- simple application
- To: mathgroup at smc.vnet.net
- Subject: [mg106016] Re: [mg105982] Re: Replace and ReplaceAll -- simple application
- From: DrMajorBob <btreat1 at austin.rr.com>
- Date: Wed, 30 Dec 2009 04:15:13 -0500 (EST)
- References: <200912270006.TAA12080@smc.vnet.net> <hh72dp$kud$1@smc.vnet.net>
- Reply-to: drmajorbob at yahoo.com
I fully agree.
We should be told that -I is an atom, hence it does NOT contain I as a
part.
a - I is NOT an atom, since we get a match in
Cases[a - I, -I, {0, Infinity}]
{-I}
AtomQ[a - I]
False
yet 1 - I _is_ an atom, so
Cases[1 - I, -I, {0, Infinity}]
{}
AtomQ[1 - I]
True
I don't know that this ever hinders me in practice, but the rationale
escapes me.
Bobby
On Tue, 29 Dec 2009 00:18:40 -0600, AES <siegman at stanford.edu> wrote:
> In article <hh9vfo$1rk$1 at smc.vnet.net>,
> Richard Fateman <fateman at cs.berkeley.edu> wrote:
>
>> Scot T. Martin wrote:
>> > The problem is that "-1." is not "1".
>
>> And furthermore, this bug has been declared a feature. And defended,
>> repeatedly. Just as I does not occur in -I.
>> It can be fixed, should be fixed, and has been fixed in other CAS.
>
>
> I'll defer to Richard Fateman (and/or other computer algebra experts) as
> to whether this general behavior (!! especially with respect to -I !!)
> is a bug that should be fixed, or a feature.
>
> But if it's gonna be considered a feature in Mathematica, I'd strongly
> suggest that it should be strongly **featured** or, if you like, it
> should be warned about!! in the Mathematica documentation, especially
> at the more elementary levels of documentation, **so that ordinary users
> can't miss being warned about this unexpected behavior** ^HH^H^H^H
> sorry, so that they can't miss being warned about "this feature".
>
> For example, right after the first line of the Help for I, which says
>
> I represents the imaginary unit Sqrt[-1].
>
> there might be a warning line which says
>
> WARNING: I is a number and not a symbol. Attempting to replace
> or reverse the sign of I using a rule, such as expr/.{I->-I) may
> produce unexpected results if the expr contains -I.
>
> (By the way, note the wording above: "I represents", not "I is".)
>
> There are a very large number of other places in the Mathematica
> documentation where emphasized WARNINGs like this would be helpful.
>
> (I suppose there could even be optional warning messages that appear any
> time a cell executes a rule containing a number on the LHS of a
> ReplaceAll symbol. And, the documentation for ReplaceAll could have an
> initial WARNING message that it is not just the kind of Global Search
> and Replace command that many ordinary users, familiar with many other
> word processing and software apps, might well think it is. )
>
--
DrMajorBob at yahoo.com
- References:
- Replace and ReplaceAll -- simple application
- From: amannuc1 <Anthony.J.Mannucci@jpl.nasa.gov>
- Replace and ReplaceAll -- simple application