Re: Re: More /.{I->-1} craziness

• To: mathgroup at smc.vnet.net
• Subject: [mg106552] Re: [mg106525] Re: More /.{I->-1} craziness
• From: Daniel Lichtblau <danl at wolfram.com>
• Date: Sat, 16 Jan 2010 06:10:50 -0500 (EST)
• References: <hhf5s3\$h4o\$1@smc.vnet.net> <hhhmhl\$o48\$1@smc.vnet.net> <201001150820.DAA29809@smc.vnet.net>

```AES wrote:
> In article <hhhmhl\$o48\$1 at smc.vnet.net>,
>  Valeri Astanoff <astanoff at gmail.com> wrote:
>
>> Imho, when applying a rule  lhs -> rhs
>> it's a risky practice to use the same symbol
>> in 'lhs' and 'rhs', because, very often, there is
>> no easy way to check what has been done.
>>
>> Anyway, for occasional users, you're right : it's crazy!
>>
>
> Thank you -- that's really my primary point.
>
> And I'd add: it's damaging (to users, and to Mathematica).
>
> As for writing rules, I'd not even try writing something compound, like
> a + b or especially something like 1 + I, on the lhs, because I'd have
> no intuition as to how this would work (how spaces would be handled,
> etc.)
>
> But a single character on the lhs?  It works correctly AFAIK for every
> other single-character in the alphabet.  Why shouldn't one expect it to
> work for I?

"[W]ork for me" (let's try to use correct grammar here).

In[1]:= D[Sin[x], x] /. D -> f
Out[1]= Cos[x]

In[3]:= Exp[I*Pi] /. Pi -> E
Out[3]= -1

In[4]:= Exp[I*Pi] /. I -> E
Out[4]= -1

In[5]:= N[Pi] /. N -> P
Out[5]= 3.14159

In[14]:= Cos[x] + O[x]^2 /. O -> P
Out[14]= SeriesData[x, 0, {1}, 0, 2, 1]

Daniel Lichtblau
Wolfram Research

```

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