Re: Just another Mathematica "Gotcha"
- To: mathgroup at smc.vnet.net
- Subject: [mg120844] Re: Just another Mathematica "Gotcha"
- From: Murray Eisenberg <murray at math.umass.edu>
- Date: Fri, 12 Aug 2011 05:07:05 -0400 (EDT)
- Delivered-to: l-mathgroup@mail-archive0.wolfram.com
- References: <201108091119.HAA15770@smc.vnet.net> <201108110910.FAA07160@smc.vnet.net>
- Reply-to: Murray Eisenberg <murray at math.umass.edu>
Yes, in actuality I often do that, too -- more often than looking up the
precedence in the docs. Don't know why I suggested the latter and not
the former. (Perhaps to hammer home my point.)
On 8/11/11 5:10 AM, DrMajorBob wrote:
> No reading or experimentation is necessary.
>
> When I wonder what operator has higher precedence -- as I often do -- I
> double-click on each operator in turn, and automatic selection expansion
> tells me what I need to know.
>
> Bobby
>
> On Wed, 10 Aug 2011 05:46:13 -0500, Murray Eisenberg
> <murray at math.umass.edu> wrote:
>
>> There you go again, insinuating that something is wrong with Mathematica
>> when it's just something you don't understand (or simply just don't
>> like). And ratcheting up the rhetoric with language such as "by any
>> normal rules of interpretation or ordinary interpretations".
>>
>> If you really do want to understand what's going on here, you could take
>> a moment to experiment or read the documentation and treat it as a
>> "teachable moment".
>>
>> You could try something simpler, e.g.:
>>
>> expr = a + b x;
>> expr // f /. b -> 0
>> expr /. b -> 0 // f
>>
>> Or look at the FullForms of the latter two expressions (after wrapping
>> each in Hold).
>>
>> Or try forcing the order of precedence with the first expression:
>>
>> (Series[a + (b1 + b2) x, {x, 0, 1}] // Normal) /. {b2 -> 0}
>>
>> Or search the Documentation Center for "order of precedence", say, and
>> in the first hit peruse the table documenting order of precedence in
>> tutorial/OperatorInputForms.
>>
>> Yes, there are dangers in deviating from straightforward head[[expr]]
>> syntax. You can either stick with that or else learn what you need to
>> know to avoid, or at least deal with, any surprises.
>>
>> On 8/9/11 7:19 AM, AES wrote:
>>> Seems as if the following two expression should yield the same output
>>> -- seems that way to me anyway -- but they don't. I'll hide the
>>> actual outputs down below so Mathematica gurus (or "ordinary users")
>>> can make their predictions as to which one does what.
>>>
>>> In[1]:= Series[a+(b1+b2)x,{x,0,1}] //Normal /.{b2->0}
>>>
>>> In[2]:= Series[a+(b1+b2)x,{x,0,1}] /.{b2->0} //Normal
>>>
>>> My conclusions:
>>>
>>> 1) By any normal rules of interpretation or ordinary interpretations
>>> of these statements, they both should do the same same thing.
>>>
>>> 2) This is just another Mathematica "Gotcha" -- and not a
>>> particularly forgivable one....
>>
>
>
--
Murray Eisenberg murray at math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower phone 413 549-1020 (H)
University of Massachusetts 413 545-2859 (W)
710 North Pleasant Street fax 413 545-1801
Amherst, MA 01003-9305
- References:
- Just another Mathematica "Gotcha"
- From: AES <siegman@stanford.edu>
- Re: Just another Mathematica "Gotcha"
- From: DrMajorBob <btreat1@austin.rr.com>
- Just another Mathematica "Gotcha"