Re: Just another Mathematica "Gotcha"
- To: mathgroup at smc.vnet.net
- Subject: [mg120802] Re: Just another Mathematica "Gotcha"
- From: Bill Rowe <readnews at sbcglobal.net>
- Date: Wed, 10 Aug 2011 06:48:51 -0400 (EDT)
- Delivered-to: l-mathgroup@mail-archive0.wolfram.com
On 8/9/11 at 7:19 AM, siegman at stanford.edu (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.
If and only if the // and /. operators had the same precedence
this would be true. But since /. has higher precedence than //
Series[a+(b1+b2)x,{x,0,1}] //Normal /.{b2->0}
is equivalent to
Series[a+(b1+b2)x,{x,0,1}] //(Normal /.{b2->0})
rather thab
(Series[a+(b1+b2)x,{x,0,1}])//Normal /.{b2->0}
The problem here is exactly analogous to why
a + b * c
doesn't yield (a+b)*c
>2) This is just another Mathematica "Gotcha" -- and not a
>particularly forgivable one.
No, this is simply a failure on your part to understand operator precedence.