       Re: Just another Mathematica "Gotcha"

• To: mathgroup at smc.vnet.net
• Subject: [mg120786] Re: Just another Mathematica "Gotcha"
• From: Roger Wilson <rogerhw999 at gmail.com>
• Date: Thu, 11 Aug 2011 03:27:09 -0400 (EDT)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• References: <j1r5d3\$fi4\$1@smc.vnet.net>

```On Aug 9, 12:24 pm, AES <sieg... at stanford.edu> 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:= Series[a+(b1+b2)x,{x,0,1}] //Normal /.{b2->0}
>
> In:= 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.
>
> Out= a+(b1+b2) x
>
> Out= a+b1 x

Its a case of operator priority.  /. has a higher priority than // so,
in the first case, you're doing Normal /. {b2->0} first which
evaluates to Normal.

(Series[a+(b1+b2)x,{x,0,1}]//Normal)/.b2->0   gives   a+b1 x

```

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