       Re: Just another Mathematica "Gotcha"

• To: mathgroup at smc.vnet.net
• Subject: [mg120806] Re: Just another Mathematica "Gotcha"
• From: Heike Gramberg <heike.gramberg at gmail.com>
• Date: Wed, 10 Aug 2011 06:49:35 -0400 (EDT)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• References: <201108091119.HAA15770@smc.vnet.net>

```Not a gotcha IMO, just a case of /. taking precedence over //, so the first example is interpreted as

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

and the second one as

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

You can check this by looking at TracePrint:

First case:

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

out:

(Normal/. b2->0)[Series[a+(b1+b2) x,{x,0,1}]]
Normal/. b2->0
ReplaceAll
Normal
b2->0
Rule
b2
0
b2->0
=85...

Second case

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

out:

Normal[Series[a+(b1+b2) x,{x,0,1}]/. {b2->0}]
Normal
Series[a+(b1+b2) x,{x,0,1}]/. {b2->0}
ReplaceAll
Series[a+(b1+b2) x,{x,0,1}]
Series
a+(b1+b2) x
Plus
=85.

In both cases, the ReplaceAll is evaluated first

Heike.

On 9 Aug 2011, at 13:19, 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:= 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.
>
>
>
>
>
>
>
>
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>
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>
> Out= a+(b1+b2) x
>
> Out= a+b1 x
>

```

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