Re: Mathematica language issues

• To: mathgroup at smc.vnet.net
• Subject: [mg52997] Re: Mathematica language issues
• From: David Bailey <dave at Remove_Thisdbailey.co.uk>
• Date: Sun, 19 Dec 2004 06:14:42 -0500 (EST)
• References: <200412171020.FAA16185@smc.vnet.net> <cq0tm1\$2m2\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Andrzej Kozlowski wrote:
> On 17 Dec 2004, at 19:20, Maxim wrote:
>
>
>>In[5]:=
>>Unevaluated[1 + 1]*2
>>2*Unevaluated[1 + 1]
>>
>>Out[5]=
>>4
>>
>>Out[6]=
>>2*Unevaluated[1 + 1]
>>
>
>
> This is not a glitch but works exactly as one woudl expect.  You can
> see the difference and the reason by looking at Trace in both cases
> (although there is no need for that, if you understand Unevaluated you
> can see it right away).
>
> First:
> 2*Unevaluated[1+1]//Trace
>
>
> {2 (1+1),2 Unevaluated[1+1]}
>
>
> First Unevaluated is stipped away and Mathematica attempts ot evaluate
> 2*(1+1). Since it knows no rule to apply and the expression has not
> changed Unevaluated is restored and evaluation is completed with the
> output you see.
>
>
>
> Unevaluated[1+1]*2//Trace
>
> {(1+1) 2,2 (1+1),{1+1,2},2 2,4}
>
> As before, first Unevaluated is stripped away and Mathematica tires to
> evaluate 2*(1+1). It now knows a rule to apply, which is given by the
> Orderless attribute and the canonical ordering, so it converts the
> expression into the form 2 (1+1). But now Unevaluated is not restored
> because the expression has changed so evaluation continues with 1+1
> evaluationg to 2 and finally you obtain 4.
>
> Now, I have honstly considered this case only because I could see at
> once what what was going on. I do not knwo if any of the others are
> glitches but jusdging by my experience with the past "language
> glitches" you have reported (unlike the more serious problems desribed
> in your last posting) I rather doubt it. However I have no time to
> spend on this just to prove a point (again).
>
>
>
> Andrzej Kozlowski
> Chiba, Japan
> http://www.akikoz.net/~andrzej/
> http://www.mimuw.edu.pl/~akoz/
>
To be fair to Maxim,

You have to read between the lines of the documentation a little to
appreciate that Unevaluated should work this way - perhaps this could be
considered a 'documentation glitch'!

David Bailey
dbaileyconsultancy.co.uk

```

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