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Re: trivial question

• To: mathgroup at smc.vnet.net
• Subject: [mg69809] Re: [mg69798] trivial question
• From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
• Date: Sat, 23 Sep 2006 23:44:57 -0400 (EDT)

On 23 Sep 2006, at 17:44, dimmechan at yahoo.com wrote:

> Hi.
>
> I think this is rather trivial, but I can think something right now.
>
> Why the commands below result in different output?
>
> (HoldForm[+##1]&)@@list
> 1+2+3
>
> HoldForm[Plus]@@list
> Plus[1,2,3]
>
> Thanks in advance.
>


If you look at your two outputs in InputForm the reason should be  
pretty clear. In the first case you have


HoldForm[1 + 2 + 3]

This has FullForm HoldForm[Plus[1,2,3]]. Now when you convert the  
first expression to StandardForm (or TraditionalForm) the head  
HoldForm is hidden the the expression inside HoldForm is displayed in  
the usual way as 1+2+3 instead Plus[1,2,3], because in StandardForm  
or in TraditionalForm you always get:

In[12]:=
Plus[a,b,c]

Out[12]=
a+b+c


In the second case you get HoldForm[Plus][1, 2, 3] (which is the same  
as its FullForm). In StandardForm (or TraditionalForm) again HoldForm  
is hidden, but this time the expression Plus inside HoldForm  
obviously cannot be displayed in any other way, since Plus alone is  
not never displayed as +, e.g.

In[13]:=
Plus//TraditionalForm

Out[13]//TraditionalForm=
Plus


Andrzej Kozlowski