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RE: Prefix[f[x]], why is it not working

  • To: mathgroup at smc.vnet.net
  • Subject: [mg38919] RE: [mg38909] Prefix[f[x]], why is it not working
  • From: "David Park" <djmp at earthlink.net>
  • Date: Sun, 19 Jan 2003 00:33:06 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

Ashraf,

I think it is because Prefix is a formatting function that affects how the
expression is displayed. But the prefix header is still there.

Prefix[s[a_]]
% // FullForm
s@a_
Prefix[s[Pattern[a, Blank[]]]]

So when you try to make an assignment with it, Mathematica objects because
Prefix is protected.

On the other hand...

s@a_
% // FullForm
s[a_]
s[Pattern[a, Blank[]]]

and you can make an assignment because s is not protected.

I have to admit that this is a little tricky and confused me at first.

David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/



From: Ashraf El Ansary [mailto:Elansary at btopenworld.com]
To: mathgroup at smc.vnet.net


Hi all,
Does anyone why f@a_=2 a gives a proper answer for f[1]
and not in the case of Prefix[s[a_]]= 2 a eventhough that Prefix[s[a_]]=
f@a_???

Thanks
In[1]:=
f@a_=2 a
f[1]
Out[1]=
2 a
Out[2]=
2
In[3]:=
Prefix[s[a_]]
Prefix[s[a_]]=2 a
s[1]
Out[3]=
s@a_
Set::write: Tag Prefix in s@a_ is Protected.
Out[4]=
2 a
Out[5]=
s[1]





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