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MathGroup Archive 2004

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Re: unevaluated, hold, holdform

  • To: mathgroup at smc.vnet.net
  • Subject: [mg50939] Re: unevaluated, hold, holdform
  • From: Paul Abbott <paul at physics.uwa.edu.au>
  • Date: Tue, 28 Sep 2004 00:59:14 -0400 (EDT)
  • Organization: The University of Western Australia
  • References: <vas7uiw8wg82@legacy> <cj86l4$78b$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

In article <cj86l4$78b$1 at smc.vnet.net>, highegg at seznam.cz (highegg) 
wrote:

> if you want to prevent an argument from evaluation,
> Wrap it with the _Unevaluated_ function:
> Numerator[Unevaluated[(4*x^2)/(2*x)]]
> 
> but be careful, if you use this as a function:
> f[x_]:=Numerator[Unevaluated[x]],
> 
> f[(4*x^2)/(2*x)] won't work, the argument is pre-evaluated again!
> 
> the best way to sort this out is to assign an argumant to the function
> f:
> AppendTo[Attributes[f],HoldAll]
> 
> now f[(4*x^2)/(2*x)] will give us what we want!
> (but note that f[x_]:=Numerator[x] still won't work!)

Another way to do this is to use Function with the attribute HoldFirst:

 f = Function[x, Numerator[Unevaluated[x]], HoldFirst]; 

Cheers,
Paul

-- 
Paul Abbott                                   Phone: +61 8 6488 2734
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