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Re: few easy questions (HoldPattern)


  • To: mathgroup@smc.vnet.net
  • Subject: [mg11527] Re: few easy questions (HoldPattern)
  • From: Paul Abbott <paul@physics.uwa.edu.au>
  • Date: Fri, 13 Mar 1998 12:22:16 -0500
  • Organization: University of Western Australia
  • References: <6e85du$ngs@smc.vnet.net>

Arturas Acus wrote:

> Why this two substitutions act differently 
>
>In[9]:= {Dt[a], Sin[a]} //. {Dt[x_] :> 1, Sin[x_] :> 1} 
>
>Out[9]= {Dt[a], 1}

Because Dt[x_] is evaluated _before_ the pattern-matching occurs.  You
can get around this using HoldPattern:

In[10]:= {Dt[a], Sin[a]} //. {HoldPattern[Dt[x_]]:> 1, Sin[x_] :> 1} 
Out[10]= {1,1}

Cheers,
	Paul 

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Paul Abbott                                   Phone: +61-8-9380-2734
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            God IS a weakly left-handed dice player
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