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Re: ReplaceAll question

• To: mathgroup at smc.vnet.net
• Subject: [mg82534] Re: ReplaceAll question
• From: Peter Pein <petsie at dordos.net>
• Date: Wed, 24 Oct 2007 04:22:41 -0400 (EDT)
• References: <ffkf46$sh6$1@smc.vnet.net>

Yaroslav Bulatov schrieb:
> b = 3;
> Hold[a[[b]]] /. {b -> c}
> 
> ReplaceAll has no effect, why? What is the recommended way of carrying
> out the replacement above?
> 
> 

the "b" in the rule is evaluated. Use HoldPattern:

In[1]:=
b = 3;
Hold[a[[b]]] /. {HoldPattern[b] -> c}

Out[2]= Hold[a[[c]]]

Peter