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Re: Problem with SetDelayed lhs:=rhs

  • To: mathgroup at smc.vnet.net
  • Subject: [mg128684] Re: Problem with SetDelayed lhs:=rhs
  • From: Roland Franzius <roland.franzius at uos.de>
  • Date: Sat, 17 Nov 2012 03:47:55 -0500 (EST)
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  • References: <k466r8$a8g$1@smc.vnet.net>

Am 29.09.2012 09:09, schrieb zahava.ginny at gmail.com:
> An assignment using SetDelayed, lhs := rhs, maintains rhs in completely unevaluated form.
> Thus, if k=5, say, the assignment
>
> In[1] = f[k,j_]:=rhs[k,j]
>
> yields
>
> In[2] = ?f
> Global`f
> f[5,j_]:=rhs[k,j]
>
> How can I force the k on the right hand side to be evaluated so that ?f will yield
> f[5,j_]:=rhs[5,j] ?
>
> Of course, I could define f[5,j_] manually, but I want to automate the procedure over a range of values of k.
>

You can always override Hold-attibutes by Evaluate

f[5,j_]:=Evaluate[rhs[k,j]]

or simply using Set

f[5,j_]=rhs[k,j]

if rhs contains no variables with different values at definition and run 
time.

In a most elegant way - a bit obscure for beginners - automated 
definition procedures prepare Lists with the two arguments of SetDelayed 
and apply SetDelayed to the inner List of pairs


SetDelayed@@@(  ({f[#,j_],rhs[#,j]}&) /@ Range[20] )

or

(  (sd[f[#,j_],rhs[#,j]]&) /@ Range[20] )/. {sd -> SetDelayed}

-- 

Roland Franzius



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