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MathGroup Archive 2002

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Re: memoizing function again

  • To: mathgroup at smc.vnet.net
  • Subject: [mg32501] Re: [mg32495] memoizing function again
  • From: BobHanlon at aol.com
  • Date: Thu, 24 Jan 2002 05:20:54 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

In a message dated 1/23/02 5:22:08 AM, erich.neuwirth at univie.ac.at writes:

>i have the following function
>
>f[x_] /; x <= 2 := f[x] = 1
>f[x_] /; x > 2 := f[x] = f[x - 1] + f[x - 2]
>
>
>it remembers what it already calculated
>i want to be able to throw away rules with values
>between calculations
>
>In[3]=DownValues[f]
>produces
>
>Out[3]={HoldPattern[f[x_]/;x\[LessEqual]2]\[RuleDelayed](f[x]=1),
>  HoldPattern[f[x_]/;x>2]\[RuleDelayed](f[x]=f[x-1]+f[x-2])}
>
>
>after f[4]
>
>we have 
>
>In[5]:=
>DownValues[f]
>
>Out[5]=
>{HoldPattern[f[1]]\[RuleDelayed]1,HoldPattern[f[2]]\[RuleDelayed]1,
>  HoldPattern[f[3]]\[RuleDelayed]2,HoldPattern[f[4]]\[RuleDelayed]3,
>  HoldPattern[f[x_]/;x\[LessEqual]2]\[RuleDelayed](f[x]=1),
>  HoldPattern[f[x_]/;x>2]\[RuleDelayed](f[x]=f[x-1]+f[x-2])}
>
>
>\[RuleDelayed] is ascii for :>, i think
>
>so
>
>DownValues[f] = Take[DownValues[f], -2]
>
>removes all the rules giving calculated values
>but i would like to throw away the rules following the pattern
>
>HoldPattern[f[x_Integer]:>y_Integer
>
>but i have not been able wo write an expression using Cases and a
>pattern
>which gets rid of the rules i want to get rid of
>

DownValues[f] = Select[DownValues[f], !FreeQ[#, x]&]

or

DownValues[f] = Cases[DownValues[f], _?(!FreeQ[#, x]&)]

or

DownValues[f] = DeleteCases[DownValues[f], _?(FreeQ[#, x]&)]


Bob Hanlon
Chantilly, VA  USA


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