Services & Resources / Wolfram Forums
-----
 /
MathGroup Archive
2006
*January
*February
*March
*April
*May
*June
*July
*August
*September
*October
*November
*December
*Archive Index
*Ask about this page
*Print this page
*Give us feedback
*Sign up for the Wolfram Insider

MathGroup Archive 2006

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: DownValues and Cases

  • To: mathgroup at smc.vnet.net
  • Subject: [mg70789] Re: DownValues and Cases
  • From: dh <dh at metrohm.ch>
  • Date: Fri, 27 Oct 2006 00:28:47 -0400 (EDT)
  • References: <ehpm23$kid$1@smc.vnet.net>

Hi Bruce,
I do not know the exact answer, I can only guess.
It looks like nested HoldPattern are stripped, they have the same effect 
as a single HoldPattern. This prevents the use of "HoldPattern" as a 
pattern. What we can nevertheless do to get {x,y }is a 2 step replacement:
Cases[h, (x_ :> y_) -> {x, y}] /. _[_[x_]] -> x

Daniel

Bruce Colletti wrote:
> Re Mathematica 5.2 under WinXP.
> 
> The results of the first two Cases statements leads me to anticipate {{1,3.5}, {2,5}, {_,0}} as the output of the third.  
> 
> The output of the fourth Cases statement confirms this expectation.
> 
> Unfortunately, the output of the third statement is the empty list.  
> 
> Why so?  I've reviewed a related mid-July MathGroup thread, but don't see the answer (if it's there at all). 
> 
> Thankx.
> 
> Bruce
> 
> --------------------------
> 
> f at 1=3.5;
> f@2=5;
> f[_]:=0;
> h=DownValues@f
> 
> Out[4]={HoldPattern[f[1]] :> 3.5, HoldPattern[f[2]] :> 5, HoldPattern[f[_]] :> 0}
> 
> Cases[h,HoldPattern[f[x_]] -> x,Infinity]
> Out[5]={1,2,_}
> 
> Cases[h,(_ :> y_) -> y,Infinity]
> Out[6]={3.5,5,0}
> 
> Cases[h,(HoldPattern[f[x_]] :> y_) -> {x,y},Infinity]
> Out[7]={}
> 
> Cases[{a :> 4,b :> 5, c :> 6},(x_ :> y_) -> {x,y}]
> Out[8]={{a,4},{b,5},{c,6}}
> 


  • Prev by Date: Re: Defining two functions at once?
  • Next by Date: Mathlink and Builder C++
  • Previous by thread: Re: DownValues and Cases
  • Next by thread: Re: Re: DownValues and Cases