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}} >