Re: Problem overriding simple built-in functions

*To*: mathgroup at smc.vnet.net*Subject*: [mg28834] Re: Problem overriding simple built-in functions*From*: Jean-Christophe Deschamps <jchd at worldnet.fr>*Date*: Tue, 15 May 2001 00:59:05 -0400 (EDT)*References*: <9dnr16$stj@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Thanks to those who took the pain to answer, pointing out that due the Listable attribute of GCD, the behavior was not surprising. I must admit my sloppiness in somehow making up the example in my previous post, living out many details like that. The corrected example below is more explicit about my question. I still must be missing something obvious, but I fail to see what. In[1]:= Unprotect[GCD] Out[1]= {"GCD"} In[2]:= ClearAttributes[GCD, Listable] In[3]:= GCD[v_?VectorQ] := Apply[GCD, v] In[4]:= ?? GCD "GCD[n1, n2, ... ] gives the greatest common divisor of the integers ni." \!\(\* InterpretationBox[GridBox[{ {\(Attributes[GCD] = {Flat, Orderless}\)}, {" "}, {GridBox[{ {\(GCD[v_?VectorQ] := GCD @@ v\)} }, GridBaseline->{Baseline, {1, 1}}, ColumnWidths->0.999, ColumnAlignments->{Left}]} }, GridBaseline->{Baseline, {1, 1}}, ColumnAlignments->{Left}], Definition[ GCD], Editable->False]\) In[5]:= v = {2, 4}; VectorQ[v] Out[6]= True In[7]:= GCD[v] $RecursionLimit::"reclim": "Recursion depth of \!\(256\) exceeded." Out[7]= GCD[{2, 4}] In[8]:= Apply[GCD, v] $RecursionLimit::"reclim": "Recursion depth of \!\(256\) exceeded." $RecursionLimit::"reclim": "Recursion depth of \!\(256\) exceeded." $RecursionLimit::"reclim": "Recursion depth of \!\(256\) exceeded." General::"stop": "Further output of \!\($RecursionLimit :: \"reclim\"\) will \ be suppressed during this calculation." Out[8]= 2 The answer finally pops up, but why the recursion? [Trace-ing of this has been left off here to conserve space.] LCM behaves as GCD in Mathematica 4. |> Consequently, define your own function in these circumstances No problem in this case: the scope of this code is very strictly limited and not any kind of a general setup. I was just wondering which new internal mecanism has been thrown in, that causes my own previous code to break pityfully. I believe it's worth understanding precisely why, in such a simple case, so as to benefit of the introduced goodies in possibly more subtle contexts where it might prove useful. JcD