Re: Help: Why no output?

*To*: mathgroup at smc.vnet.net*Subject*: [mg34952] Re: [mg34941] Help: Why no output?*From*: Andrzej Kozlowski <andrzej at bekkoame.ne.jp>*Date*: Sat, 15 Jun 2002 02:27:35 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

A correction to my earlier message. The first lines should have been: g[x___]:=h[a,x,b] In[2]:= g[] Out[2]= h[a,b] In[3]:= Clear[g] In[4]:= g[x___]:=h[a,1*x,b] In[5]:= g[] Out[5]= h[a,1,b] and the rest as I originally wrote. On Friday, June 14, 2002, at 06:09 PM, Andrzej Kozlowski wrote: > The explanation lies in the following behaviour : > > In[1]:= > g[x___]:=h[a,x,b] > > In[2]:= > g[] > > Out[2]= > h[a,b] > > In[3]:= > g[1*x] > > Out[3]= > h[a,x,b] > > This behaviour is analogous to what you would get if you inserted used > Sequence[] as your second argument: > > In[4]:= > h[a,Sequence[],b] > > Out[4]= > h[a,b] > > In[5]:= > h[a,x*Sequence[],b] > > Out[5]= > h[a,x,b] > > However, it does not seem that Sequence is actually used, since > > In[6]:= > SetAttributes[h,SequenceHold] > > In[7]:= > g[] > > Out[7]= > h[a,b] > > In[8]:= > h[a,Sequence[],b] > > Out[8]= > h[a,Sequence[],b] > > In your case what happens is this. Let's just consider f[{1,2}]. As > there is no second argument the function If turns into > If[Length[{1,2}==1,Length[{1,2}]] (since there was no n, it vanished, > just as if you had Sequence[] as the second argument). But this > evaluates to If[False,Length[{1,2}]]. Since nothing is specified to be > returned from If in case of False, nothing is returned. However, in the > other case you get If[Length[{1,2}]==1,1,Length[{1,2}]] , which of > course evaluates to If[False,1,Length[{1,2}]] which returns > Length[{1,2}] which is 2. > > Andrzej Kozlowski Toyama International University JAPAN http://platon.c.u-tokyo.ac.jp/andrzej/ > > On Friday, June 14, 2002, at 03:38 PM, Kezhao Zhang wrote: > >> The behavior of the following function is puzzling to me: >> >> In[1]:=f[x_, n___Integer] := If[Length[{n}] == 1, n , Length[x]] >> In[2]:=f[0.5] (* Nothing returned. *) >> In[3]:=f[{1,2}] (* Nothing returned. The length of x should be >> returned *) >> (* However, with the modification to f, everything works as intended >> *) >> In[4]:=f[x_, n___Integer] := If[Length[{n}] == 1, 1*n , Length[x]] >> ^^^^^^ >> In[5]:=f[0.5] >> Out[5]:=0 >> In[6]:=f[{1,2}] >> Out[6]:=2 >> >> Could anyone help me understand why changing n to (1*n) in the If[] >> statement makes such difference? Why doesn't f[] defined in the In[1] >> work? >> >> Thanks for your help. >> >> Kezhao Zhang >> >> >> >