RE: Help: Why no output?

*To*: mathgroup at smc.vnet.net*Subject*: [mg34955] RE: [mg34941] Help: Why no output?*From*: "DrBob" <majort at cox-internet.com>*Date*: Sat, 15 Jun 2002 02:27:40 -0400 (EDT)*Reply-to*: <drbob at bigfoot.com>*Sender*: owner-wri-mathgroup at wolfram.com

Kezhao, When you give no second argument to f, n is Sequence[]... a list of arguments to place in some function, but a list of zero length! Hence in that case If[Length[{n}] == 1, n, Length[x]] is just: If[Length[{}] == 1, Length[x]] (NO arguments are inserted at the position of n.) Now, also 1*Sequence[] resolves to Times[1, Sequence[]] and hence to Times[1] and hence to 1 Therefore, when n=Sequence[], the following If[Length[{n}] == 1, 1*n, Length[x]] resolves to If[Length[{}] == 1, 1, Length[x]] which is just Length[x] Bobby Treat -----Original Message----- From: Kezhao Zhang [mailto:kzhang at flashmail.com] To: mathgroup at smc.vnet.net Subject: [mg34955] [mg34941] Help: Why no output? 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