Re: Help: Why no output?

*To*: mathgroup at smc.vnet.net*Subject*: [mg34960] Re: [mg34941] Help: Why no output?*From*: Tomas Garza <tgarza01 at prodigy.net.mx>*Date*: Sat, 15 Jun 2002 02:27:50 -0400 (EDT)*References*: <200206140638.CAA04787@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

I think you'll be able to understand what's going on if you use Trace. In the first case, since no n is given as input, the If expression reduces to If[Length[{}]==1, Length[0.5]], and this in turn will eventually lead to a Null statement. In the second case, the If expression remains as If[Length[{}]==1, Times[1], Length[0.5], which eventually leads to 0, since Length[expr] returns 0 whenever AtomQ[expr] is True (see the Help browser). Tomas Garza Mexico City ----- Original Message ----- From: "Kezhao Zhang" <kzhang at flashmail.com> To: mathgroup at smc.vnet.net Subject: [mg34960] [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 >

**References**:**Help: Why no output?***From:*kzhang@flashmail.com (Kezhao Zhang)