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MathGroup Archive 2002

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Re: Help: Why no output?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg34947] Re: [mg34941] Help: Why no output?
  • From: Andrzej Kozlowski <andrzej at platon.c.u-tokyo.ac.jp>
  • Date: Sat, 15 Jun 2002 02:27:29 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

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
>
>
>



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