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MathGroup Archive 1993

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RE: M.Trott

  • To: MathGroup at yoda.physics.unc.edu
  • Subject: RE: M.Trott
  • From: reiszig at e-technik.tu-dresden.dbp.de
  • Date: Tue, 6 Apr 1993 13:47:20 +0200

Hi,
Michael Trott asked why
   (1)     {a,b}/.{x___,a,y___}:> Print[z;x]
prints "z" but
   (2)     {a,b}/.{x___,a,y___}:> Print[z;xx]
prints "xx".
That is because while applying the 1st rule, x is assigned 
the actual value of x___, namely a sequence of length 0 (I 
guess:  Sequence[]).
 
CompoundExpression[z, Sequence[]] seems to become always z, so Print[z;x] in
(1) becomes Print[z]:

 LogBook V. 1.2 opened (OpenWrite) for In and Out  1993-04-05  18:45:32

 Mma-Version:      386 2.1 (July 21, 1992)
 Operating System: MSDOS (386)
 Packages:         {Global`, System`}
 
In[3]:=  {a,b}/.{x___,a,y___}:> Print[z;x]

z		(* Printed *)

In[4]:=  CompoundExpression[ z,Sequence[]]

Out[4]=  z

In[5]:=  FullForm[z;Sequence[]]

Out[5]//FullForm=  z

In[6]:=  CloseLog[];

 LogBook V. 1.2 closed  1993-04-05  18:45:33
 
Gunther Reiszig




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