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MathGroup Archive 2004

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Re: Undiscovered Bug?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg46890] Re: Undiscovered Bug?
  • From: drbob at bigfoot.com (Bobby R. Treat)
  • Date: Fri, 12 Mar 2004 23:40:12 -0500 (EST)
  • References: <c2rnrf$p4h$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Part[expr,{a,b,c,d}] returns:

Head[expr][expr[[a]],expr[[b]],expr[[c]],expr[[d]]]

So, look at your final example:

(x/y)[[{2, 1, 1, 0, 2, 2, 0}]]
(Times^2*x^2)/y^3

To explain it, we first need to see what the expression really is:

x/y//FullForm
Times[x,Power[y,-1]]

The Head is Times, (x/y)[[2]] is Power[y,-1], (x/y)[[1]] is x, and
(x/y)[[0]] is Times. So the result is:

Times[Power[y,-1],x,x,Times,Power[y,-1],Power[y,-1],Times]

and that simplifies to the answer above.

Bobby

Harold.Noffke at wpafb.af.mil (Harold Noffke) wrote in message news:<c2rnrf$p4h$1 at smc.vnet.net>...
> MathGroup:
> 
> In Mathematica 5.0.1 with Windows 2000, I mistakenly used the [[{}]]
> operator on x/y.  I was expecting an error, but I kept getting unusual
> results as shown in In/Out's 2 to 5 below.
> 
> Have I stumbled on a bug, or is there a way to make sense out of this?
> 
> 
> In[1]:= TreeForm[x/y]
> 
> Out[1]//TreeForm= Times[x, |           ]
>                            Power[y, -1]
> 
> In[2]:= (x/y)[[{2,2}]]
> 
> 
> Out[2]=  -2
>         y
> 
> In[3]:= (x/y)[[{1,2,1}]]
> 
>          2
>         x
> Out[3]= --
>         y
> 
> In[4]:= (x/y)[[{2, 2, 2, 2}]]
> 
>          -4
> Out[4]= y
> 
> In[5]:= (x/y)[[{2, 1, 1, 0, 2, 2, 0}]]
> 
>              2  2
>         Times  x
> Out[5]= ---------
>             3
>            y
> 		   
> 		   
> Regards,
> Harold


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