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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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