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MathGroup Archive 1998

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Re: Getting rid of annoying zeroes in algebraic expressions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg15249] Re: [mg15228] Getting rid of annoying zeroes in algebraic expressions
  • From: jtischer at col2.telecom.com.co (Cl-Jurgen Tischer)
  • Date: Sun, 27 Dec 1998 03:58:40 -0500
  • Organization: Universidad del Valle
  • Sender: owner-wri-mathgroup at wolfram.com

Sean,
first, on my computer your two versions where equivalent.

In[1]:= xx=0. + (a*f)/(0. + f) + (b*f)/(0. + f) + (c*f)/(0. + f);

In[2]:=  xx /. 0. + (q_) -> q

Out[2]= (a*f)/(0. + f) + (b*f)/(0. + f) + (c*f)/(0. + f)

In[3]:= 0. + (a*f)/(0. + f) + (b*f)/(0. + f) + (c*f)/(0. + f) /. 
  0. + (q_) -> q

Out[3]= (a*f)/(0. + f) + (b*f)/(0. + f) + (c*f)/(0. + f)

Second, it's easy to get what you want:

In[4]:= xx //. 0. + (q_) -> q

Out[4]= a+b+c

or if you like it more

In[5]:= Chop[xx]

Out[5]= a+b+c


Jurgen


"Ross, Sean" wrote:
> 
> Let xx be the result of some algebraic manipulations which, for some
> reason, mathematica thinks the real zeroes ought to be kept and I think
> they ought to be dropped.
> 
> xx=0. + (a*f)/(0. + f) + (b*f)/(0. + f) + (c*f)/(0. + f)
> 
> xx/.Plus[0.,q_]->q
> 
> returns
> 
> 0. + (a*f)/(0. + f) + (b*f)/(0. + f) + (c*f)/(0. + f)
> 
> while
> 
> 0. + (a*f)/(0. + f) + (b*f)/(0. + f) + (c*f)/(0. + f)/.Plus[0.,q_->q]
> 
> returns the expected
> 
> a+b+c.
> 
> xx is supposed to be equivalent to the expression, but in this case, it
> is not.
> I can't write a function to automatically drop zeroes if expressions
> behave differently when they are alone on an input line and when they
> are set equal to a symbol.  Does anyone know how to eliminate this kind
> of behavior?  I am sure it has something to do with some obscure step
> in the "standard evaluation cycle" and that there is probably a long,
> convoluted explanation as to why someone would want this behavior, but
> I don't and I would appreciate it if someone could tell me how to get
> rid of it or circumvent it.
> 
> Thanks,
> 
> Sean Ross
> 
> Please reply to rosss at plk.af.mil as I no longer subscribe to the
> mathgroup.
> 



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