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Re: replacement x->y except in Exp[x]

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  • Subject: [mg110885] Re: replacement x->y except in Exp[x]
  • From: Andrzej Kozlowski <akozlowski at>
  • Date: Sat, 10 Jul 2010 04:00:20 -0400 (EDT)
  • References: <i14aij$g0p$> <> <i16vm6$iuj$> <>

On 10 Jul 2010, at 05:35, Richard Fateman wrote:

> Andrzej Kozlowski wrote:
> ....
>> Suppose in the expression 2/3 I + x/y I you wish to replace all fractions (that is 2/3 and x/y) by r and I by d. Without worrying about evaluation you can do this as follows:
>> Unevaluated[Unevaluated[2/3 I + x/y I] /. HoldPattern[x_/y_] -> r] /.
>> HoldPattern[I] -> d
>> 2 d r
>> ...
>> All this is perfectly reasonable, logical and a great deal easier than almost anything in an undergraduate math syllabus at a reasonable university.
> Really?
> So if someone gives you a basket of fruit and says to replace the apples with bananas, you explain that it is easy:  First you tell that person to throw away that basket and come up with a new basket in which he has  wrapped the whole basket in a layer of aluminum foil. You then replace
> "apple-wrapped-in-foil" by banana.
> You then proclaim the problem solved.
> Unfortunately, if I give you an already unwrapped basket of fruit, it is impossible.  (that is, an already simplified Mathematica expression)
> Which is presumably the usual state of the basket.

Impossible? For example (just one of many ways to do this):

expr = 1 + I;

 ToExpression[ToString[expr, TraditionalForm], InputForm, Hold] /.
  HoldPattern[I] -> d]



> A discussion and  partial definition of BetterRules, a Mathematica program that does things like this, is in

It's amazing what some people call "papers" these days.

Andrzej Kozlowski

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