Re: Replace and ReplaceAll -- simple application

*To*: mathgroup at smc.vnet.net*Subject*: [mg106457] Re: Replace and ReplaceAll -- simple application*From*: AES <siegman at stanford.edu>*Date*: Wed, 13 Jan 2010 05:58:25 -0500 (EST)*Organization*: Stanford University*References*: <200912270006.TAA12080@smc.vnet.net> <hh72dp$kud$1@smc.vnet.net> <hh9vfo$1rk$1@smc.vnet.net> <200912290618.BAA02632@smc.vnet.net> <hhf5kg$go6$1@smc.vnet.net> <200912310814.DAA24681@smc.vnet.net> <hhkj1a$4tl$1@smc.vnet.net>

In article <hhkj1a$4tl$1 at smc.vnet.net>, Murray Eisenberg <murray at math.umass.edu> wrote: > That's an instructive example on the point being discussed. As you note, > > R + I w L /. I -> -I > > works "as expected". But so does: > > I /. I -> -I > > What does NOT work is either of: > > R - I w L /. I -> -I > -I /. I -> -I > What you say is certainly correct -- but I believe it bypasses the core point. An individual with a physics background may manipulate the impedance for an RL circuit by writing R + I w L /. I -> -I while one with an EE background may very naturally write this as R + I 2 Pi f L /. I -> -I (or, in many textbooks, R + 2Pi I L /. I -> -I) And, **one of these will get a wrong answer**. [And if this "erroneous" input is buried in a sequence of compound expressions or definitions early in a notebook, that error may not even become apparent until hours later, way down in the notebook, in the form of bizarre and puzzling behavior in some much more complex derived result.] Same problem for two individuals, one of whom likes to use half width at half maximum linewidths and writes a complex lorentzian as 1 + I (x-x0)/dxHwhm and another who likes full width at half max linewidth, and so writes 1 + 2 I (x-x0)/dxFwhm R + I w L/.I->-I R + 2 Pi I f L/.I->-I 1+I ((x-x0)/deltax)^2/.I->-I 1+I (2(x-x0)/deltax)^2/.I->-I

**Follow-Ups**:**Re: Re: Replace and ReplaceAll -- simple application***From:*Murray Eisenberg <murray@math.umass.edu>