Re: ReplaceAll and ReplaceRepeated Strange Behavior

*To*: mathgroup at smc.vnet.net*Subject*: [mg111484] Re: ReplaceAll and ReplaceRepeated Strange Behavior*From*: Eric Loots <eric.loots at gmail.com>*Date*: Tue, 3 Aug 2010 06:35:48 -0400 (EDT)

Hi, I'm new to this group, but I'll have a go at this. The rule dm*Pi -> L/Tan[a] must look for matches of the pattern. I guess that the pattern just doesn't match in all cases. When you look at the output after entering the first line of input, you see that one of the dm * Pi terms is rearranged. A simple solution to your problem is to apply another for of the rule: dm -> L / Pi / Tan[a] Adding Simplify on top of that: (L*(-L*mu + dm*Pi*Cos[b]))/(dm*Pi*(dm*Pi*mu + L*Cos[b])) /. dm -> L / =CF=80/Tan[a] // Simplify gives: (Cos[b] - mu Tan[a])/(Cos[b] + mu Cot[a]) With respect to copy/paste from a Notebook to the group, I'm using regular mail to post this reply, and an ordinary copy/past just works. Regards -- Eric On 02 Aug 2010, at 13:04, blamm64 wrote: > Given the expression: > > eff == (L*(-L*mu+dm*Pi*Cos[b]))/(dm*Pi*(dm*Pi*mu+L*Cos[b])) > > After this I enter > > eff /. {dm*Pi -> L/Tan[a]} > > and get a result that does not, for a reason I cannot fathom and > believe to be a bug, replace one instance of dm*Pi. The result of the > evaluation yields > > L*(-L*mu+L*Cos[b]*Cot[a])/(dm*Pi*(L*Cos[b]+L*mu*Cot[a])) > > Precisely the same result occurs with eff //.{dm*Pi->L/Tan[a]} > > Why is the one remaining dm*Pi not replaced with L/Tan[a]? > > In fact, when define manually define eff1 as > > eff1 == (L*(-L*mu + L*Cos[b]*Cot[b]))/(dm*Pi*(L*Cos[b]+L*mu*Cot[a])) > > which is as you can see the result of the first ReplaceAll execution, > and then enter > > eff1 /. {dm*Pi->L/Tan[a]} > > Mathematica returns eff1 unaltered! > > Now, I say this is a bug, so of course I'm probably missing something > and it isn't a bug. I have to jump through syntactic hoops to get the > 'right' answer: > > FullSimplify[eff /.{dm*Pi->L/Tan[a]}]/.{dm*Pi->L/Tan[a]}//FullSimplify > > and FullSimplify[eff /. {dm*Pi->L/Tan[a]} does no good either. > > It is not at all obvious to me what I might be missing with such an > apparently simple replacement. > > By the way, how do you guys copy input or output from Mathematica > notebook into this usergroup window? I had to do all this by hand, > copy it from here into a notebook, and execute the notebook cells just > to make sure I was getting it correct. I ran across this originally > when I was constructing a simple notebook for doing some Acme power > screw calculations. > > -Brian Lamm > (Windows XP x64 Version 2003 SP2, Mathematica 7.0.1) >

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