Re: Complex Oddity

*To*: mathgroup at smc.vnet.net*Subject*: [mg57483] Re: [mg57455] Complex Oddity*From*: Chris Chiasson <chris.chiasson at gmail.com>*Date*: Sun, 29 May 2005 01:03:44 -0400 (EDT)*References*: <200505280939.FAA21738@smc.vnet.net>*Reply-to*: Chris Chiasson <chris.chiasson at gmail.com>*Sender*: owner-wri-mathgroup at wolfram.com

Is the expression, z, atomic (AtomQ[z])? Otherwise, the replacement code may not work. Since you said the expression was complicated, I assume that it is not atomic. You could always numerically evaluate z and then use the replacement code. The other (exact) alternative is Im[z] and Re[z], where you will have to work with Im[x] and Im[y] because you probably have not told Mathematica anything about x and y. If you know that x and y are, for example, real, you can try: {imagpart,realpart}=Refine[{Re@#,Im@#}&@z,{Element[x,Reals],Element[y,Reals]] Disclaimer: I didn't test any of the above code. Regards, On 5/28/05, John Reed <nospamjreed at alum.mit.edu> wrote: > I was trying to separate the real and imaginary parts of a complicated > expression, and ended up with something strange. Here is a short version of > what happened. > > Let z = x + I y, then realPart = z /. {Complex[a_,b_]->a} gives realPart = > x. Great! > > Now, try imagPart = z /. {Complex[a_,b_]->b} returns with imagPart = x + y. > Oops > > In my original expression, it was harder to see, but the same error was > occuring. What I tried first was using Re[z] and Im[z], but then I have to > work with Im[y] and Im[x]. It seems to me two things need to be done here. > First, be able to assign variables so that they always stay real or else > indicate an error is occuring if they turn out to be complex, and second do > something to avoid errors like the above. I have to say that I don't trust > Mathematica's answers as much as I did before this came up. Now I feel like > I better have a good idea of what the answer is before I trust Mathematica. > > John Reed > > -- Chris Chiasson http://chrischiasson.com/ 1 (810) 265-3161

**References**:**Complex Oddity***From:*"John Reed" <nospamjreed@alum.mit.edu>