Re: Complex Oddity
- To: mathgroup at smc.vnet.net
- Subject: [mg57491] Re: Complex Oddity
- From: Jean-Marc Gulliet <jeanmarc.gulliet at 9online.fr>
- Date: Sun, 29 May 2005 21:00:08 -0400 (EDT)
- Organization: New York University
- References: <d79enu$lbl$1@smc.vnet.net>
- Reply-to: jmg336 at nyu.edu
- Sender: owner-wri-mathgroup at wolfram.com
John Reed 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
>
Hi John,
the expression z = a + I y may not be translated internally as you expect:
In[1]:= z = x + I y
Out[1]= x+\[ImaginaryI] y
In[2]:= FullForm[z]
Out[2]= Plus[x,Times[Complex[0,1],y]]
You could try
In[3]:= imagPart = z/.{a_+I b_\[Rule]b}
Out[3]= y
In[4]:= realPart=z/.{a_+I b_\[Rule]a}
Out[4]= x
Best regards,
/J.M.