Re: Redefine Arg to return a value from 0 to 2 pi
- To: mathgroup at smc.vnet.net
- Subject: [mg80421] Re: Redefine Arg to return a value from 0 to 2 pi
- From: markholtuk at googlemail.com
- Date: Wed, 22 Aug 2007 04:41:13 -0400 (EDT)
- References: <faeash$f5i$1@smc.vnet.net>
I've always got the impression from this list that it's not
necessarily a good idea to mess around with built-in commands. The
thing to do is to define your own command e.g. myArg as follows:
myArg[z_] := Module[{z0 = z}, If[Im[z0] < 0,
ArcTan[Re[z0], Im[z0]] + 2*Pi, ArcTan[Re[z0],
Im[z0]]]];
This will then do what you want:
Plot[myArg[2*Exp[I*t]], {t, 0, 4*Pi}]
I'm sorry I can't explain why your approach doesn't work, but I'm sure
one of our resident guru's can/will.
Best wishes,
Mark
On Aug 21, 10:22 am, chuck009 <dmili... at comcast.com> wrote:
> I'm trying to redefine Arg to return a value from 0 to 2 pi. However when I do so and then try and plot Arg[z], Mathematica seems to still use the old definition. Can anybody help me? This is my code:
>
> (* redefine Arg to return a value from 0 to 2 pi *)
>
> Unprotect[Arg]
> Arg[z_] := Module[{z0 = z}, If[Im[z0] < 0,
> ArcTan[Re[z0], Im[z0]] + 2*Pi, ArcTan[Re[z0],
> Im[z0]]]];
> Protect[Arg]
>
> (* check new definition -- this is correct *)
>
> ListPlot[Table[{t, Arg[2*Exp[I*t]]}, {t, 0, 4*Pi, 0.1}]]
>
> (* try and plot new definition of Arg -- Plot used the old definition (-pi to pi) *)
>
> Plot[Arg[2*Exp[I*t]], {t, 0, 4*Pi}]
>
> (* resore old definition of Arg *)
>
> Unprotect[Arg]
> Arg[z_] =.
> Protect[Arg]
Mark R. Holt, BA, PhD,
Room 3.28
Molecular Cardiology,
Cardiovascular Division and
Randall Division of Cell and Molecular Biophysics,
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King's College London,
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Phone: +44 20 7848 6438
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