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MathGroup Archive 2007

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Re: Redefine Arg to return a value from 0 to 2 pi

  • To: mathgroup at smc.vnet.net
  • Subject: [mg80463] Re: Redefine Arg to return a value from 0 to 2 pi
  • From: dh <dh at metrohm.ch>
  • Date: Thu, 23 Aug 2007 01:03:24 -0400 (EDT)
  • References: <faeash$f5i$1@smc.vnet.net>


Hi Chuck,

it is bad practise to redefine built in funcions, you may run into 

troubles you did not anticipate. I think it is better to write your own 

version, what can be done e.g. by

myArg[x_]:=Mod[Arg[x],2Pi]

hope this helps, Daniel



chuck009 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]

> 




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