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MathGroup Archive 1997

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Re: Not a machine-size real number?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg9272] Re: [mg9260] Not a machine-size real number?
  • From: seanross at worldnet.att.net
  • Date: Mon, 27 Oct 1997 02:46:44 -0500
  • Sender: owner-wri-mathgroup at wolfram.com

Pedro Soria-Rodriguez wrote:
> 
> Hello, I am trying to do the following in Mahtematica 3.0.0 for Linux
> 
> BB[I_,a_,NN,d_,z_]= (I*NN/(2*d))*(z/Sqrt[a^2+z^2])
> Plot[BB[1,0.05,100,0.01,z],{z,-1,1}]
> 
> Instead of getting the plot, I get the following error message:
> 
> Plot::"plnr":
> BB[1,0.05,100,0.01,z] ] is not a machine-size real  number at z = -1 ..
> 
> Plot::"plnr":
> BB[1,0.05,100,0.01,z] ] is not a machine-size real  number at z =
> -0.918866
> 
> Plot::"plnr":
> BB[1,0.05,100,0.01,z] ] is not a machine-size real  number at z =
> -0.830382 ..
> 
> General::"stop":
> Further output of Plot::"plnr" will be suppressed during this
>   calculation.
> 
> ---------------------------------------------------
> 
> What does this mean?  Is the definition of my function incorrect?
> Thank you very much in advance
> 
> --
> 
> --
> Pedro Soria-Rodriguez
> sorrodp at ece.wpi.edu


1) general programming practice is to evaluate your function and see
what kind of object mathematica returns before you attempt plotting it.

2) Thhe message about not a machine size real number is sort of the
default plot error message. It could mean that the function is complex,
has symbols in it, has an error in it etc.

3)  I evaluated your function at a few values.  BB[1,0.05,100,0.01,1]
returns itself.  The reason is that you forgot the underscore after the
NN.  This makes BB defined if and only if the third argument is NN. 
Anything else will not do.  If you want NN to be a variable, then you
need the underscore.  Also, it is more common to use the :=(delayed
assignment) in one line functions. The =(immediate assignment) is
normally for assigning values to constants.   It is also dangerous to
make a variable have the same name as a built in mathematica symbol, in
your case I is normally used for the square root of -1.  By making a
pattern have the same name, you effectively preclude the use of complex
numbers in your function.  Since all mathematica built in symbols start
with capital letters, it is normally a good standard practice to make
yours start with small letters.


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