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MathGroup Archive 1997

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Re: Functions and evaluation, returning themselves

  • To: mathgroup at smc.vnet.net
  • Subject: [mg7265] Re: Functions and evaluation, returning themselves
  • From: Paul Abbott <paul at physics.uwa.edu.au>
  • Date: Tue, 20 May 1997 02:59:15 -0400 (EDT)
  • Organization: University of Western Australia
  • Sender: owner-wri-mathgroup at wolfram.com

Red wrote:

> I am trying to write a function in Mathematica
> that can give a message and return the
> expression entered.
> 
> For example
> In[1] :=  Sin[]
> 
> ``Sin::"argx":
>  "Sin called with 0 arguments; 1 argument is expected.
> 
> Out[1]=  Sin[]

The following rule issues an error message (using the built-in Message
mechanism) if f is called with an incorrect number (i.e. != 1) of
arguments:

	f[args___]:= Null/; Length[{args}] != 1 &&
Message[f::"argx",f,Length[{args}]]

As a side-effect of evaluating the Condition -- entered using the infix 
shorthand /; which can be read as such that -- the rule on the
right-hand side computes the number of arguments using Length and
(denoted here by &&) prints a message when it is not equal to one (i.e., 
Length[{args}] != 1):

	f[1,2]

	f::"argx": "f called with 2 arguments; 1 argument is expected."

     f[1,2]

Cheers,
	Paul 

_________________________________________________________________ 
Paul Abbott
Department of Physics                       Phone: +61-8-9380-2734 
The University of Western Australia           Fax: +61-8-9380-1014
Nedlands WA  6907                         paul at physics.uwa.edu.au 
AUSTRALIA                           http://www.pd.uwa.edu.au/Paul

          God IS a weakly left-handed dice player
_________________________________________________________________


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