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Re: Checking function syntax

  • To: mathgroup at
  • Subject: [mg64605] Re: Checking function syntax
  • From: Paul Abbott <paul at>
  • Date: Thu, 23 Feb 2006 00:35:54 -0500 (EST)
  • Organization: The University of Western Australia
  • References: <dthh6f$nai$>
  • Sender: owner-wri-mathgroup at

In article <dthh6f$nai$1 at>,
 Chris Rodgers <rodgers at> wrote:

> How can I make my own functions give an error if they are called with 
> the wrong syntax?

This is a FAQ -- but not so easy to find (a search on argx is a good way 
to find the answer).

> For example, if I define this function
> In[37]:=
> MyFunc1[x_,y_]:=Tuples[{x,y},2]
> and then Map MyFunc2 over the result
> In[38]:=
> Map[MyFunc2,MyFunc1[a,b]]
> I get this (correct, desired) output.
> Out[38]=
> {MyFunc2[{a,a}],MyFunc2[{a,b}],MyFunc2[{b,a}],MyFunc2[{b,b}]}
> However, if I inadvertently added a third argument
> In[39]:=
> Map[MyFunc2,MyFunc1[a,b,c]]
> then I get this (correct, but undesired) output
> Out[39]=
> MyFunc1[MyFunc2[a],MyFunc2[b],MyFunc2[c]]
> I would prefer to get some sort of error message in the second case, 
> such as the built-in function Sin[...] produces when called with the 
> wrong number of arguments.
> In[41]:=
> Sin[1.]
> Out[41]=
> 0.841471
> In[42]:=
> Sin[2,3]
>  From In[42]:=
> Sin::argx : Sin called with 2 arguments; 1 argument is expected. Out[42]=
> Sin[2,3]

The following rule issues an error message (using the built-in Message 
mechanism via "argrx" -- look this up in the Help Browser) if f is 
called with an incorrect number (e.g. != 2) of arguments:   

  f[args___]:= Null/; Length[{args}] != 2 && 
    Message[f::"argrx", f, Length[{args}], 2] 

As a side-effect of evaluating the Condition the rule on the right-hand 
side computes the number of arguments using Length and prints a message 
when it is not equal to two.


 f::"argrx": f called with 3 arguments; 2 arguments are expected. More...

 f[1, 2, 3]


Paul Abbott                                      Phone:  61 8 6488 2734
School of Physics, M013                            Fax: +61 8 6488 1014
The University of Western Australia         (CRICOS Provider No 00126G)    

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