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Re: Conditional evaluations of functions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg96514] Re: Conditional evaluations of functions
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Sun, 15 Feb 2009 03:21:34 -0500 (EST)
  • Organization: The Open University, Milton Keynes, UK
  • References: <gn5udh$gsv$1@smc.vnet.net>

In article <gn5udh$gsv$1 at smc.vnet.net>, Asim <maa48 at columbia.edu> 
wrote:


> I am not sure why the function g does not work, but the function f
> works as expected on Mathematica 7.0 on Windows XP.  I want to get the
> sum of a vector only if the argument supplied is a numeric vector.
> 
> In[1]:= g[x_?VectorQ[ x, NumericQ]] := Total[x]
> 
> In[2]:= g[{a, b , c}]
> 
> Out[2]= g[{a, b, c}]
> 
> In[3]:= g[{1, 2, 3}]
> 
> Out[3]= g[{1, 2, 3}]

[snip]

The test function that follows the question mark operator '?' is 
expected to be a pure function. A *pattern*, like the one used for the 
function f after the condition operator '/;' (which expect a pattern) is 
*not* a pure function.  Try
 
  In[1]:= 

    g[(x_)?(VectorQ[#1, NumericQ] & )] := Total[x]
    g[{a, b, c}]
    g[{1, 2, 3}]

  Out[2]= g[{a, b, c}]

  Out[3]= 6

Regards,
--Jean-Marc


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