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Re: Unevaluated functions

• To: mathgroup at smc.vnet.net
• Subject: [mg109358] Re: Unevaluated functions
• From: Rui <rui.rojo at gmail.com>
• Date: Fri, 23 Apr 2010 03:48:51 -0400 (EDT)
• References: hqou0u$9g5$1@smc.vnet.net

On Apr 22, 4:31 am, mokambo <alexandrepassosalme... at gmail.com> wrote:
> Hello,
>
> In Mathematica when a function returns unevaluated the "return value"
> is the name of the function with arguments.
> How do we control such output? Can we suppress it? How do we change
> the usual string for something like "N/A"?
> Advise please.
>
> Alex

Totally agree with Albert.
I wanted to note too that Mathematica is symbolic. That means that
some results you expect to get are symbols and numbers combined in a
certain way (Imagine expanding (x+1)^2 to get x^2+2x+1 as a result)
Therefore, you cannot tell Mathematica something like
"All results that are of the form   something[something] should be
replaced by "N/A""
because you would get "N/A" in everything you do.

However, I'm guessing perhaps it is possible (don't know if it makes
sense) to tell Mathematica to return "N/A" every time it tries to
replace an expression and finds no rule to do so. Perhaps setting $Pre
to a function that stores the expression unevaluated and $Post to
another one that compares the result and decides what to do. Or
perhaps with a general Format, not sure.