Re: How to get non-printing heads?

• To: mathgroup at smc.vnet.net
• Subject: [mg46968] Re: How to get non-printing heads?
• From: bobhanlon at aol.com (Bob Hanlon)
• Date: Thu, 18 Mar 2004 01:24:53 -0500 (EST)
• References: <c388k0\$bnb\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Clear[a,b,f, g, y, isConstant];

Constant is an attribute not a head

SetAttributes[a, Constant]

Dt[#,t]& /@ {a, b, y, E, 3}

{0, Dt[b, t], Dt[y, t], 0, 0}

isConstant[x_] := NumericQ[x] || !FreeQ[Attributes[x], Constant];

The test for NumericQ is needed since Attributes does not know how to handle
numbers that are not numeric symbols (such as Pi and E)

isConstant /@ {a, b, y, E, 3}

{True, False, False, True, True}

{False, True, False, False, False}

f /@ {a, b, y, E, 3}

{f[a], 0, f[y], f[E], f[3]}

g /@ {a, b, y, E, 3}

{0, 0, g[y], 0, 0}

Bob Hanlon

In article <c388k0\$bnb\$1 at smc.vnet.net>, kj <socyl at 987jk.com> wrote:

<< How can I give a Mathematica object a different head?

I tried

But, as shown below, this doesn't work as desired:

In[2]:= MatchQ[a, _Constant]

Out[2]= False

In[3]:= MatchQ[a, _Symbol]

Out[3]= True

I even tried

In[4]:= a/: MatchQ[a, _Constant] = True;

In[5]:= ??a
Global`a

a /: MatchQ[a, _Constant] = True

...but

In[6]:= MatchQ[a, _Constant]

Out[6]= False

Is there any way to give "a" a user-defined head?

My immediate goal is to be able to define functions that perform
algebraic manipulations on Mathematica expressions according to
the types of the objects they contain.  I want to leave these
objects pretty much undefined, other than specifying their types.
E.g.:

myDerivative[x_Constant] := 0;

myDerivative[a]

==> 0

myDerivative[b]

==> myDerivative[b]    (* Derivative doesn't know how to handle Symbol *)
>><BR><BR>

```

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