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Re: Flat: Problems & Workarounds Sweden.
*To*: mathgroup at smc.vnet.net
*Subject*: [mg8492] Re: Flat: Problems & Workarounds Sweden.
*From*: Robert Villegas <villegas>
*Date*: Tue, 2 Sep 1997 16:15:36 -0400
*Organization*: Wolfram Research
*Sender*: owner-wri-mathgroup at wolfram.com
> This is the result from my work trying to construct a associative function.
> Assume I want a associative function h[] which treats numerical arguments
> specially. Starting from the template in Roman Maeders "Programming in
> Mathematica", 3rd ed. I define h[] as
>
> In[3]:= SetAttributes[h,{Flat,OneIdentity}]
> h[x_,y_?NumericQ] := f[y,x]
> h[x_?NumericQ,y_] := f[x,y]
> h[x_] := x
> h[] = 1
When I want an associative function h that acts as the identity on
singletons, I ditch the use of attributes like Flat and OneIdentity
and write a definition to make h self-flattening. If I want h to
recognize certain argument arrangements and re-group them, I control
this myself with additional definitions, since the pattern-matcher under
the influence of Flat might try groupings and orderings of them different
from what I want.
Here is how I recommend formulating your h:
ClearAll[h];
h[elems___] /; MemberQ[Unevaluated[{elems}], _h] :=
Flatten[Unevaluated @ h[elems], Infinity, h];
h[elems__, n_?NumberQ] := f[n, h[elems]];
h[n_?NumberQ, elems__] := f[n, h[elems]];
h[singleton_] := singleton;
h[] = 1;
This h does what you asked for the examples you cited, and its behavior
seems reasonable in related cases:
In[37]:=
{h[], h[1], h[a], h[1, a], h[a, 1], h[a, b], h[2, a, 3], h[2, 3, a],
h[a, 2, 3]}
Out[37]=
{1, 1, a, f[1, a], f[1, a], h[a, b], f[3, f[2, a]], f[2, f[3, a]],
f[3, f[2, a]]}
Your function isn't precisely specified, so this may not be exactly what
you want, but it shouldn't be difficult to modify this to do something a
bit different.
The key definition in h was the following
h[elems___] /; MemberQ[Unevaluated[{elems}], _h] :=
Flatten[Unevaluated @ h[elems], Infinity, h]
which canonicalizes the arguments of h so that other definitions have the
luxury of knowing that none of h's arguments have head h. From there, make
a list of the precise arrangements you want to recognize and re-group, e.g.
(1) A bunch of arguments followed by a final NumericQ argument
(2) An initial NumericQ argument followed by a bunch of arguments
then add the definitions for these.
It is occasionally necessary to use HoldPattern on the left-hand sides of
rules, or to notice the automatic ordering of rules, but not usually.
Robby Villegas
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