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Re: pattern bugs and comment on intuitive syntax for the New Year

  • To: mathgroup at smc.vnet.net
  • Subject: [mg115187] Re: pattern bugs and comment on intuitive syntax for the New Year
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Tue, 4 Jan 2011 04:22:23 -0500 (EST)
  • References: <ifs30a$oor$1@smc.vnet.net> <4D21F917.2020209@cs.berkeley.edu>

I have skipped most of the tired and empty rhetorics.

 
On 3 Jan 2011, at 17:28, Richard Fateman wrote:

>
> So to call Lisp compiled and Mathematica not compiled is either
> over-simplified or ignorant.

Please check:

http://en.wikipedia.org/wiki/Compiled_language

I quote: "A compiled language is a programming language whose 
implementations are typically compilers..."

Among the list of "compiled languages" you will find Lisp.  My point was 
obvious: since Lisp is a compiled language it is not surprising that 
programs written in Lips can be compiled. Is it?
Neither I not anyone else ever asserted that it would not be possible to 
write a CAS like Mathematica in which purely symbolic code could be 
compiled. But that it is not the way Mathematica works and that is all 
that this list is concerned with. All your comments on this matter are 
irrelevant and are simply displays of certain aspects of your 
personality. They provide no other information and the one that they do 
is already very well known.
>
>
>
> $ but then
> $  nobody wanting to make sense would use a BlankNullSequence in
> $  this situation: it is an artificial examples made up purely
> $  to cause puzzlement and an appearance of something like a
> $  "bug". In the case
>
> $  x /. x + c___ :> aha[c]
>
> $  x
>
> $  one gets a different outcome because no match is found, but
> $  there is nothing at all surprising here since the function
> $  Plus with its attributes is not explicitly involved and one
> $  would not expect Mathematica to replace all instances of
> $  single symbols by Plus[x] in order to make a match.
>
> Of course Mathematica would not have to use your proposed mechanism
> to make this match. Indeed, I doubt that it uses your proposed
> mechanism of rewriting as Plus[x,y,Plus[]], either.  But one does
> not have to replace "all instances of single symbols by Plus[x].
> Only when trying to match against a pattern that looks like Plus[...].

In fact I was wrong on some points of detail concerning the role of the 
Flat and OneIdenitity attributes although it was you who quite 
spuriously introduced them into this issue.

Observe:

ClearAll[f]

f[a] /. f[a_, c___] :> aha[c]

aha()

This is without any f having any attributes.

In[3]:= Attributes[f]

Out[3]= {}

The point is that the very presence of BlankNullSequence in the pattern 
makes Mathematica re-write f[a] as f[a, ]. If f has the Flat attribute 
(like Plus) then f[a, ] is equivalent (as a pattern) to f[a,f[]], which 
is why I wrote Plus[x,y,Plus[]] (as I thought writing Plus[x,y, ] would 
like like some sort of error). But the Flat attribute plays no role 
here. In any case, the point is that the basic principle of Mathematica 
pattern matching is that if you use a certain pattern Mathematica 
assumes that you wish to find a match. So when you use ___ instead of _ 
you are inviting matches involving no arguments i.e. f[a, ]. This is 
true in general and so it is also true for Plus, even though Plus[a,b, ] 
does not make mathematical sense (a blank space is not 0). This is 
exactly why I think using this pattern in this context shows lack of 
understanding of Mathematica.
>
> Ok, here's why I tried c___, seeing the construction c_.  (or
> in this example, b_., fail. Desperation? Flailing?)
>
> Construct a test to see if an expression is a quadratic in a
> given variable.
>
> quadp[a_. x_^2+ b_. x_+c_., x_]:= qq[c,b,a] /; FreeQ[{a,b,c},x]
>
> for example,
> quadp[5+4*x +3*x^2,x]  produces qq[5,4,3].
> quadp[  4*x +3*x^2,x]  produces qq[0,4,3].
> quadp[    x +  x^2,x]  produces qq[0,1,1].
>
> quadp[5     +3*x^2,x]  no match  oops.


But the problem is obvious. The default for multiplication is 1 not 0. 
So, as you noted yourself, you need more patterns.

> Now, using 2 rules seems to help:
> quadp[a_. x_^2+ b_. x_+c_., x_]:= qq[c,b,a] /; FreeQ[{a,b,c},x]
> quadp[a_. x_^2        +c_., x_]:= qq[c,0,a] /; FreeQ[{a,c},x]

but this clearly shows a (shocking!) lack of understanding of what is 
going on! Try

quadp[x + 1, x]

quadp(x+1,x)

You need  to account for all coefficients being either 0 or 1. That 
means 8 rules. E.g.:


quadp[a_. x_^2 + b_. x_ + c_., x_] /; FreeQ[{a, b, c}, x] :=
 qq[c, b, a]

quadp[a_:0 x_^2 + b_.  x_ + c_., x_] /; FreeQ[{a, b, c}, x] :=
 qq[c, b, a]

quadp[a_. x_^2 + b_:0  x_ + c_., x_] /; FreeQ[{a, b, c}, x] :=
 qq[c, b, a]

....

and so on.

quadp[1 + x^2, x]

qq(1,0,1)

It would certainly be nice if one one could do it all using Alternatives 
but I don't think it can be done.

>
> Now this pattern collection also fails, e.g. for  r*x^2+s*x^2+1  which 
most people would think is a quadratic.  (I tried various other
> tricks like Alternatives, to get everything into one neat rule,
> but decided the 2 rules were probably better.  I'm open to suggestions
> of neater patterns.

Well, it fails on the simpler case above. As for this case, if will work 
if you first use Collect[r*x^2+s*x^2+1,x]. Actually you can incorporate 
doing Collect automatically into your function. But obviously the right 
way is to do it without pattern matching which seem to have realised 
after wasting a lot of time (perhaps yours but certainly mine).

Andrzej


>
> Here is a better program. Something like
>
> quadp=Function[{ex,var}, Module[{ans=CoefficientList[ex,var]},
>  If [Length[ans]==3 && FreeQ[ans,var],  Apply[qq,ans], 
notqq[ex]]]]
>
> It also works for r*x^2+s*x^2.
> RJF
>
>
>
>
>
> $   Andrzej Kozlowski
>
>



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