Date: Apr 17, 2013 2:30 AM Author: David Park Subject: Re: How to apply tags to expression terms? Alexei,

The answers you received recommending TraditionalForm are probably the way

to go in most cases. But suppose that one prefers to work in StandardForm,

or for one reason or another TraditionalForm is also not the order you want

for terms or factors? Then I think a convenience routine might be useful.

I don't think the idea of using tags and trying to compute with such

expressions is too great. One would have to almost rewrite Mathematica

algebra! But we can write a useful routine for final display expressions.

So here is a routine that allows you to reorder terms in a Plus expression

or factors in a Times expression.

HoldOrderForm::usage =

"HoldOrderForm[permutation][expr] will reorder the terms or factors \

of a Plus or Times expression according to the permutation and put \

the result in a HoldForm.";

SyntaxInformation[HoldOrderForm] = {"ArgumentsPattern" -> {_}};

HoldOrderForm[permutation_?PermutationListQ][(f : Plus | Times)[

args__]] /; Length[{args}] == Length[permutation] :=

(HoldForm @@ {(List @@ f[args])[[permutation]]}) /. List -> f

Then for your simple case:

-1 + x;

HoldOrderForm[{2, 1}]@%

x - 1

Here is a case where we first reorder terms and then reorder the resulting

factors:

(1 - x + Exp[I x]) (x - 1)

Inner[#1[#2] &, {HoldOrderForm[{2, 1, 3}], HoldOrderForm[{2, 1}]},

List @@ %, Times]

HoldOrderForm[{2, 1}]@%

(1 + E^(I x) - x) (-1 + x)

(x - 1) (E^(I x) + 1 - x)

(E^(I x) + 1 - x) (x - 1)

If you like it I'll put it in Presentations. Presemtations contains a number

of convenience routines for manipulating expressions to special forms. Some

of them just seemed useful to me but many of them came from questions on

MathGroup. If the ideas seem useful and not too specialized then I consider

including them. And they are in a common place where Presentations users can

find them.

David Park

djmpark@comcast.net

http://home.comcast.net/~djmpark/index.html

From: Alexei Boulbitch [mailto:Alexei.Boulbitch@iee.lu]

Dear Community members,

I often see on this site an at the StackExchange the repeating questions of

how to rearrange some expression, that Mathematica "likes" to keep in one

form, but the user prefers another one. It is like in this trivial example:

(x^2 - 1)/(x + 1) // Simplify

that Mathematica returns -1+x, rather than x-1 that might be wished by the

user.

I have seen many answers to this questions, and gave few mine. The problem

here is that the answers are non-universal: they strongly depend upon the

expression in question. Besides, they require some additional programming,

and the more complex is the formula to sort, the longer will be the part of

the sorting code.

It seems that the problem of sorting terms of analytic expressions in the

desired order might be solved, if one could assign tags to the terms to be

sorted, and then sort the terms according to a specified list of such tags.

Now comes my question, do you know how to apply tags to expression terms?

I have seen an analogous functionality in the Presentation Master, the

package of David Park. There, however, the tags are used to be assigned to

sub expressions, the held part of the expression in question. David, is it

possible to assign tags to expressions that are not held?

Thank you in advance.

Alexei

Alexei BOULBITCH, Dr., habil.

IEE S.A.

ZAE Weiergewan,

11, rue Edmond Reuter,

L-5326 Contern, LUXEMBOURG

Office phone : +352-2454-2566

Office fax: +352-2454-3566

mobile phone: +49 151 52 40 66 44

e-mail: alexei.boulbitch@iee.lu