Re: Re: Re: Hold and Equal

• To: mathgroup at smc.vnet.net
• Subject: [mg73837] Re: [mg73771] Re: [mg73747] Re: Hold and Equal
• From: "Chris Chiasson" <chris at chiasson.name>
• Date: Thu, 1 Mar 2007 06:25:34 -0500 (EST)
• References: <erufqm\$s7j\$1@smc.vnet.net> <200702271048.FAA24024@smc.vnet.net>

```In[1]:= Attributes@formEquation=List@HoldFirst(*or one could just
supply Unevaluated arguments*)

Out[1]= {HoldFirst}

In[2]:= formEquation[expr_,op_]:=With[{result=op@expr},HoldForm[expr=result]]

In[3]:= formEquation[Integrate[x^2,x],Identity]

Out[3]= \[Integral]x^2\[DifferentialD]x=x^3/3

On 2/28/07, Murray Eisenberg <murray at math.umass.edu> wrote:
> Aha!  I believe this approach _almost_ allows me to accomplish what I
> was REALLY trying to accomplish.  It certainly works in the example I
> gave.  If I encapsulate this in a function...
>
>    formEquation[expr_, op_]:= HoldForm[expr=z]/.z\[Rule]op[expr]
>
> ... then
>
>    formEquation[(a+b)^2,Identity]
>
> will produce exactly what I want.
>
> However, if I try something like the example I was really after (which I
> didn't mention in my original post, since I gave something simpler), it
> works in the direct version...
>
>    HoldForm[Integrate[x^2,x] = z] /. z\[Rule]Integrate[x^2,x]
>
> but not with the encapsulating function:
>
>    formEquation[Integrate[x^2, x], Identity]
>
> The latter produces the equation
>
>    x^3/3 = x^3/2
>
> whereas I want the left-hand side to be the unevaluated integral expression.
>
> You can tell I'm struggling with Hold!  (One of the "last frontiers" in
> my Mathematica education.)
>
> bghiggins at ucdavis.edu wrote:
> > Murray,
> >
> > Try this
> >
> >
> > HoldForm[(a + b)^2 = z] /. z -> Expand[(a + b)^2]
> >
> >
> > (a + b)^2 = a^2 + 2*a*b + b^2
> >
> > Cheers,
> >
> > Brian
> >
> >
> >
> > On Feb 26, 3:20 am, Murray Eisenberg <mur... at math.umass.edu> wrote:
> >> How can I produce in an Output cell (under program control) an
> >> expression like the following,
> >>
> >>    (a+b)^2 = a^2+ 2 a b + b^2
> >>
> >> where instead of the usual Equal (==) I get a Set (=), as in traditional
> >> math notation?  I want to input the unexpanded (a+b)^2 and have the
> >> expansion done automatically.
> >>
> >> Of course, I can try something like the following:
> >>
> >>    (a+b)^2 == Expand[(a+b)^2])
> >>
> >> So how do I convert the == to =?  Of course
> >>
> >>    ((a + b)^2 == Expand[(a + b)^2]) /. Equal -> Set
> >>
> >> gives a Set::write error.  And
> >>
> >>    (Hold[(a + b)^2 == Expand[(a + b)^2]]) /. Equal -> Set
> >>
> >> doesn't actually evaluate the Expand part and leaves the "Hold" wrapper.
> >>
> >> --
> >> Murray Eisenberg                     mur... at math.umass.edu
> >> Mathematics & Statistics Dept.
> >> Lederle Graduate Research Tower      phone 413 549-1020 (H)
> >> University of Massachusetts                413 545-2859 (W)
> >> 710 North Pleasant Street            fax   413 545-1801
> >> Amherst, MA 01003-9305
> >
> >
> >
>
> --
> Murray Eisenberg                     murray at math.umass.edu
> Mathematics & Statistics Dept.
> Lederle Graduate Research Tower      phone 413 549-1020 (H)
> University of Massachusetts                413 545-2859 (W)
> 710 North Pleasant Street            fax   413 545-1801
> Amherst, MA 01003-9305
>
>

--
http://chris.chiasson.name/

```

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