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Re: Problems with easy simplifications
*To*: mathgroup at smc.vnet.net
*Subject*: [mg54175] Re: [mg54143] Problems with easy simplifications
*From*: Chris Chiasson <chris.chiasson at gmail.com>
*Date*: Sat, 12 Feb 2005 01:58:04 -0500 (EST)
*References*: <200502110834.DAA09274@smc.vnet.net>
*Reply-to*: Chris Chiasson <chris.chiasson at gmail.com>
*Sender*: owner-wri-mathgroup at wolfram.com
The problem is that there is a type of hold on the function that isn't
showing itself. I found it when I checked the attributes of the If
command.
The following commands do what you want globally, but I haven't been
able to selectively mute the HoldRest.
H[x_, a_, b_] = If[x < a, a + b, a - b]
If[x < a, a + b, a - b]
Attributes[If]
{HoldRest, Protected}
Unprotect[If]
{If}
ClearAttributes[If, HoldRest]
H[x, 0, 0]
If[x < 0, 0, 0]
On Fri, 11 Feb 2005 03:34:47 -0500 (EST), Antonio González
<gonferh2o at esi.us.es> wrote:
> I have found some weird behaviors of Mathematica (5.0) related to the
> simplifications of easy expressions.
>
> Take, for instance the function
>
> H[x_, a_, b_] = If[x < a, a + b, a - b]
>
> If I try now to evaluate
>
> H[x,0,0]
>
> the result
>
> If[x < 0, 0 + 0, 0 - 0]
>
> (the values of a and b are irrelevat, a similar result is obtained, for
> instance, with a=1, b= 2, or any integer, real or complex values). I
> cannot force (at least in a simple way) Mathematica to make the addition
> or the substraction. The action of Simplify or FullSimplify leaves the
> expression unchanged while a Map of Simplify
>
> MapAll[Simplify, H[x, 0, 0]]
>
> produces
>
> If[x < 0,
> Simplify[Simplify[0] + Simplify[0]], Simplify[Simplify[
> 0] + Simplify[Simplify[-1] Simplify[0]]]]
>
> Something even worse happens if a use values already defined. For instance
>
> c = 0; d = 0; H[x_] = If[x < c, c + d, c - d]
>
> produces
>
> If[x < 0, c + d, c - d]
>
> In this case, the addition is not necessary.
>
> c = 0; d = 0; H[x_] = If[x < c, c, d]
>
> leads to
>
> If[x < 0, c, d]
>
> Similar problems arise using the function Which.
>
> Any explanation or help with this malfunction would be welcome.
>
> --
>
> Antonio
>
> (Eliminar el agua para responder por e-mail - Remove water to reply by
> e-mail)
>
>
--
Chris Chiasson
Kettering University
Mechanical Engineering
Graduate Student
1 810 265 3161
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