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Re: Approximate Zero Times A Symbol

  • To: mathgroup at smc.vnet.net
  • Subject: [mg127068] Re: Approximate Zero Times A Symbol
  • From: Bob Hanlon <hanlonr357 at gmail.com>
  • Date: Thu, 28 Jun 2012 04:01:20 -0400 (EDT)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <201206270811.EAA18698@smc.vnet.net>

Zero times anything is zero  (Times help: "0 x evaluates to 0, and 0.0
x evaluates to 0.0."). For an extreme example,

0 * Plot[x, {x, 0, 1}]

0


Another approach:

Manipulate[
 theta = angle Degree;
 NumberForm[Row[{
     Sin[theta], " ", x, " + ",
     Cos[theta], " ", y}] // Chop,
  {7, 6}],
 {{angle, 0, "   angle\n{degrees)"}, 0, 90, 0.5,
  Appearance -> "Labeled"}]


Bob Hanlon


On Wed, Jun 27, 2012 at 12:25 PM, djmpark <djmpark at comcast.net> wrote:
> Thanks Bob, but I think I didn't express myself clearly enough. What I have
> is a dynamic display where the coefficients are varied and I want to retain
> x and y in the display even when their coefficients are zero.
>
> The following is what I ended up doing, but it does seem like a roundabout
> method to obtain the result.
>
> angle = 0.;
> Slider[Dynamic[angle], {0., 90.}, Appearance -> "Labeled"]
> Dynamic@NumberForm[
>  HoldForm[aa x + bb y] /.
>    Thread[{aa, bb} -> {Sin[angle Degree], Cos[angle Degree]}] //
>   Chop, {7, 6}]
>
> And I still don't understand why Mathematica should drop the x if x is not a
> numeric quantity.
>
> David Park
> djmpark at comcast.net
> http://home.comcast.net/~djmpark/index.html
>
>
>
>
> From: Bob Hanlon [mailto:hanlonr357 at gmail.com]
>
>
> Use Rationalize
>
> expr = 0. x + 1. y;
>
> expr // Rationalize
>
> y
>
> expr // Rationalize[#, 0] &
>
> y
>
> If there are any rational factors remaining you can use N with any desired
> precision to return to reals.
>
>
> Bob Hanlon
>
>
> On Wed, Jun 27, 2012 at 4:11 AM, djmpark <djmpark at comcast.net> wrote:
>>
>> What is the justification for the following?
>>
>>
>>
>> 0. x + 1. y
>>
>>
>>
>> 0. + 1. y
>>
>>
>>
>> I want to display a dynamic weighted sum of x and y and sometimes one
>> of the coefficients becomes zero. I would like to keep both terms (for
>> a steady
>> display) and format with NumberForm.  If Mathematica is going to drop
>> the x, why doesn't it at least also drop the approximate zero?
>>
>>
>>
>> If I use SetPrecision we obtain:
>>
>>
>>
>> SetPrecision[0. x + 1. y, 10]
>>
>>
>>
>> 1.000000000 y
>>
>>
>>
>> which is at least more consistent, but not what I want either.
>>
>>
>>
>> David Park
>>
>> djmpark at comcast.net
>>
>> http://home.comcast.net/~djmpark/index.html
>



--
Bob Hanlon



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