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Re: Why is FindRoot[] HoldAll?

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  • Subject: [mg81981] Re: [mg81957] Why is FindRoot[] HoldAll?
  • From: "Szabolcs HorvÃt" <szhorvat at>
  • Date: Mon, 8 Oct 2007 00:07:24 -0400 (EDT)
  • References: <>

On 10/7/07, Andrzej Kozlowski <akoz at> wrote:
> I am not sure from your message if you are aware of this:
> FindRoot[NIntegrate[Exp[a x], {x, 0, 1}] == 2, {a, 1}, Evaluated ->
> False]
> {a -> 1.25643}

Thanks for the reply!  I did not know about this option.  It is a new
option in Mathematica 6, but it seems to be undocumented (or at least
I cannot find it anywhere in the documentation).

These plotting functions accept this option too:
{ContourPlot, ContourPlot3D, DensityPlot, FindRoot, ParametricPlot, \
ParametricPlot3D, Plot, Plot3D, RegionPlot, RegionPlot3D}

Most of them have the default setting Evaluated -> Automatic (which
works with FindRoot too, though I have no idea how it decides whether
to evaluate the argument or not).

> No error messages. Of course, one would expect that using this option
> will in most cases have a negative impact ton the performance.
> Why the HoldAll attribute? Well, presumably without the HoldAll
> attribute the function would always be evaluated irrespective of any
> option settings so one could not use this approach. Not having the
> HoldAll or HoldFirst argument means that it will always be evaluated,
> before the function is applied. having it does not mean that the
> argument will not be evaluated; it only means it will not be "pre-
> evaluated". It may still be evaluated by the function itself.

With the Evaluated option, it all makes sense.

> Andrzej Kozlowski


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