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MathGroup Archive 2006

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Re: x=2;Composition[f,FindMinimum][x+1,{x,a}]

  • To: mathgroup at smc.vnet.net
  • Subject: [mg68327] Re: [mg68282] x=2;Composition[f,FindMinimum][x+1,{x,a}]
  • From: "Carl K. Woll" <carlw at wolfram.com>
  • Date: Tue, 1 Aug 2006 06:59:58 -0400 (EDT)
  • References: <200607310745.DAA26874@smc.vnet.net> <44CDF8C9.3070607@wolfram.com> <acbec1a40607311814sf84582wcf4a6d608a0196f1@mail.gmail.com>
  • Sender: owner-wri-mathgroup at wolfram.com

Chris Chiasson wrote:
> The crash confused matters for me. Thank you for clearing that up. Now
> that the source is known, perhaps someone could crack the "original"
> question I am trying to solve:
> 
> x=2;{Composition[Hold][x+1,{x,a}],Composition[Hold,Hold][x+1,{x,a}]}
> 
> produces
> 
> {Hold[x+1,{x,a}],Hold[Hold[3,{2,a}]]}
> 
> Why is it that in Composition[Hold], Composition does not effectively
> block the HoldAll attribute of Hold? The same thing happens with
> Identity.
> 

It's because Mathematica evaluates the head first and then the body. In 
the first case the head evaluates to Hold:

In[3]:= Composition[Hold]
Out[3]= Hold

So,

Composition[Hold][x+1,{x,a}] ---> Hold[x+1,{x,a}]

and Mathematica doesn't evaluate the body because of the Hold. In the 
second case the head doesn't evaluate to anything:

In[4]:= Composition[Hold, Hold]
Out[4]= Composition[Hold,Hold]

So, the body gets evaluated, and then the Composition takes place

Composition[Hold,Hold][x+1,{x,a}] --->
Composition[Hold,Hold][3,{2,a}] --->
Hold[Hold[3,{2,a}]]

This can be seen by looking at the Trace.

Carl Woll
Wolfram Research

> On 7/31/06, Carl K. Woll <carlw at wolfram.com> wrote:
>> Chris Chiasson wrote:
>> > The above command crashes my kernel on Windows XP and Linux. Anybody 
>> else?
>> >
>>
>> The crash comes when evaluating
>>
>> FindMinimum[3,{2,a}]
>>
>> Presumably you did not intend to evaluate this.
>>
>> In the development version, the above code does not cause a crash.
>>
>> Carl Woll
>> Wolfram Research
>>
> 
> 


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