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MathGroup Archive 2001

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Re: Re: Re: Thread and MapThread

  • To: mathgroup at smc.vnet.net
  • Subject: [mg29903] Re: [mg29882] Re: [mg29870] Re: Thread and MapThread
  • From: Andrzej Kozlowski <andrzej at tuins.ac.jp>
  • Date: Wed, 18 Jul 2001 02:08:46 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

In general this is not a good idea. There is a reason why Thread evaluates
its arguments and giving it the HoldFirst attribute will change its
behaviour and make many constructions fail. Just compare these two (out of
many possible) examples:

Normal behaviour:

In[1]:=
Thread[Through[p[g,h][x]],g]

Out[1]=
g[p[x,h[x]]]

After adding the HoldFirst attribute:

SetAttributes[Thread, HoldFirst]

In[3]:=
Thread[Through[p[g,h][x]],g]

Out[3]=
p[g[x],h[x]]

If you must change the attributes of Thread (or any built-in function) it is
better to use the anonymous function Function[y, Thread[y], HoldFirst],
which is equivalent to  Thread with the HoldFirst attribute but does not
affect the behaviour of Thread itself.

-- 
Andrzej Kozlowski
Toyama International University
JAPAN

http://platon.c.u-tokyo.ac.jp/andrzej/
http://sigma.tuins.ac.jp/~andrzej/


on 01.7.17 2:00 PM, Mianlai Zhou at lailai at nikhef.nl wrote:

> There is another alternative to fulfill that:
> 
> SetAttributes[Thread, HoldFirst];
> Thread[ ftest[{x1, x2, x3}, {y1, y2, y3}] ]
> 
> Then it will give the answer you want:
> 
> {{f1[x1], f2[y1]}, {f1[x2], f2[y2]}, {f1[x3], f2[y3]}}
> 
> Enjoy it.
> 
> Mianlai Zhou
> Theory Group, NIKHEF
> Amsterdam, The Netherlands
> 
> On Sun, 15 Jul 2001, Allan Hayes wrote:
> 
>> David,
>> 
>> Here are the relevant evaluation steps of
>> Thread[ftest[{x1, x2, x3}, {y1, y2, y3}]]
>> 
>> Thread[ {f1[{x1, x2, x3}], f2[{y1, y2, y3}]}]
>> (*ftest[...] evaluates before Thread acts*)
>> 
>> {f1[{x1, x2, x3}], f2[{y1, y2, y3}]}]
>> (*Thread does nothing since it has not, for example, been told to thread
>> over f1}*)
>> 
>> Compare
>> 
>> Thread[ftest[{x1,x2,x3},{y1,y2,y3}],f1]
>> 
>> f1[{{x1, x2, x3}, f2[{y1, y2, y3}]}]
>> 
>> 
>> There are many ways of avoiding premature evaluation, here are a few
>> 
>> Thread[Unevaluated[ftest[{x1,x2,x3},{y1,y2,y3}]]] (*caution:x1, ... are
>> held*)
>> 
>> {{f1[x1], f2[y1]}, {f1[x2], f2[y2]}, {f1[x3], f2[y3]}}
>> 
>> 
>> Block[{ftest},Thread[ftest[{x1,x2,x3},{y1,y2,y3}]]]
>> 
>> {{f1[x1], f2[y1]}, {f1[x2], f2[y2]}, {f1[x3], f2[y3]}}
>> 
>> 
>> Evaluate[Thread[#[{x1,x2,x3},{y1,y2,y3}]]]&[ftest]
>> 
>> {{f1[x1], f2[y1]}, {f1[x2], f2[y2]}, {f1[x3], f2[y3]}}
>> 
>> --
>> Allan
>> ---------------------
>> Allan Hayes
>> Mathematica Training and Consulting
>> Leicester UK
>> www.haystack.demon.co.uk
>> hay at haystack.demon.co.uk
>> Voice: +44 (0)116 271 4198
>> Fax: +44 (0)870 164 0565
>> 
> 
> 
> 




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