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MathGroup Archive 1999

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[Q] Thread[] and Hold[]

  • To: mathgroup at smc.vnet.net
  • Subject: [mg18785] [Q] Thread[] and Hold[]
  • From: "Kevin Jaffe" <kj0 at mailcity.com>
  • Date: Tue, 20 Jul 1999 01:33:37 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

to comp.soft-sys.math.mathematica, I learned a neat, but rather
puzzling, trick:

In[4]:= Thread[Hold[{a,b,c}]]  (* This I understand *)

Out[4]= {Hold[a], Hold[b], Hold[c]}

In[5]:= Thread[%, Hold]  (* This baffles me *)

Out[5]= Hold[{a, b, c}]


I can't explain why the form in [5] would be the inverse of the form
in [4], but be that as it may, with this maneuver one can do nifty
things like:

In[16]:= Thread[{Hold[1 + 1], Hold[3^2], Hold[3*6]}, Hold]

                       2
Out[16]= Hold[{1 + 1, 3 , 3 6}]

Neither under Thread nor Hold could I find any explanation in the
Mathematica Book for the behavior in [5].  Does anybody know an
explanation for it?

Thanks,

KJ



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