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Re: What is f[1]? Advanced question

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  • Subject: [mg131323] Re: What is f[1]? Advanced question
  • From: John Doty <noqsiaerospace at gmail.com>
  • Date: Fri, 28 Jun 2013 04:13:53 -0400 (EDT)
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On Thursday, June 27, 2013 4:25:04 AM UTC-6, amannucci wrote:
> Thanks. This is very helpful. I remain puzzled by this:
>
>
>
> u // FullForm
>
> FullForm[Symbol["u"]]
>
> FullForm[Unevaluated[Symbol["u"]]
>
> Head[u]
>
>
>
> FullForm[{1, 2, 3}]
>
> Head[{1, 2, 3}]
>
>
>
> In other words, Head and FullForm seem consist for the list, and inconsistent for the variable u. u's full form is just, literally "u", not Symbol["u"]. But the list's FullForm starts with "List". Head of Symbol["u"] is not Symbol, but "u". What gives?

If FullForm did what you expected, its output would be infinite, because every expression in Mathematica has a head. Remember that the head of an expression is itself an (arbitrary!) expression. So, FullForm "bottoms out" when further expansion would expose an elementary head like Symbol or Integer.



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