Manipulating slots in a pure function

Question

How does one manipulate the slot numbers in a pure function? Some trick with Evaluate or Hold? I'm aiming for something along the lines of:

(Slot /@ Range[19, 164, 29])& @@ {...}

EDIT

It turns out my actual problem is: why does

Evaluate[{Slot /@ Range[19, 164, 29]}] & @@ Range[164]

yield {{19, 48, 77, 106, 135, 164}} but

{Evaluate[Slot /@ Range[19, 164, 29]]} & @@ Range[164]

yields {{#19, #48, #77, #106, #135, #164}}?

This answer to a previous question seems relevant. – m_goldberg Apr 25 '14 at 09:23 — m_goldberg, Apr 25 '14 at 09:23

Mr.Wizard · Accepted Answer · 2014-04-25T02:00:03.457

12

Responding to your updated question, which should probably be closed as a duplicate once someone takes the time to find the original: Evaluate only works when it is the explicit head of an argument. In other words Evaluate[ . . . ] must appear as one of the arguments of the Head who's Hold attribute you wish to override. You should read this paper, which teaches this among many other useful things:

Working with Unevaluated Expressions - Robby Villegas

As an example consider these lines:

Hold[1 + 1, Evaluate[2 + 2]]
Hold[1 + 1, {Evaluate[2 + 2]}]
Hold[1 + 1, Evaluate @@ {2 + 2}]

Hold[1 + 1, 4]
Hold[1 + 1, {Evaluate[2 + 2]}]
Hold[1 + 1, Evaluate @@ {2 + 2}]

Note that only the first form evaluates. On lines two and three the Heads of the second arguments are List and Apply rather than Evaluate.

A common method to get around this is to use With to inject an expression into the body of the function:

With[{body = Slot /@ Range[19, 164, 29]}, {body} &]

% @@ Range[164]

{{#19, #48, #77, #106, #135, #164}} &
{{19, 48, 77, 106, 135, 164}}

If you want to evaluate the entire body every time you can just Apply Function to a List of the components that form the Function:

(Function @@ {{Slot /@ Range[19, 164, 29]}}) @@ Range[164]

{{19, 48, 77, 106, 135, 164}}

edited Apr 25 '14 at 02:00

answered Apr 25 '14 at 01:30

Mr.Wizard

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+1 for detailed explanation, concur that part is duped somewhere... – ciao Apr 25 '14 at 01:33
@rasher Thanks. Let me know if you find that duplicate. – Mr.Wizard Apr 25 '14 at 01:37
@MrWizard Thanks, this is very helpful. Also scanned for the dup but didn't see it – mfvonh Apr 25 '14 at 01:42
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Thanks for the link. The Villegas paper is a "must read". – m_goldberg Apr 25 '14 at 09:19
Perhaps this is the previous answer you are trying to remember? – m_goldberg Apr 25 '14 at 09:25
@m_goldberg Thanks. That could be the one. I don't think these are quite similar enough to close; do you agree? – Mr.Wizard Apr 25 '14 at 09:41
I wouldn't close. The cited answer is related and should be linked to this one, which I have done (both here and ina comment to the question) – m_goldberg Apr 25 '14 at 10:01
Related: (25266) – Mr.Wizard Feb 07 '16 at 14:48

score 3 · Answer 2 · answered Apr 25 '14 at 00:50

3

E.g.,

Evaluate[Slot /@ Range[1, 5, 2]] & @@ {1, 2, a, 4, 5, 6, 7}

(*  {1,a,5}  *)

answered Apr 25 '14 at 00:50

ciao

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Thanks for the reply. It made me realize I wasn't asking the right question; I've edited the post. – mfvonh Apr 25 '14 at 01:04
@mfvonh: Look at the FullForm of each. One produces a function with a list of slots, the other produces a function with a list with only one entry: Evaluate[Map[Slot, Range[19, 164, 29]]] giving you the list of unevaluated slots... – ciao Apr 25 '14 at 01:15

Manipulating slots in a pure function

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