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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}}?

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  • $\begingroup$ This answer to a previous question seems relevant. $\endgroup$
    – m_goldberg
    Commented Apr 25, 2014 at 9:23

2 Answers 2

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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:

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}}
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  • $\begingroup$ +1 for detailed explanation, concur that part is duped somewhere... $\endgroup$
    – ciao
    Commented Apr 25, 2014 at 1:33
  • $\begingroup$ @rasher Thanks. Let me know if you find that duplicate. $\endgroup$
    – Mr.Wizard
    Commented Apr 25, 2014 at 1:37
  • $\begingroup$ @MrWizard Thanks, this is very helpful. Also scanned for the dup but didn't see it $\endgroup$
    – mfvonh
    Commented Apr 25, 2014 at 1:42
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    $\begingroup$ Thanks for the link. The Villegas paper is a "must read". $\endgroup$
    – m_goldberg
    Commented Apr 25, 2014 at 9:19
  • $\begingroup$ Perhaps this is the previous answer you are trying to remember? $\endgroup$
    – m_goldberg
    Commented Apr 25, 2014 at 9:25
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E.g.,

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

(*  {1,a,5}  *)
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  • $\begingroup$ Thanks for the reply. It made me realize I wasn't asking the right question; I've edited the post. $\endgroup$
    – mfvonh
    Commented Apr 25, 2014 at 1:04
  • $\begingroup$ @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... $\endgroup$
    – ciao
    Commented Apr 25, 2014 at 1:15

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