4

consider this example:

list = {a, b, c};
Table[{Sin[i], Cos[j]}, {i, 1, 3}, {j, list[[i ;;]]}]

(*{{{Sin[1], Cos[a]}, {Sin[1], Cos[b]}, {Sin[1], Cos[c]}}, {{Sin[2], 
   Cos[b]}, {Sin[2], Cos[c]}}, {{Sin[3], Cos[c]}}}*)

for the following code:

{Sin[#], Cos[#]} & /@ list[[# ;;]] & /@ {1, 2, 3}

we can get same result if the slot inside Sin (Sin[#]) takes the values from the outer range ({1,2,3})

so in general, is it possible to specify which range that slot takes from?

Basheer Algohi
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3 Answers3

7

My first suggestion would be to localize the outer Slot, like this:

With[{indx = #}, {Sin[indx], Cos[#]} & /@ list[[indx ;;]]] & /@  Range[3]

(* {{{Sin[1], Cos[a]}, {Sin[1], Cos[b]}, {Sin[1], Cos[c]}}, {{Sin[2], 
   Cos[b]}, {Sin[2], Cos[c]}}, {{Sin[3], Cos[c]}}} *)

You could also rework the process to get the full set of tuples of {Sin[1],Sin[2],Sin[3]} and {Cos[a],Cos[b],Cos[c]} and keep the ones you want, using something like:

MapThread[#1[[#2 ;;]] &, {Outer[{Sin[#1], Cos[#2]} &, Range[3], list],
   Range[3]}]
Verbeia
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6

You can nest Functions to accomplish what I believe you want to:

f = i \[Function] {Sin[i], Cos[#]} & /@ list[[i ;;]];

Array[f, 3] // Column

{{Sin[1],Cos[a]},{Sin[1],Cos[b]},{Sin[1],Cos[c]}}
{{Sin[2],Cos[b]},{Sin[2],Cos[c]}}
{{Sin[3],Cos[c]}}

This looks nicer in the Notebook:

enter image description here

Other methods include:

# /. i_ :> ({Sin[i], Cos[#]} & /@ list[[i ;;]]) & /@ {1, 2, 3}

With[{i = #}, {Sin[i], Cos[#]} & /@ list[[i ;;]]] & /@ {1, 2, 3}  (* Verbeia's *)

Nevertheless I feel that the Table construct is at least as clear as any of these.

Mr.Wizard
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5

is it possible to specify which range that slot takes from?

The short answer is "no" (not counting solutions with named arguments since they aren't Slots) but if you really, really want to do it you can write it like I've done below, then #1 refers to the inner Map and #2 refers to the outer Map:

Map[
 Map[
   {Sin[#], Cos[#2]} & @@ # &,
   Thread@{#, list[[# ;;]]}
   ] &,
 {1, 2, 3}
 ]

This technique can be used for more levels I think, so that if you had four Map inside each other you could use #1, #2, #3 and #4 to refer to the different levels. But it isn't worth the trouble to start thinking about it.

C. E.
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