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I need to use:

Composition[G][x]

in the elements of the following list:

j = {{a, b, c}, {c, b, a}};

That is, I could do:

Table[Composition[j[[n]]][x], {n, 1, 2}]

which produces:

{{a, b, c}[x], {c, b, a}[x]}

but Composition doesn't seem to work with lists. I can't evaluate Composition[{a,b,c}][x]; it accepts only Composition[a,b,c][x].

Is there a simple way to convert Composition[{a,b,c}] to Composition[a,b,c]?

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4 Answers 4

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$\begingroup$ 
lis = {a, b, c}
Composition[Sequence @@ lis][x]

Mathematica graphics

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  • $\begingroup$ Doesn't just Composition @@ lis work just fine? $\endgroup$
    – LSpice
    Nov 22, 2021 at 2:12
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    $\begingroup$ @LSpice yes, and it needs to be enclosed by parentheses before applying to an argument, e.g. (Composition @@ lis)[x] $\endgroup$
    – polfosol
    Nov 22, 2021 at 15:27
  • $\begingroup$ @polfosol or just x // Composition @@ lis $\endgroup$
    – AsukaMinato
    Nov 29, 2021 at 18:49
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Apply is designed for this purpose:

Apply[Composition][{a, b, c}][x]
(*  a[b[c[x]]]  *)

Or

Apply[Composition, {a, b, c}][x]
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    $\begingroup$ Shorthand for Apply is @@. E.g. (Composition @@ {a,b,c})[x] or x // Composition @@ {a,b,c}. $\endgroup$
    – Džuris
    Nov 20, 2021 at 11:52
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    $\begingroup$ @Džuris Yep, I usually use @@. Except, a personal preference, I dislike parentheses, which led me to avoid @@ in these situations. With command completion, the length of typing is about the same, and no parentheses feels easier to type & read for me. $\endgroup$
    – Michael E2
    Nov 20, 2021 at 21:24
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With:

alist = {a, b, c}

A variant using Fold could be:

Fold[Composition, alist][x]

a[b[c[x]]]

Another variant using ComposeList could be:

ComposeList[Reverse@alist, x]

{x, c[x], b[c[x]], a[b[c[x]]]}

from which the last item can be extracted.

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    $\begingroup$ x // Fold[Composition] /@ {{a, b, c}, {c, b, a}} // Through shows how naturally Fold and Composition can express the solution to OP's problem. Or myFuncList = Fold[Composition] /@ {{a, b, c}, {c, b, a}}; which can be used later, either as individual functions or in Through[myFuncList[x]] $\endgroup$
    – Michael E2
    Nov 20, 2021 at 15:52
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    $\begingroup$ A variation of this solution is Fold[ReverseApplied[Construct], x, Reverse@{a, b, c}]. $\endgroup$
    – J. M.'s lack of A.I. ♦
    Dec 9, 2021 at 20:36
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Since no one seems to have answered how to Apply Composition to a List like j given by OP, here it is:

Composition[##][x]&@@#&/@j

{a[b[c[x]]],c[b[a[x]]]}

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    $\begingroup$ Or Composition[##][x]&@@@j if always a list of lists of functions. :) $\endgroup$
    – Michael E2
    Nov 21, 2021 at 14:09
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    $\begingroup$ A simpler version is: #[x]& /@ Composition@@@j $\endgroup$
    – polfosol
    Nov 22, 2021 at 15:34

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