performance tuning - How to find all the positions of max value of a list efficiently?

For Example, list=RandomInteger[{1,100},2000]. Yes,I know Position[list,Max[list]] can do. But it's based on pattern matching! Ordering[list,-1] could find one position but not all. So how to find all the positions of max value of list in a more efficient way?Thank you.

Answer

A fast uncompiled alternative without pattern matching is to use the NonzeroPositions property of SparseArray, as long as you're dealing with numerical data.

list = RandomInteger[{1, 100}, 10^7];

SparseArray[Unitize[list - Max[list]] - 1]["NonzeroPositions"]; // RepeatedTiming
(* 0.0855 *)

Position[list, Max[list]] // RepeatedTiming
(* 0.509 *)

compPos[list, Max[list]] // RepeatedTiming (* Marius' solution *)
(* 0.0366 *)


SparseArray[Unitize[list - Max[list]], Automatic, 1][
   "NonzeroPositions"]; // RepeatedTiming (* MichaelE2's solutions *)
(* 0.0663 *)

Comments

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