PositionIndex

PositionIndex[list] gives an association between unique elements in list and the positions at which they occur.

• PositionIndex[assoc] gives an association whose keys are the distinct values in assoc, and whose values are lists of the keys with which they are associated.

Examples

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

PositionIndex[<|"x" -> 1, "y" -> 2, "z" -> 1|>]

Please visit the official Wolfram Language Reference for more details.