GroupBy

GroupBy[{elem1, elem2, ...}, f] gives an association that groups elements into lists associated with distinct keys f[elemi].

Examples

Group by a function:

GroupBy[{1, 2, 3, 4, 5, 6}, EvenQ]
(* <|False -> {1, 3, 5}, True -> {2, 4, 6}|> *)

Group strings by first letter:

GroupBy[{"apple", "banana", "avocado", "blueberry"}, StringTake[#, 1] &]
(* <|"a" -> {"apple", "avocado"}, "b" -> {"banana", "blueberry"}|> *)

Please visit the official Wolfram Language Reference for more details.