CountsBy

CountsBy[{e1, e2, ...}, f] gives an association whose keys are the distinct values of the f[ei], and whose values give the number of times these f[ei] values appear.

CountsBy[f] represents an operator form of CountsBy that can be applied to an expression.

Examples

Count by absolute value:

CountsBy[{-2, -1, 1, 2, 2}, Abs]
(* <|2 -> 3, 1 -> 2|> *)

Count words by length:

CountsBy[{"cat", "dog", "bird", "fish"}, StringLength]
(* <|3 -> 2, 4 -> 2|> *)

Please visit the official Wolfram Language Reference for more details.