Returns a map with the unique values of a list as keys and their frequencies as the values.

Use Iterable.toSet() to get the unique values of the list.
Use Map.fromIterable(), Iterable.where() and Iterable.length to generate a map with the unique values as keys and their frequencies as values.

代码实现

Map<T, int> frequencies<T>(Iterable<T> itr) {
  return Map.fromIterable(itr.toSet(),
      value: (i) => itr.where((v) => v == i).length);
}

使用样例

frequencies(['a', 'b', 'a', 'c', 'a', 'a', 'b']); // { a: 4, b: 2, c: 1 }