groupBy function

Map<dynamic, List> groupBy(Iterable collection, [ dynamic iteratee ])

Groups the elements of an Iterable collection based on a specified iteratee.

The groupBy function takes an Iterable collection and an optional iteratee. It returns a Map where the keys are derived from the elements of the collection according to the iteratee, and the values are lists of elements that correspond to each key.

The iteratee can be:

• null: If no iteratee is provided, the elements themselves are used as keys.
• A Function: If a function is provided, it is applied to each element to determine the corresponding key.
• A String: If a string is provided, it is used as a key to access properties of elements (assuming elements are maps).

Throws an ArgumentError if the iteratee is neither a function nor a string, or if the iteratee is a string and the elements are not maps.

Example usage:

var numbers = [1, 2, 2, 3, 3, 3];
var groupedByValue = groupBy(numbers); // {1: [1], 2: [2, 2], 3: [3, 3, 3]}

var people = [
  {'name': 'Alice', 'age': 30},
  {'name': 'Bob', 'age': 30},
  {'name': 'Charlie', 'age': 40},
];
var groupedByAge = groupBy(people, 'age'); // {30: [{'name': 'Alice', 'age': 30}, {'name': 'Bob', 'age': 30}], 40: [{'name': 'Charlie', 'age': 40}]}

var words = ['apple', 'banana', 'cherry'];
var groupedByFirstLetter = groupBy(words, (word) => word[0]); // {'a': ['apple'], 'b': ['banana'], 'c': ['cherry']}

• Parameters:

  • collection: The Iterable collection to group.
  • iteratee: An optional iteratee which can be a Function or a String.

• Returns: A Map where the keys are derived from the elements of the collection according to the iteratee, and the values are lists of elements that correspond to each key.

• Throws: ArgumentError if the iteratee is invalid.

Implementation

Map<dynamic, List<dynamic>> groupBy(Iterable collection, [dynamic iteratee]) {
  var result = <dynamic, List<dynamic>>{};

  for (var element in collection) {
    var key;
    if (iteratee == null) {
      key = element;
    } else if (iteratee is Function) {
      key = iteratee(element);
    } else if (iteratee is String) {
      if (element is Map) {
        key = element[iteratee];
      } else {
        throw ArgumentError('Invalid element type for property access');
      }
    } else {
      throw ArgumentError('Invalid iteratee: must be a function or a string');
    }

    if (!result.containsKey(key)) {
      result[key] = [];
    }
    result[key]!.add(element);
  }

  return result;
}