by<K, V> method

@useResult Map<K, V> by<K, V>(Select<E, K> by, { required V as(V? previous, E current), })

Groups the iterable's elements by keys returned by by before being folded using as.

The order of elements passed to as is non-deterministic when the iterable is unordered, i.e. HashSet.

Example

final list = [('a', 1), ('a', 2), ('b', 4)];
final counts = list.group.by((e) => e.$1, as: (count, e) => (count ?? 0) + e.$2);

print(counts); // {'a': 3, 'b': 4}

Implementation details

Computing K is assumed to be cheap. Hence, Ks are recomputed each time rather than cached.

Implementation

@useResult Map<K, V> by<K, V>(Select<E, K> by, {required V Function(V? previous, E current) as}) {
  final results = <K, V>{};
  for (final element in _iterable) {
    final key = by(element);
    results[key] = as(results[key], element);
  }

  return results;
}