add mapValues to Map<K, V>

[polux]

I often find myself writing

Map newMap = new Map();
m.forEach((k, v) {
  newMap[k] = f(v);
});
...

It would be great if Map had a map or mapValues method instead:

Map newMap = m.mapValues(f);

[DartBot]

This comment was originally written by @seaneagan

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If we had issue #7088, then could do:

var newMap = new Map.fromPairs<K, V>(m.pairs.map((pair) => new Pair(pair.key, f(pair.value))));

and if Map instead directly extended Iterable<Pair<K, V>> it could be:

var newMap = new Map.from(m.map((pair) => new Pair(pair.key, f(pair.value))));

[DartBot]

This comment was originally written by [email protected]

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Added Area-Library label.

[anders-sandholm]

Added Triaged label.

[DartBot]

This comment was originally written by @seaneagan

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FWIW I've been using this in the meantime:

Map mapValues(Map map, var valueMapper(key)) => map.keys.fold({}, (result, key) {
  result[key] = valueMapper(map[key]);
  return result;
});

[DartBot]

This comment was originally written by @seaneagan

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An alternative would be to add optional key and value callbacks to Map.from similar to Map.fromIterable:

factory Map.from(Map other, {K key(key), V value(value)});

[kasperl]

Issue #21780 has been merged into this issue.

[lrhn]

You can use Map.fromIterables:

  var mappedMap = new Map.fromIterables(map.keys, map.values.map(transform));

There is a limit to how many specialized transformers we want to add.

Another alternative is to create a wrapper. It's not that complex:

  import "dart:collection";
  class MapMap<K,V> extends UnmodifiableMapBase<K,V> {
    final Map<K, dynamic> _map;
    final Function _valueTransform; // Should map null to null.
    MapMap(Map<K, dynamic> map, V valueTransform(oldValue))
         : _map = map, _valueTransform = valueTransform;
    V operator[](K key) => _valueTransform(_map[key]);
    Iterable<K> get keys => _map.keys;
  }
    

[srawlins]

Map.map has landed.