Mapping two list of Objects by key-CodePudding

I have two list of objects like so :

class KeyValue {
  KeyValue({required this.key, required this.value});

  final String key;
  final String value;
}

class CustomClass {
  CustomClass({
    required this.key,
    required this.name,
    this.value,
  });

  final String key;
  final String name;
  final String? value;
}

var listCustomClass = [
  CustomClass(key: "1234", name: "name1"),
  CustomClass(key: "1234", name: "name2"),
  CustomClass(key: "12345", name: "name3"),
  CustomClass(key: "12345678", name: "name4"),
];

final listKeyValue = [
  KeyValue(key: "1234", value: "someValue1"),
  KeyValue(key: "12345", value: "someValue2"),
  KeyValue(key: "12345678", value: "someValue3"),
  KeyValue(key: "123456789", value: "someValue4"),
];

I'd like to map listCustomClass & listKeyValue by key in order to create create a new List<CustomClass> with each CustomClass() taking its value parameter from its matching KeyValue() object (the one with the same key)

it would create newListCustomClass like so :


newListCustomClass = [
  CustomClass(key: "1234", name: "name1", value: "someValue1"),
  CustomClass(key: "1234", name: "name2", value: "someValue1"),
  CustomClass(key: "12345", name: "name3", value: "someValue2"),
  CustomClass(key: "12345678", name: "name4", value: "someValue3"),
];

CodePudding user response：

If listCustomClass or listKeyValue are large, you would be better off converting one of them to a Map first. Since listCustomClass apparently can have elements with the same key, that one should stay a List.

  var mapKeyValue = {
    for (var keyValue in listKeyValue) keyValue.key: keyValue.value,
  };

  var newListCustomClass = [
    for (var customClass in listCustomClass)
      CustomClass(
        key: customClass.key,
        name: customClass.name,
        value: mapKeyValue[customClass.key],
      ),
  ];

If listCustomClass has m elements and listKeyValues has n elements, then the above code would be O(m n). If you store listKeyValues as a Map in the first place, then the code to convert from List to Map could be removed, and it would be just O(m).

CodePudding user response：

You can do it like

  String getValue(String key) =>
      listKeyValue.firstWhere((element) => element.key == key).value;

  for (int i = 0; i < listCustomClass.length; i  ) {
    listCustomClass[i] =
        listCustomClass[i].copyWith(value: getValue(listCustomClass[i].key));
  }

And the class

class CustomClass {
  CustomClass({
    required this.key,
    required this.name,
    this.value,
  });

  final String key;
  final String name;
  final String? value;

  @override
  String toString() => 'CustomClass(key: $key, name: $name, value: $value)';

  CustomClass copyWith({
    String? key,
    String? name,
    String? value,
  }) {
    return CustomClass(
      key: key ?? this.key,
      name: name ?? this.name,
      value: value ?? this.value,
    );
  }
}