Mapping two list of Objects by key

Question

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"),
];

jamesdlin · Accepted Answer · 2022-07-27 18:16:07Z

2

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).

answered Jul 27, 2022 at 18:16

jamesdlin

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2 Comments

Sunshine Over a year ago

hi thanks for your answer 🙏truth is my list is quite small & I actually have two values for each KeyValue(key, value1, value2) so converting them to Map won't work - but I will up vote : )

jamesdlin Over a year ago

@Sunshine Converting to a Map would still work with multiple values; you'd just need the Map value to be a List of values or a KeyValue object itself.

Md. Yeasin Sheikh · Accepted Answer · 2022-07-27 12:01:51Z

1

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,
    );
  }
}

answered Jul 27, 2022 at 12:01

Md. Yeasin Sheikh

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3 Comments

Sunshine Over a year ago

Thank you 🙏 do I have to keep listCustomClass as var for it to be modified with the new values from your for loop or can I make it final ? 🙂

Md. Yeasin Sheikh Over a year ago

Yes listCustomClass can be final, we are modifying elements here

jamesdlin Over a year ago

Note that this is O(m * n) with respect to the lengths of listCustomClass and listKeyValue. That should be okay if the lists are small, but it will become inefficient if they're large.

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