I am streaming api. With the API, I get 1 item each and add to the list. The fact is that the api stream works in a circle, and duplicates are added to the list. How can I eliminate duplicates?
Code add list:
groupData.map((dynamic item) => GetOrder.fromJson(item))
.where((element) {
if (element.orderId != null) {
if (!list.contains(element)) {
list.add(element);
}
return true;
} else {
return false;
}
}).toList();
If elements are primitives, you can use a Set:
final myList = ['a', 'b', 'a'];
Set.from(myList).toList(); // == ['a', 'b']
but if elements are objects, a Set wouldn't work because every object is different from the others (unless you implement == and hashCode, but that goes beyond this answer)
class TestClass {
final String id;
TestClass(this.id);
}
...
final myClassList = [TestClass('a'), TestClass('b'), TestClass('a')];
Set.from(myClassList).toList(); // doesn't work! All classes are different
you should filter them, for example creating a map and getting its values:
class TestClass {
final String id;
TestClass(this.id);
}
...
final myClassList = [TestClass('a'), TestClass('b'), TestClass('a')];
final filteredClassList = myClassList
.fold<Map<String, TestClass>>({}, (map, c) {
map.putIfAbsent(c.id, () => c);
return map;
})
.values
.toList();
That said, this should work for you
groupData
.map((dynamic item) => GetOrder.fromJson(item))
.fold<Map<String, GetOrder>>({}, (map, element) {
map.putIfAbsent(element.orderId, () => element);
return map;
})
.values
.toList();
You can use Set instead
A Set is an unordered List without duplicates
If this is not working, then chances are that u have different object for the same actual object. (meaning, you have in 2 different places in memory)
In this case .contains or Set will not work
Related
i'm new in flutter and need to help:
I have already got
final List<Genres> genres = [{1,"comedy"}, {2,"drama"},{3,"horror"}]
from api.
class Genres {
final int id;
final String value;
Genres({this.id,this.value});
}
In another method I get genres.id.(2) How can I convert it to genres.value ("drama")?
Getting a Genre from an id is inconvenient when your data structure is a List. You have no choice but to iterate over the list and compare the id value to the id of each element in the list:
final id = 2;
final genre = genres.firstWhere((g) => g.id == id, orElse: () => null);
The problem with this code is that it's slow and there could be multiple matches (where the duplicates after the first found would be ignored).
A better approach would be to convert your list to a Map when you first create it. Afterwards, you can simply use an indexer to get a Genre for an ID quickly and safely.
final genresMap = Map.fromIterable(genres, (item) => item.id, (item) => item);
// later...
final id = 2;
final genre = genresMap[id];
This way, there is guaranteed to not be any duplicates, and if an ID doesn't exist then the indexer will simply return null.
you could iterate over the json result of the api and map them to the Gener class like so,
void fn(id) {
final gener = geners.firstWhere((gener) => gener['id'] == id);
// now you have access to your gener
}
You can find the item inside the List<Genres> like this
Genres element = list.firstWhere((element) => element.id == 2); // 2 being the id you give in the question as an exaple. You should make it dynamic
print(element.value);
I am trying to understand the code which is present in the line
list = (json.decode(response.body) as List)
.map((data) => PhotoData.fromJson(data))
.toList();
I have the following questions
a) What does .map do ?
b) What is 'data' in the above code
c) Could you please simplify the code into long form so that I can better grasp it.
The complete function is listed as below:
_fetchData() async {
http.Response response =
await http.get("https://jsonplaceholder.typicode.com/photos");
print("Fetching data...");
list = (json.decode(response.body) as List)
.map((data) => PhotoData.fromJson(data))
.toList();
setState(() {
isLoading =false;
});
}
factory PhotoData.fromJson(Map<String, dynamic> json) {
return PhotoData( json['id'], json['title'],
json['thumbnailUrl'], json['url']);
}
map is a method inside class Iterable. Since class List implements EfficientLengthIterable<E> therefore it inherits the method map.
According to the docs, the map does the following:
Returns a new lazy Iterable with elements that are created by calling f on each element of this Iterable in iteration order.
This is the implemenation of the map method:
Iterable<T> map<T>(T f(E e)) => MappedIterable<E, T>(this, f);
Iterable<T> => this means that the map method will return an Iterable
f(E e) => means that map will contain a function with element of type E
=> the arrow is a shorthand of writing return
Example:
class Person
{
String firstName;
String lastName;
Person(this.firstName, this.lastName);
}
void main() {
List<Person> people = new List<Person>();
people.add(new Person("Joe", "Smithers"));
people.add(new Person("Patrick", "Thomas"));
var mappedNames = people.map<String>((Person n) => 'Mr. ${n.firstName} ${n.lastName}');
print(mappedNames);
}
In this example map returns an iterable of type String, thus you can add map<String>, and variable n is of type Person, thus you can write Person n.
The type of n is equivalent to the type of variable people. Since here basically map method is iterating inside the list people and returning a new Iterable according to what you wrote in the return statement.
Therefore print(mappedNames) will give you the following:
(Mr. Joe Smithers, Mr. Patrick Thomas)
Note: you dont have to add the types since dart infers the type.
In your code you have the following:
list = (json.decode(response.body) as List)
.map((data) => PhotoData.fromJson(data))
.toList();
(json.decode(response.body) as List) which means that you are casting the json object to type List, then using map you iterate inside of it and return an iterable.
Then you use toList() to create a List from the returned iterable
Did several google searches, nothing helpful came up. Been banging my head against some errors when trying to do something that should be pretty simple. Convert a map such as {2019-07-26 15:08:42.889861: 150, 2019-07-27 10:26:28.909330: 182} into a list of objects with the format:
class Weight {
final DateTime date;
final double weight;
bool selected = false;
Weight(this.date, this.weight);
}
I've tried things like: List<Weight> weightData = weights.map((key, value) => Weight(key, value));
There's no toList() method for maps, apparently. So far I'm not loving maps in dart. Nomenclature is confusing between the object type map and the map function. Makes troubleshooting on the internet excruciating.
Following on Richard Heap's comment above, I would:
List<Weight> weightData =
mapData.entries.map( (entry) => Weight(entry.key, entry.value)).toList();
Don't forget to call toList, as Dart's map returns a kind of Iterable.
List<Weight> weightData = List();
weights.forEach((k,v) => weightData.add(Weight(k,v)));
Sometimes the typecast will fail and you can enforce it by doing:
List<Weight> weightData =
weightData.entries.map<Weight>( (entry) => Weight(entry.key, entry.value)).toList();
Example from my project where it wasn't working without typecast:
List<NetworkOption> networkOptions = response.data['data']['networks']
.map<NetworkOption>((x) => NetworkOption.fromJson(x))
.toList();
Use the entries property on the map object
This returns a List of type MapEntry<key,value>.
myMap.entries.map((entry) => "${entry.key} + ${entry.value}").toList();
You can also use a for collection to achieve the same.
var list = [for (var e in map.entries) FooClass(e.key, e.value)];
Details
Flutter 1.26.0-18.0.pre.106
Solution
/libs/extensions/map.dart
extension ListFromMap<Key, Element> on Map<Key, Element> {
List<T> toList<T>(
T Function(MapEntry<Key, Element> entry) getElement) =>
entries.map(getElement).toList();
}
Usage
import 'package:myApp/libs/extensions/map.dart';
final map = {'a': 1, 'b': 2};
print(map.toList((e) => e.value));
print(map.toList((e) => e.key));
You can do this:
List<Weight> weightData = (weights as List ?? []).map((key, value) => Weight(key,value)).toList()
or you can try:
List<Weight> weightData = List.from(weights.map((key, value) => Weight(key, value)))
If you need to convert Map values to a list, the simplest oneline code looks like this:
final list = map.values.toList();
Vidor answer is correct .any way this worked for me
List<String> list = new List();
userDetails.forEach((k, v) => list.add(userDetails[k].toString()));
its very simple just initialize a list of your custom object like this
List<CustomObject> list=[];
for (int i = 0; i < map.length; i++) {
CustomObject customObject= CustomObject(
date:map[i]['key'],
weight:map[i]['key']
);
list.add(CustomObject);
}
hope it works for you thanks
You simply don't need to. the values property is an Iterable<> of your objects. You can iterate over this or you can convert it to a list. For example,
// ignore_for_file: avoid_print
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';
void main() {
testWidgets("convert Map to List of Objects", (tester) async {
final weight1 = Weight(const ValueKey("1"), DateTime.now(), 1);
final weight2 = Weight(const ValueKey("2"), DateTime.now(), 2);
final map = {weight1.key: weight1, weight2.key: weight2};
//You don't have to convert this to a list
//But you can if you want to
final list = map.values.toList();
list.forEach((w) => print("Key: ${w.key} Weight: ${w.weight} "));
});
}
class Weight {
final Key key;
final DateTime date;
final double weight;
bool selected = false;
Weight(this.key, this.date, this.weight);
}
Object Class
class ExampleObject {
String variable1;
String variable2;
ExampleObject({
required this.variable1,
required this.variable2,
});
Map<String, dynamic> toMap() {
return {
'variable1': this.variable1,
'variable2': this.variable2,
};
}
factory ExampleObject.fromMap(Map<String, dynamic> map) {
return ExampleObject(
variable1: map['variable1'] as String,
variable2: map['variable2'] as String,
);
}
}
Convert Map to Object List
List<ExampleObject> objectList = List<ExampleObject>.from(mapDataList.map((x) => ExampleObject.fromMap(x)));
I'm currently working with a Map in which the values are of type integer but I need to update the value of a key every time an action takes place. Example: if the Map is { "key1": 1 } after the actions takes place it should be {"key1":2} and so on. Here's my code:
void addToMap(Product product) {
if (_order.containsKey(product.name)) {
_order.update(product.name, (int) => _order[product.name]+1);
}
_order[product.name] = 1;
}
Where _order is the Map
You may use the following idiomatic approach in Dart:
map.update(
key,
(value) => ++value,
ifAbsent: () => 1,
);
This uses the built-in update method along with the optional ifAbsent parameter that helps set the initial value to 1 when the key is absent in the map. It not only makes the intent clear but also avoids pitfalls like that of forgetting to place the return statement that had been pointed out in the other answer.
Additionally, you may also wrap up the above method as an Extension to Map<dynamic, int>. This way also makes the call site look much less cluttered, as visible from the following demo:
extension CustomUpdation on Map<dynamic, int> {
int increment(dynamic key) {
return update(key, (value) => ++value, ifAbsent: () => 1);
}
}
void main() {
final map = <String, int>{};
map.increment("foo");
map.increment("bar");
map.increment("foo");
print(map); // {foo: 2, bar: 1}
}
Add return or the map will always get overridden by _order[product.name] = 1;
void addToMap(Product product) {
if (_order.containsKey(product.name)) {
_order.update(product.name, (int) => _order[product.name]+1);
return;
}
_order[product.name] = 1;
}
I need to check whether myItemsList contains myitem.itemId or not, If it exists need to add itemQuantity, if it not exists need to add myitem object to myItemsList.
List<MyItem> myItemsList = new List();
MyItem myitem = new MyItem (
itemId: id,
itemName: name,
itemQuantity: qty,
);
if (myItemsList.contains(myitem.itemId)) {
print('Already exists!');
} else {
print('Added!');
setState(() {
myItemsList.add(myitem);
});
}
MyItem class
class MyItem {
final String itemId;
final String itemName;
int itemQuantity;
MyItem ({
this.itemId,
this.itemName,
this.itemQuantity,
});
}
above code is not working as expected, please help me to figure out the issue.
Contains() compares the whole objects.
Besides overriding == operator or looping over, you can use list's singleWhere method:
if ((myItemsList.singleWhere((it) => it.itemId == myitem.itemId,
orElse: () => null)) != null) {
Edit:
As Dharaneshvar experienced and YoApps mentioned in the comments .singleWhere raises StateError when more elements are found.
This is desired when you expect unique elements such as in the case of comparing IDs.
Raised error is the friend here as it shows that there is something wrong with the data.
For other cases .firstWhere() is the right tool:
if ((myItemsList.firstWhere((it) => it.itemName == myitem.itemName,
orElse: () => null)) != null) {
// EO Edit
Whole example:
List<MyItem> myItemsList = new List();
class MyItem {
final String itemId;
final String itemName;
int itemQuantity;
MyItem({
this.itemId,
this.itemName,
this.itemQuantity,
});
}
void main() {
MyItem myitem = new MyItem(
itemId: "id00",
itemName: "name",
itemQuantity: 50,
);
myItemsList.add(myitem);
String idToCheck = "id00";
if ((myItemsList.singleWhere((it) => it.itemId == idToCheck,
orElse: () => null)) != null) {
print('Already exists!');
} else {
print('Added!');
}
}
As already said before, contains compares two Objects with the == operator. So you currently compare MyItem with String itemId, which will never be the same.
To check whether myItemsList contains myitem.itemId you can use one of the following:
myItemsList.map((item) => item.itemId).contains(myitem.itemId);
or
myItemsList.any((item) => item.itemId == myitem.itemId);
You're using contains slightly wrong.
From: https://api.dartlang.org/stable/2.2.0/dart-core/Iterable/contains.html
bool contains(Object element) {
for (E e in this) {
if (e == element) return true;
}
return false;
}
You can either override the == operator, see: https://dart-lang.github.io/linter/lints/hash_and_equals.html
#override
bool operator ==(Object other) => other is Better && other.value == value;
Or you can loop over your list and search the normal way one by one, which seems slightly easier.
One more way to check does list contain object with property or not
if (myList.firstWhereOrNull((val) => val.id == someItem.id) != null) {}