How can I use firstWhereOrNull with maps in Flutter?
In other words, how can I do this:
final myVariable1 = myList.firstWhereOrNull(
(myVariable2) =>
!myList.containsValue(myVariable2));
Instead of using a list (myList), I'd like to do the same with a map (Map<String,int>).
Map<String,int> myMap = {};
myMap("stuff1") = 1;
myMap("stuff2") = 2;
myMap("stuff3") = 3;
Thanks
There is no such firstWhereOrNull method for Maps, but you can easily come up with one using extension methods:
extension ExtendedMap on Map {
/// The first entry satisfying test, or null if there are none.
MapEntry? firstWhereOrNull(bool Function(MapEntry entry) test) {
for (var entry in this.entries) {
if (test(entry)) return entry;
}
return null;
}
}
Here is how you can use it:
final map = <String, int>{
'stuff1': 1,
'stuff2': 2,
'stuff3': 3,
};
final test = map.firstWhereOrNull((entry) => entry.value == 2);
print(test); // Prints MapEntry(stuff2: 2)
final nullTest = map.firstWhereOrNull((entry) => entry.key == "stuff5");
print(nullTest); // Prints null
So, I created this implementation, I don't think it's the most optimized, also because it was necessary to use the cast since because of the sound null safety, it's not possible to return any value. But it works for you.
var myMap = {"key1": "value", "key2": 3};
var result = myMap.entries
.cast<dynamic>()
.firstWhere((e) => e.key == "key", orElse: () => null);
print(result);
I hope this helps in some way!
Related
Prior to null-safe dart, the following was valid syntax:
final list = [1, 2, 3];
final x = list.firstWhere((element) => element > 3, orElse: () => null);
if (x == null) {
// do stuff...
}
Now, firstWhere requires orElse to return an int, opposed to an int?, therefore I cannot return null.
How can I return null from orElse?
A handy function, firstWhereOrNull, solves this exact problem.
Import package:collection which includes extension methods on Iterable.
import 'package:collection/collection.dart';
final list = [1, 2, 3];
final x = list.firstWhereOrNull((element) => element > 3);
if (x == null) {
// do stuff...
}
You don't need external package for this instead you can use try/catch
int? x;
try {
x = list.firstWhere((element) => element > 3);
} catch(e) {
x = null;
}
A little bit late but i came up with this:
typedef FirstWhereClosure = bool Function(dynamic);
extension FirstWhere on List {
dynamic frstWhere(FirstWhereClosure closure) {
int index = this.indexWhere(closure);
if (index != -1) {
return this[index];
}
return null;
}
}
Example use:
class Test{
String name;
int code;
Test(code, this.name);
}
Test? test = list.frstWhere(t)=> t.code==123);
An alternative is that you set a nullable type to the list.
Instead of just [1, 2, 3], you write <int?>[1, 2, 3], allowing it to be nullable.
void main() {
final list = <int?>[1, 2, 3];
final x = list.firstWhere(
(element) => element != null ? (element > 3) : false,
orElse: () => null);
print(x);
}
This should work, and it's a better solution:
extension IterableExtensions<T> on Iterable<T> {
T? firstWhereOrNull(bool Function(T element) comparator) {
try {
return firstWhere(comparator);
} on StateError catch (_) {
return null;
}
}
}
To add to #Alex Hartfords answer, and for anyone who doesn't want to import a full package just for this functionality, this is the actual implementation for firstWhereOrNull from the collection package that you can add to your app.
extension FirstWhereExt<T> on List<T> {
/// The first element satisfying [test], or `null` if there are none.
T? firstWhereOrNull(bool Function(T element) test) {
for (final element in this) {
if (test(element)) return element;
}
return null;
}
}
I am new in flutter. I want to do for each to check my array JSON. I know typescript is doing like this:
Object.keys(res).forEach(key => {
if (res[key].name === 'Max') {
match = true;
}
});
if (match) {
console.log ('Username has been taken');
} else {
console.log('Username is available');
}
My question is for dart language in Flutter, how to do that?
Please find the below sample code for forEach in dart, which can used in flutter also.
main() {
List<String> list = new List<String>();
list.add('apple');
list.add('ball');
list.add('cat');
list.forEach((element) => print(element));
Set<String> set = Set.from(list);
set.forEach((element) => print(element));
}
try this
var decodedData= json.decode(response.body);
for (var item in decodedDat){
if(item['name'] == 'Max'){
// if true
}
}
The Darty way to test whether an Iterable (and by extension List) contains an element that matches a predicate is with any.
bool match = list.any((s) => s == 'Max');
How can I initialize a list inside a map?
Map<String,Map<int,List<String>>> myMapList = Map();
I get an error :
The method '[]' was called on null.
I/flutter (16433): Receiver: null
I/flutter (16433): Tried calling: [](9)
By just adding the elements when you create the map, or later by adding them. For example:
var myMapList = <String, Map<int, List<String>>>{
'someKey': <int, List<String>>{
0: <String>['foo', 'bar'],
},
};
or
var m2 = <String, Map<int, List<String>>>{}; // empty map
m2['otherKey'] = <int, List<String>>{}; // add an empty map at the top level
m2['otherKey'][2] = <String>[]; // and an empty list to that map
m2['otherKey'][2].add('baz'); // and a value to that list
print(m2);
prints {otherKey: {2: [baz]}}
Try initializing using {}
Map<String,Map<int,List<String>>> myMapList = {}; // just add {}
it will create an empty map and works perfectly fine.
In case for list of int as value
void main() {
List<int> dividends = [99,101,176,182];
Map map = new Map<int, List<int>>();
dividends.forEach((i) {
if(i > 0) {
map[i] = <int>[];
for(int j=1; j <= i;j++) {
if(i%j == 0) {
map[i].add(j);
}
}
// print('$i, $map[$i]');
}
});
print(map);
}
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;
}