Here is my Dart code
var mp = new Map();
mp[[1,2]] = "Hi";
mp[[3,5]] = "sir";
mp.remove([3,5]);
print(mp);
Output here is null
How can i access value at mp[[3,5]]?
Two list instances containing the same elements is not equal to each other in Dart. This is the reason your example does not work.
If you want to create a Map which works like your example, you can use LinkedHashMap from dart:collection (basically the same when you are using Map()) to create an instance with its own definition of what it means for keys to be equal and how hashCode is calculated for a key.
So something like this if you want to have keys to be equal if the list contains the same elements in the same order. It should be noted it does not support nested lists:
import 'dart:collection';
void main() {
final mp = LinkedHashMap<List<int>, String>(
equals: (list1, list2) {
if (list1.length != list2.length) {
return false;
}
for (var i = 0; i < list1.length; i++) {
if (list1[i] != list2[i]) {
return false;
}
}
return true;
},
hashCode: Object.hashAll,
);
mp[[1, 2]] = "Hi";
mp[[3, 5]] = "sir";
mp.remove([3, 5]);
print(mp); // {[1, 2]: Hi}
}
I should also add that this is really an inefficient way to do use maps and I am highly recommend to never use List as keys in maps.
You add a list instance as a key to the Map object. You need the corresponding list instance to delete it again.
There are two ways to access
First;
final mp = {};
mp[[1,2]] = "Hi";
mp[[3,5]] = "sir";
mp.removeWhere((key, value) {
if(key is List){
return key.first == 3 && key[1] == 5;
}
return false;
});
Second;
final mp = {};
final key = [3, 5];
mp[[1,2]] = "Hi";
mp[key] = "sir";
mp.remove(key);
Map countries = {
"01": "USA",
"02": "United Kingdom",
"03": "China",
"04": "India",
"05": "Brazil",
"06": "Nepal",
"07": "Russia"
};
//method 1:
var _key = countries.keys.firstWhere((k)
=> countries[k] == 'Russia', orElse: () => null);
print(key); //output: 07
Related
I have a list called selected list of type dynamic and it holds a list of objects, each objects contains from a teacher ID & index.
I want to check if this list contains the same Id & index, if it does i want to remove this object from the list.
here is my code ....
void addTeacher(int teacherId, int index) {
if (this.selectedList.contains({ **/// the problem is here** })) {
this.selectedList.remove({teacherId, index});
this.myColor = Colors.grey;
print('removed teacher => ${teacherId.toString()}');
} else {
this.selectedList.add({teacherId, index});
this.myColor = AsasColors().blue;
print('added teacher => ${teacherId.toString()}');
}
notifyListeners();
print(selectedList);
}
how can i achive this ?
Contains and remove use the == operator which in this case will return false because unless you override it for a specific class it will compare by reference.
You can use indexWhere to find out if an item is in a list based on a compare function like that (if the function returns -1 the item is not on the list:
// Index different than -1 means the item is found somewhere in the list
final teacherIndex = this.selectedList.indexWhere((teacher) => teacher['teacherId'] == teacherId);
if (teacherIndex != -1) {
this.selectedList.removeAt(teacherIndex);
this.myColor = Colors.grey;
print('removed teacher => ${teacherId.toString()}');
} else {
...
}
I have implemented it and it worked fine
[The code]
[The output:]
This is the written code:
class Subject {
int? teacherID;
int? subjectID;
Subject(this.teacherID, this.subjectID);
#override
String toString() => "Subject {teacherID: $teacherID, subjectID: $subjectID";
//TODO: Change your needed criteria here..
#override
bool operator ==(Object other) =>
other is Subject &&
teacherID == other.teacherID &&
subjectID == other.subjectID;
}
void addSubject(List<Subject> list, Subject subject) {
if (list.contains(subject)) {
list.remove(subject);
} else {
list.add(subject);
}
}
void main() {
List<Subject> selectedList =
List.generate(10, (index) => Subject(index + 1, index + 1));
print("SelectedList = $selectedList");
addSubject(selectedList, Subject(11, 11));
addSubject(selectedList, Subject(11, 12));
addSubject(selectedList, Subject(12, 11));
addSubject(selectedList, Subject(12, 12));
print("SelectedList2 = $selectedList");
addSubject(selectedList, Subject(12, 12));
print("SelectedList3 = $selectedList");
}
Sincerely, accept the answer if it worked for you.
In my app, at many places I have used Lists like this:-
List<int> nums = [];
// initializing list dynamically with some values.
nums.length = 12; // increasing length of list
// setting these values afterward using nums[i] at different places.
Now after migrating to null-safety obviously nums.length = 4 is giving me a runtime error, so I was wondering is there any method to set the length of the list with default values such that, after if the length of the list was smaller than before then with new length extra elements are added with some default value.
Note: Of course I know we can use for loop, but I was just wondering if there is any easier and cleaner method than that.
var num = List<int>.generate(4, (i) => i);
You can read this.
Another approach:
extension ExtendList<T> on List<T> {
void extend(int newLength, T defaultValue) {
assert(newLength >= 0);
final lengthDifference = newLength - this.length;
if (lengthDifference <= 0) {
return;
}
this.addAll(List.filled(lengthDifference, defaultValue));
}
}
void main() {
var list = <int>[];
list.extend(4, 0);
print(list); // [0, 0, 0, 0];
}
Or, if you must set .length instead of calling a separate method, you could combine it with a variation of julemand101's answer to fill with a specified default value instead of with null:
class ExtendableList<T> with ListMixin<T> {
ExtendableList(this.defaultValue);
final T defaultValue;
final List<T> _list = [];
#override
int get length => _list.length;
#override
T operator [](int index) => _list[index];
#override
void operator []=(int index, T value) {
if (index >= length) {
_list.extend(index + 1, defaultValue);
}
_list[index] = value;
}
#override
set length(int newLength) {
if (newLength > length) {
_list.extend(newLength, defaultValue);
} else {
_list.length = newLength;
}
}
}
(I also made its operator []= automatically grow the ExtendableList if the specified index is out-of-bounds, similar to JavaScript.)
Your problem is that the List in Dart does not have the concept of adding more space while you promise that you are not going to use this new capacity before it is set.
But you can easily make your own List implementation which does this:
import 'dart:collection';
void main() {
List<int> nums = ExtendableList();
nums.length = 3;
nums[2] = 1;
nums[0] = 1;
nums[1] = 1;
print(nums); // [1, 1, 1]
nums.add(2);
print(nums); // [1, 1, 1, 2]
print(nums.runtimeType); // ExtendableList<int>
}
class ExtendableList<T> with ListMixin<T> {
final List<T?> _list = [];
#override
int get length => _list.length;
#override
T operator [](int index) => _list[index] as T;
#override
void operator []=(int index, T value) => _list[index] = value;
#override
set length(int newLength) => _list.length = newLength;
}
As you can see we are using a null type behind the scene but from the outside it will work like the list contains non-nullable. This only works because we assume the [] operator will not be called while a null value are in the list (which happens if we extend the list and does not set the value).
I should add that using such a List implementation does comes with great risk since you don't get any warning/error from the analyzer if you are using it wrongly.
You have to use a list of nullable element to make it longer.
List<int?> nums = [];
nums.length = 4; // OK
print(nums); // [null, null, null, null]
You can also use filled method. Here growable is false by default.
void main() {
var a = List<int>.filled(3, 0, growable: true);
print(a);
// [0, 0, 0]
}
Refer: https://api.flutter.dev/flutter/dart-core/List/List.filled.html
How can I type something like "print(list[1,4]);" in Dart?
For example:
int main() {
var products = new List(5);
products[0] = "Laptop";
products[1] = "Mouse";
products[2] = "Keyboard";
products[3] = "Monitor";
products[4] = "Microphone";
print(products[1]); // Mouse
print(products[1,3]); // I want to see 'Mouse,Monitor'
}
This is not directly supported in the SDK but you can easily make a extension on e.g. List to add this feature:
void main() {
final products = List<String>(5);
products[0] = "Laptop";
products[1] = "Mouse";
products[2] = "Keyboard";
products[3] = "Monitor";
products[4] = "Microphone";
print(products[1]); // Mouse
print(products.selectMultiple([1,3]).join(',')); // Mouse,Monitor
}
extension MultiSelectListExtension<E> on List<E> {
Iterable<E> selectMultiple(Iterable<int> indexes) sync* {
for (final index in indexes) {
yield this[index];
}
}
}
You can't make it so [1,3] (as in you own example) would be valid since the [] operator does only allow one argument. So instead, we need to make a method which takes our requested indexes as argument.
class CoinData {
var _controller = StreamController<Map<String,dynamic>>();
.....
Stream<Map<String,dynamic>> getAllCurrentRates() {
int numAssets = 0;
int counter = 0;
this.listAllAssets()
.then((list) {
if (list != null) {
numAssets = list.length;
List<Map<String, dynamic>>.from(list)
.where((map) => map["type_is_crypto"] == 1)
.take(3)
.map((e) => e["asset_id"].toString())
.forEach((bitCoin) {
this.getCurrentRate(bitCoin)
.then((rate) => _controller.sink.add(Map<String,dynamic>.from(rate)))
.whenComplete(() {
if (++counter >= numAssets) _controller.close();
});
});
}
});
return _controller.stream;
}
.....
}
The length of returned list is around 2500 and this value is assumed by numAssets, however as you see that list is modified later and therefore its length is less, then the evaluation (++counter >= numAssets) is incorrect. So, is it possible to fix that code maintaining its current structure?
.take(3) is temporal, it shall be removed later.
I have List List<MyType>, my type contains Age and RandomID
Now I want to find the maximum age from this list.
What is the simplest and most efficient way?
Assuming you have access to LINQ, and Age is an int (you may also try var maxAge - it is more likely to compile):
int maxAge = myTypes.Max(t => t.Age);
If you also need the RandomID (or the whole object), a quick solution is to use MaxBy from MoreLinq
MyType oldest = myTypes.MaxBy(t => t.Age);
Okay, so if you don't have LINQ, you could hard-code it:
public int FindMaxAge(List<MyType> list)
{
if (list.Count == 0)
{
throw new InvalidOperationException("Empty list");
}
int maxAge = int.MinValue;
foreach (MyType type in list)
{
if (type.Age > maxAge)
{
maxAge = type.Age;
}
}
return maxAge;
}
Or you could write a more general version, reusable across lots of list types:
public int FindMaxValue<T>(List<T> list, Converter<T, int> projection)
{
if (list.Count == 0)
{
throw new InvalidOperationException("Empty list");
}
int maxValue = int.MinValue;
foreach (T item in list)
{
int value = projection(item);
if (value > maxValue)
{
maxValue = value;
}
}
return maxValue;
}
You can use this with:
// C# 2
int maxAge = FindMaxValue(list, delegate(MyType x) { return x.Age; });
// C# 3
int maxAge = FindMaxValue(list, x => x.Age);
Or you could use LINQBridge :)
In each case, you can return the if block with a simple call to Math.Max if you want. For example:
foreach (T item in list)
{
maxValue = Math.Max(maxValue, projection(item));
}
int max = myList.Max(r => r.Age);
http://msdn.microsoft.com/en-us/library/system.linq.enumerable.max.aspx
var maxAge = list.Max(x => x.Age);
thelist.Max(e => e.age);
Easiest way is to use System.Linq as previously described
using System.Linq;
public int GetHighestValue(List<MyTypes> list)
{
return list.Count > 0 ? list.Max(t => t.Age) : 0; //could also return -1
}
This is also possible with a Dictionary
using System.Linq;
public int GetHighestValue(Dictionary<MyTypes, OtherType> obj)
{
return obj.Count > 0 ? obj.Max(t => t.Key.Age) : 0; //could also return -1
}
Simplest is actually just Age.Max(), you don't need any more code.
How about this way:
List<int> myList = new List<int>(){1, 2, 3, 4}; //or any other type
myList.Sort();
int greatestValue = myList[ myList.Count - 1 ];
You basically let the Sort() method to do the job for you instead of writing your own method. Unless you don't want to sort your collection.