I am not sure how to write comparator for Collectionos.binarySearch(). Can anyone help ? sample code:
List<Object> list1 = new ArrayList<>();
List<List<Object>> list2 = new ArrayList<>();
//loop starts
// adds elements into list1
list1.add(values);//values is an object containing elements like [3, John, Smith]
if (list2.size() == 0) {
list2.add(list1);//first element
} else {
if (index >= 0) {
int index = Collections.binarySearch(list2, list1, comparator);
list2.add(index, list1);//I want to add these elements in ascending order ?
}
}
//loop ends
How do I write comparator, so that elements in list 2 are added in ascending or descending order.
You can use an anonymous class which implements a Comparator<List<Object>>:
int index = Collections.binarySearch(list2, list1, new Comparator<List<Object>>() {
#Override
public int compare(List<Object> o1, List<Object> o2) {
// Your implementation here
return 0;
}
});
You could implement an IComparer<List<Object>> class, or use a lambda expression.
You just need to create a class that implements the Comparator interface.
For example, you can do this inline with an anonymous class:
Comparator<List<Object>> comparator = new Comparator<List<Object>>() {
#Override
public int compare(List<Object> x, List<Object> y) {
// custom logic to compare x and y here. Return a negative number
// if x < y, a positive number if x > y, and 0 otherwise
}
};
Collections.binarySearch(list, comparator);
I don't like using an indexed array for no reason other than I think it looks ugly. Is there a clean way to sum with an anonymous function? Is it possible to do it without using any outside variables?
Dart iterables now have a reduce function (https://code.google.com/p/dart/issues/detail?id=1649), so you can do a sum pithily without defining your own fold function:
var sum = [1, 2, 3].reduce((a, b) => a + b);
int sum = [1, 2, 3].fold(0, (previous, current) => previous + current);
or with shorter variable names to make it take up less room:
int sum = [1, 2, 3].fold(0, (p, c) => p + c);
This is a very old question but
In 2022 there is actually a built-in package.
Just import
import 'package:collection/collection.dart';
and call the .sum extension method on the Iterable.
FULL EXAMPLE
import 'package:collection/collection.dart';
void main() {
final list = [1, 2, 3, 4];
final sum = list.sum;
print(sum); // prints 10
}
If the list is empty, .sum returns 0.
You might also be interested in list.average...
I still think this is cleaner and easier to understand for this particular problem.
num sum = 0;
[1, 2, 3].forEach((num e){sum += e;});
print(sum);
or
num sum = 0;
for (num e in [1,2,3]) {
sum += e;
}
There is not a clean way to do it using the core libraries as they are now, but if you roll your own foldLeft then there is
main() {
var sum = foldLeft([1,2,3], 0, (val, entry) => val + entry);
print(sum);
}
Dynamic foldLeft(Collection collection, Dynamic val, func) {
collection.forEach((entry) => val = func(val, entry));
return val;
}
I talked to the Dart team about adding foldLeft to the core collections and I hope it will be there soon.
Starting with Dart 2.6 you can use extensions to define a utility method on the List. This works for numbers (example 1) but also for generic objects (example 2).
extension ListUtils<T> on List<T> {
num sumBy(num f(T element)) {
num sum = 0;
for(var item in this) {
sum += f(item);
}
return sum;
}
}
Example 1 (sum all the numbers in the list):
var numbers = [1, 2, 3];
var sum = numbers.sumBy((number) => number);
Example 2 (sum all the Point.x fields):
var points = [Point(1, 2), Point(3, 4)];
var sum = points.sumBy((point) => point.x);
I'd just like to add some small detail to #tmaihoff's answer (about using the collection.dart package):
The sum getter he talks about only works for iterables of num values, like List<int> or Set<double>.
If you have a list of other object types that represent values (like Money, Decimal, Rational, or any others) you must map it to numbers. For example, to count the number of chars in a list of strings you can do:
// Returns 15.
['a', 'ab', 'abc', 'abcd', 'abcde'].map((e) => e.length).sum;
As of 2022, another way of doing it, is using the sumBy() method of the fast_immutable_collections package:
// Returns 15.
['a', 'ab', 'abc', 'abcd', 'abcde'].sumBy((e) => e.length), 15);
Note: I'm the package author.
I suggest you to create this function in any common utility file.
T sum<T extends num>(T lhs, T rhs) => lhs + rhs;
int, double, float extends num class so you can use that function to sum any numbers.
e.g.,
List<int> a = [1,2,3];
int result = a.reduce(sum);
print(result); // result will be 6
Herewith sharing my Approach:
void main() {
int value = sumTwo([1, 4, 3, 43]);
print(value);
}
int sumTwo(List < int > numbers) {
int sum = 0;
for (var i in numbers) {
sum = sum + i;
}
return sum;
}
If when using fold gives a double TypeError, you can use reduce:
var sum = [0.0, 4.5, 6.9].reduce((a, b) => a + b);
If you are planning on doing a number of mathematical operations on your list, it may be helpful to create another list type that includes .sum() and other operations by extending ListBase. Parts of this are inspired by this response with performance tweaks from this response.
import 'dart:collection';
import 'dart:core';
class Vector<num> extends ListBase<num> {
List<num> _list;
Vector() : _list = new List<num>();
Vector.fromList(List<num> lst): _list = lst;
void set length(int l) {
this._list.length=l;
}
int get length => _list.length;
num operator [](int index) => _list[index];
void operator []=(int index, num value) {
_list[index]=value;
}
// Though not strictly necessary, for performance reasons
// you should implement add and addAll.
void add(num value) => _list.add(value);
void addAll(Iterable<num> all) => _list.addAll(all);
num sum() => _list.fold(0.0, (a, b) => a + b) as num;
/// add additional vector functions here like min, max, mean, factorial, normalize etc
}
And use it like so:
Vector vec1 = Vector();
vec1.add(1);
print(vec1); // => [1]
vec1.addAll([2,3,4,5]);
print(vec1); // => [1,2,3,4,5]
print(vec1.sum().toString()); // => 15
Vector vec = Vector.fromList([1.0,2.0,3.0,4.0,5.0]); // works for double too.
print(vec.sum().toString()); // => 15
A solution that has worked cleanly for me is:
var total => [1,2,3,4].fold(0, (e, t) => e + t); // result 10
Different ways to find the sum of all dart list elements,
Method 1: Using a loop :
This is the most commonly used method. Iterate through the list using a loop and add all elements of the list to a final sum variable. We are using one for loop here :
main(List<String> args) {
var sum = 0;
var given_list = [1, 2, 3, 4, 5];
for (var i = 0; i < given_list.length; i++) {
sum += given_list[i];
}
print("Sum : ${sum}");
}
Method 2: Using forEach :
forEach is another way to iterate through a list. We can also use this method to find out the total sum of all values in a dart list. It is similar to the above method. The only difference is that we don’t have to initialize another variable i and list.length is not required.
main(List<String> args) {
var sum = 0;
var given_list = [1, 2, 3, 4, 5];
given_list.forEach((e) => sum += e);
print("Sum : ${sum}");
}
Method 3: Using reduce :
reduce method combines all elements of a list iteratively to one single value using a given function. We can use this method to find out the sum of all elements as like below :
main(List<String> args) {
var given_list = [1, 2, 3, 4, 5];
var sum = given_list.reduce((value, element) => value + element);
print("Sum : ${sum}");
}
Method 4: Using fold :
fold() is similar to reduce. It combines all elements of a list iteratively to one single value using a function. It takes one initial value and calculates the final value based on the previous value.
main(List<String> args) {
var sum = 0;
var given_list = [1,2,3,4,5];
sum = given_list.fold(0, (previous, current) => previous + current);
print("Sum : ${sum}");
}
for more details:https://www.codevscolor.com/dart-find-sum-list-elements
extension DoubleArithmeticExtensions on Iterable<double> {
double get sum => length == 0 ? 0 : reduce((a, b) => a + b);
}
extension IntArithmeticExtensions on Iterable<int> {
int get sum => length == 0 ? 0 : reduce((a, b) => a + b);
}
Usage:
final actual = lineChart.data.lineBarsData[0].spots.map((s) => s.x).sum;
i would like to translate this request in LINQ to SQL:
SELECT * from Agir where NouvelIncident='1' AND facturable is null
My try:
public static List<Agir> GetINDEFAgir()
{
DataClassesActilogDataContext db = ContextSingleton.GetDataContext();
List<Agir> list;
var v = from i in db.Agir
where i.facturable is null && i.NouvelIncident == true
select i;
list = v.ToList();
return list;
}
Looks like "is null" is not allowed in LINQ to SQL... i have a mistake.
Thanks in advance for your help
Use ==, 'is' is to check types
public static List<Agir> GetINDEFAgir()
{
DataClassesActilogDataContext db = ContextSingleton.GetDataContext();
List<Agir> list;
var v = from i in db.Agir
where i.facturable == null && i.NouvelIncident == true
select i;
list = v.ToList();
return list;
}
Doesn't this work?
var v = from i in db.Agir
where i.facturable == null && i.NouvelIncident == true
select i;
Linq-to-SQL should translate that to the proper SQL.
I have a list of Ids and I want to get all the rows back in one query. As a list of objects(So a List of Products or whatever).
I tried
public List<TableA> MyMethod(List<string> keys)
{
var query = "SELECT * FROM TableA WHERE Keys IN (:keys)";
var a = session.CreateQuery(query).SetParameter("keys", keys).List();
return a; // a is a IList but not of TableA. So what do I do now?
}
but I can't figure out how to return it as a list of objects. Is this the right way?
List<TableA> result = session.CreateQuery(query)
.SetParameterList("keys", keys)
.List<TableA>();
Howeever there could be a limitation in this query if number of ":keys" exceed more than 1000 (incase of oracle not sure with other dbs) so i would recommend to use ICriteria instead of CreateQuery- native sqls.
Do something like this,
[TestFixture]
public class ThousandIdsNHibernateQuery
{
[Test]
public void TestThousandIdsNHibernateQuery()
{
//Keys contains 1000 ids not included here.
var keys = new List<decimal>();
using (ISession session = new Session())
{
var tableCirt = session.CreateCriteria(typeof(TableA));
if (keys.Count > 1000)
{
var listsList = new List<List<decimal>>();
//Get first 1000.
var first1000List = keys.GetRange(0, 1000);
//Split next keys into 1000 chuncks.
for (int i = 1000; i < keys.Count; i++)
{
if ((i + 1)%1000 == 0)
{
var newList = new List<decimal>();
newList.AddRange(keys.GetRange(i - 999, 1000));
listsList.Add(newList);
}
}
ICriterion firstExp = Expression.In("Key", first1000List);
ICriterion postExp = null;
foreach (var list in listsList)
{
postExp = Expression.In("Key", list);
tableCirt.Add(Expression.Or(firstExp, postExp));
firstExp = postExp;
}
tableCirt.Add(postExp);
}
else
{
tableCirt.Add(Expression.In("key", keys));
}
var results = tableCirt.List<TableA>();
}
}
}
I have the following 3 tables as part of a simple "item tagging" schema:
==Items==
ItemId int
Brand varchar
Name varchar
Price money
Condition varchar
Description varchar
Active bit
==Tags==
TagId int
Name varchar
Active bit
==TagMap==
TagMapId int
TagId int (fk)
ItemId int (fk)
Active bit
I want to write a LINQ query to bring back Items that match a list of tags (e.g. TagId = 2,3,4,7). In my application context, examples of items would be "Computer Monitor", "Dress Shirt", "Guitar", etc. and examples of tags would be "electronics", "clothing", etc. I would normally accomplish this with a SQL IN Statement.
Something like
var TagIds = new int[] {12, 32, 42};
var q = from map in Context.TagMaps
where TagIds.Contains(map.TagId)
select map.Items;
should do what you need. This will generate an In ( 12, 32, 42 ) clause (or more specifically a parameterized IN clause if I'm not mistaken).
given array of items:
var list = new int[] {2,3,4}
use:
where list.Contains(tm.TagId)
List<int> tagIds = new List<int>() {2, 3, 4, 7};
int tagIdCount = tagIds.Count;
//
// Items that have any of the tags
// (any item may have any of the tags, not necessarily all of them
//
var ItemsAnyTags = db.Items
.Where(item => item.TagMaps
.Any(tm => tagIds.Contains(tm.TagId))
);
//
// Items that have ALL of the tags
// (any item may have extra tags that are not mentioned).
//
var ItemIdsForAllTags = db.TagMap
.Where(tm => tagIds.Contains(tm.TagId))
.GroupBy(tm => tm.ItemId)
.Where(g => g.Count() == tagIdCount)
.Select(g => g.Key);
//
var ItemsWithAllTags = db.Items
.Where(item => ItemsIdsForAllTags.Contains(item.ItemId));
//runs just one query against the database
List<Item> result = ItemsWithAllTags.ToList();
You can simply use,
var TagIds = {12, 32, 42}
var prod =entities.TagMaps.Where(tagmaps=> TagIds .Contains(tagmaps.TagId));
string[] names = {"John", "Cassandra", "Sarah"};
var results = (from n in db.Names
where names.Contains(n.Name)
select n).ToList();
You may create an extension method "IN()"
public static class Extension
{
public static bool IN(this object anyObject, params object[] list)
{ return list.Contains(anyObject); }
}
to be used like this
var q = from map in Context.TagMaps
where map.TagId.IN(2, 3, 4, 7)
select map.Items;
Or just use the inline array.Contains() notation:
var q = from map in Context.TagMaps
where new[]{2, 3, 4, 7}.Contains(map.TagId)
select map.Items;