var a=[1,2,3]
var b=[1,2,3]
var c=[1,2,4]
System.print(a==b)
System.print(a==c)
How do I check if two lists are equal? I tried the equality operator == but this doesn't work; it prints false and false
There is no built-in operator or algorithm for list equality in wren - you must check the elements individually.
An example algorithm is available here: https://github.com/minirop/wren-helpers/blob/master/algorithms/algorithm.wren#L6-L17
static equal(list1, list2) {
if (list1.count != list2.count) {
return false
}
for (i in 0...list1.count) {
if (list1[i] != list2[i]) {
return false
}
}
return true
}
There is an open discussion on whether to add support for an equality list function: here https://github.com/wren-lang/wren/issues/606
I'm trying to find a value in a list of objects in kotlin, using for it "filter", but I need to return true or false if the value is found, but filter returns me a list of object in the case of match.
t.filter { it.retailerId == value }
¿How I can return a boolean when I find this value in the list of objects?
If you need that the element is exactly one:
t.filter { it.retailerId == value }.size == 1
if not:
t.any { it.retailerId == value }
With foldRight and a break when you found it:
t.foldRight(false) {val, res ->
if(it.retailerId == value) {
return#foldRight true
} else {
res
}
}
In alternative to firstOrNull you can also use any with the same predicate:
val found = t.any { it.retailerId == value }
You can use firstOrNull() with the specific predicate:
val found = t.firstOrNull { it.retailerId == value } != null
If firstOrNull() does not return null this means that the value is found.
For single element
list.first { it.type == 2 (eg: conditions) }
or
list.firstOrNull { it.type == 2 (eg: conditions) }
For list of elements
list.filter { it.type == 2 (eg: conditions) }
Kotlin has this nice extension function which u can use
if (none { it.isSelected == true }) {
first().isSelected = true
}
How to sort a list of objects by the alphabetical order of one of its properties (Not the name but the actual value the property holds)?
You can pass a comparison function to List.sort.
someObjects.sort((a, b) => a.someProperty.compareTo(b.someProperty));
In general, you can provide a custom comparison function to List.sort.
/// Desired relation | Result
/// -------------------------------------------
/// a < b | Returns a negative value.
/// a == b | Returns 0.
/// a > b | Returns a positive value.
///
int mySortComparison(SomeClass a, SomeClass b) {
final propertyA = someProperty(a);
final propertyB = someProperty(b);
if (propertyA < propertyB) {
return -1;
} else if (propertyA > propertyB) {
return 1;
} else {
return 0;
}
}
list.sort(mySortComparison);
If you're sorting some custom class you own, you alternatively could make your class implement the Comparable interface:
class MyCustomClass implements Comparable<MyCustomClass> {
...
#override
int compareTo(MyCustomClass other) {
if (someProperty < other.someProperty) {
return -1;
} else if (someProperty > other.someProperty) {
return 1;
} else {
return 0;
}
}
}
and then you can use list.sort() directly without supplying a callback.
Note that if you're sorting by a single property that already implements the Comparable interface, implementing the comparison functions is much simpler. For example:
class MyCustomClass implements Comparable<MyCustomClass> {
...
#override
int compareTo(MyCustomClass other) =>
someProperty.compareTo(other.someProperty);
}
Reversing
If you want to reverse the sort order, you can:
Make your comparison function return a value with the opposite sign.
Alternatively just explicitly reverse the list after sorting:
list = (list..sort()).reversed.toList();
Sorting by multiple properties (a.k.a. subsorting)
There are a variety of ways to sort by multiple properties.
A general way is to perform a stable sort for each property in reverse order of importance. For example, if you want to sort names primarily by surname and then subsort within surnames by given name, then you would first sort by given names, and then perform a stable sort by surname. See below for how to perform a stable sort.
Alternatively, you could sort with a comparison function that itself checks multiple properties. For example:
class Name {
Name({String? surname, String? givenName})
: surname = surname ?? "",
givenName = givenName ?? "";
final String surname;
final String givenName;
}
int compareNames(Name name1, Name name2) {
var comparisonResult = name1.surname.compareTo(name2.surname);
if (comparisonResult != 0) {
return comparisonResult;
}
// Surnames are the same, so subsort by given name.
return name1.givenName.compareTo(name2.givenName);
}
package:collection provides an extension to chain comparison functions so that combining them is a bit more straightforward and less error-prone:
import 'package:collection/collection.dart';
int compareSurnames(Name name1, Name name2) =>
name1.surname.compareTo(name2.surname);
int compareGivenNames(Name name1, Name name2) =>
name1.givenName.compareTo(name2.givenName);
final compareNames = compareSurnames.then(compareGivenNames);
My dartbag package also provides a compareIterables function that allows comparing Lists of property values in order of importance:
import 'package:dartbag/collection.dart';
int compareNames(Name name1, Name name2) =>
compareIterables(
[name1.surname, name1.givenName],
[name2.surname, name2.givenName],
);
Okay, I want a stable sort
List.sort is not guaranteed to be a stable sort. If you need a stable sort, package:collection provides insertionSort and mergeSort implementations that are stable.
But comparing might be expensive
Suppose you have a custom comparison function that looks something like:
int compareMyCustomClass(MyCustomClass a, MyCustomClass b) {
var a0 = computeValue(a);
var b0 = computeValue(b);
return a0.compareTo(b0);
}
The sorting process might call computeValue multiple times for the same object, which is particularly wasteful if computeValue() is expensive. In such cases, a Schwartzian transform could be faster (at the expense of using more memory). This approach maps your objects to directly sortable keys, sorts those keys, and extracts the original objects. (This is how Python's sort and sorted functions work.)
Here's one possible implementation:
class _SortableKeyPair<T, K extends Comparable<Object>>
implements Comparable<_SortableKeyPair<T, K>> {
_SortableKeyPair(this.original, this.key);
final T original;
final K key;
#override
int compareTo(_SortableKeyPair<T, K> other) => key.compareTo(other.key);
}
/// Returns a sorted *copy* of [items] according to the computed sort key.
List<E> sortedWithKey<E, K extends Comparable<Object>>(
Iterable<E> items,
K Function(E) toKey,
) {
final keyPairs = [
for (var element in items) _SortableKeyPair(element, toKey(element)),
]..sort();
return [
for (var keyPair in keyPairs) keyPair.original,
];
}
void main() {
final list = <MyCustomClass>[ ... ];
final sorted = sortedWithKeys(list, computeValue);
}
My dartbag package provides such a sortWithKey function (and also a sortWithAsyncKey function if the key needs to be generated asynchronously).
If you want to sort the object "objects" by the property "name" do something like this
objects.sort((a, b) {
return a.value['name'].toString().toLowerCase().compareTo(b.value['name'].toString().toLowerCase());
});
Immutable extension sortedBy for List.
extension MyIterable<E> on Iterable<E> {
Iterable<E> sortedBy(Comparable key(E e)) =>
toList()..sort((a, b) => key(a).compareTo(key(b)));
}
And use
list.sortedBy((it) => it.name);
Here is my contribution to this good question. If someone is facing difficulty to understand how the #Nate Bosch answer is working & you want to sort your custom model class list then you can do this way.
1. You have to implement Comparable abstract class in your model class.
It has the method compareTo which you have to override.
For example, I have this StudentMarks model class which has marks property in it.
class StudentMarks implements Comparable {
int marks;
StudentMarks({
this.marks,
});
#override
int compareTo(other) {
if (this.marks == null || other == null) {
return null;
}
if (this.marks < other.marks) {
return 1;
}
if (this.marks > other.marks) {
return -1;
}
if (this.marks == other.marks) {
return 0;
}
return null;
}
}
2. Now you can call compareTo method inside the sort method.
void _sortStudents({bool reversed: false}) {
_students.sort((a, b) {
return a.compareTo(b);
});
if (reversed) {
_students = _students.reversed.toList();
}
setState(() {});
}
Refer to this link you want to know more about the Comparable class
https://api.dart.dev/stable/2.1.0/dart-core/Comparable-class.html
Its worked for me:
myList..sort((a, b) => a.name.toLowerCase().compareTo(b.name.toLowerCase()));
Using Comparatorfunction, sort Users by id.
Comparator<UserModel> sortById = (a, b) => a.id.compareTo(b.id);
users.sort(sortById);
Now we can sort it in reversed/descending order.
users = users.reversed.toList();
To sort it in reverse order :
list.sort((a, b) {
return b.status.toLowerCase().compareTo(a.status.toLowerCase());
});
What's more, you can use Comparable.compare for more clear, for example:
class _Person {
final int age;
final String name;
_Person({required this.age, required this.name});
}
void _test() {
final array = [
_Person(age: 10, name: 'Dean'),
_Person(age: 20, name: 'Jack'),
_Person(age: 30, name: 'Ben'),
];
// ascend with age
// Dean Jack Ben
array.sort((p1, p2) {
return Comparable.compare(p1.age, p2.age);
});
// decend with age
// Ben Jack Dean
array.sort((p1, p2) {
return Comparable.compare(p2.age, p1.age);
});
// ascend with name
// Ben Dean Jack
array.sort((p1, p2) {
return Comparable.compare(p1.name, p2.name);
});
}
Similar to #pavel-shorokhovs answer, but strongly typed:
extension IterableExtensions<T> on Iterable<T> {
Iterable<T> sortBy<TSelected extends Comparable<TSelected>>(
TSelected Function(T) selector) =>
toList()..sort((a, b) => selector(a).compareTo(selector(b)));
Iterable<T> sortByDescending<TSelected extends Comparable<TSelected>>(
TSelected Function(T) selector) =>
sortBy(selector).toList().reversed;
}
i had fpgrowth machine learning output/result with each element of list contains another list and frequency field i was to sort by frequency in descending order so i used a bit of recursion for that try it might work i know i am late but i am posting maybe someone else could benefit.
sort(List<FrequentItem> fqItems) {
int len = fqItems.length;
if(len==2){
if(fqItems[0].frequency>fqItems[1].frequency){
sortedItems.add(fqItems[0]);
sortedItems.add(fqItems[1]);
}else{
sortedItems.add(fqItems[1]);
sortedItems.add(fqItems[0]);
}
return;
}else{
FrequentItem max = fqItems[0];
int index =0;
for(int i=0;i<len-2;i++){
if(max.frequency<fqItems[i+1].frequency){
max = fqItems[i+1];
index = i+1;
}
}
sortedItems.add(max);
fqItems.removeAt(index);
sort(fqItems);
}
}
Step 1: Add compareTo method to class:
class Student {
String? name;
int? age;
Student({this.name, this.age});
int getAge() {
if (age == null) return 0;
return age!;
}
#override
int compareTo(Student other) {
var a = getAge();
var b = other.getAge();
if (a < b) {
return -1;
} else if (a > b) {
return 1;
} else {
return 0;
}
}
}
Step 2: Sorting your list:
By ascending:
studentList.sort((a, b) {
return a.compareTo(b);
});
By descending:
studentList.sort((a, b) {
return b.compareTo(a);
});
I have a vector of pointers to class instances I created with multiple values in it called Record
it has a value called, when I access them I
bool recordDeleted;
bool recordOwnership;
vector<Record*> RecordsVec
I want to create a function that would do something like,
bool func()
{
for (auto it = RecordsVec.begin(); it < RecordsVec.end(); it++)
{
// check whether recordDeleted is true // or recordOwnership == true)
}
// if all are true
// return true
// else
// return false
}
what is the most efficient way to do this ?
Pretty simple:
bool allDeleted() {
return std::all_of(begin(RecordsVec), end(RecordsVec), [](Record *r) {
return r->recordDeleted;
});
}
And the same for your ownership flag, of course.
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Sorry if this question is not suited for SO.
I have a C++ function that approximately looks like MyFun() given below.
From this function I am calling some(say around 30) other functions that returns a boolean variable (true means success and false means failure). If any of these functions returns false, I have to return false from MyFun() too. Also, I am not supposed to exit immediately (without calling the remaining functions) if an intermediate function call fails.
Currently I am doing this as given below, but feel like there could be a more neat/concise way to handle this. Any suggestion is appreciated.
Many Thanks.
bool MyFun() // fn that returns false on failure
{
bool Result = true;
if (false == AnotherFn1()) // Another fn that returns false on failure
{
Result = false;
}
if (false == AnotherFn2()) // Another fn that returns false on failure
{
Result = false;
}
// Repeat this a number of times.
.
.
.
if (false == Result)
{
cout << "Some function call failed";
}
return Result;
}
I would replace each if statement with a more coincise bitwise AND assignment:
bool MyFun() // fn that returns false on failure
{
bool Result = true;
Result &= AnotherFn1(); // Another fn that returns false on failure
Result &= AnotherFn2(); // Another fn that returns false on failure
// Repeat this a number of times.
.
.
.
if (false == Result)
{
cout << "Some function call failed";
}
return Result;
}
Use something like a std::vector of std::function. It is a lot more maintenable.
Example: http://ideone.com/0voxRl
// List all the function you want to evaluate
std::vector<std::function<bool()>> functions = {
my_func1,
my_func2,
my_func3,
my_func4
};
// Evaluate all the function returning the number of function that did fail.
unsigned long failure =
std::count_if(functions.begin(), functions.end(),
[](const std::function<bool()>& function) { return !function(); });
If you want to stop when a function fail, you just have to use std::all_of instead of std::count_if. You dissociate the control flow from the function list and that is, in my opinion, a good thing.
You can improve this by using a map of function with name as key that will allows you to output which function failed:
std::map<std::string, std::function<bool()>> function_map;
bool MyFun() // fn that returns false on failure
{
bool Result = true;
// if need to call every function, despite of the Result of the previous
Result = AnotherFn1() && Result;
Result = AnotherFn2() && Result;
// if need to avoid calling any other function after some failure
Result = Result && AnotherFn1();
Result = Result && AnotherFn2();
return Result;
}
Instead of
if (false == AnotherFn1()) // Another fn that returns false on failure
{
Result = false;
}
if (false == AnotherFn2()) // Another fn that returns false on failure
{
Result = false;
}
if (false == AnotherFn3()) // Another fn that returns false on failure
{
Result = false;
}
begin to use booleans as what they are, truth values:
if (!AnotherFn1()) // Another fn that returns false on failure
{
Result = false;
}
if (!AnotherFn2()) // Another fn that returns false on failure
{
Result = false;
}
if (!AnotherFn3()) // Another fn that returns false on failure
{
Result = false;
}
Then, all those conditions have the same code; they are basically part of one big condition:
if ( !AnotherFn1()
| !AnotherFn2()
| !AnotherFn3())
{
Result = false;
}
For your specific problem, where you want all functions be called, even if you know early you'll return false, it is important to not use the short circuiting operators && and ||. Using the eager bitwise operators | and & is really a hack, because they are bitwise and not boolean (and thus hide intent), but work in your situation iff AnotherFn? return strict bools.
You can negate what you do inside; less negations yield better code:
Result = false;
if ( AnotherFn1()
& AnotherFn2()
& AnotherFn3())
{
Result = true;
}
and then you can rid these assignments and instead return straightly:
if ( AnotherFn1()
& AnotherFn2()
& AnotherFn3())
{
return true;
}
cout << "something bad happened";
return false;
Summary
Old:
bool MyFun() // fn that returns false on failure
{
bool Result = true;
if (false == AnotherFn1()) // Another fn that returns false on failure
{
Result = false;
}
if (false == AnotherFn2()) // Another fn that returns false on failure
{
Result = false;
}
// Repeat this a number of times.
.
.
.
if (false == Result)
{
cout << "Some function call failed";
}
return Result;
}
New:
bool MyFun() // fn that returns false on failure
{
if (AnotherFn1() &
AnotherFn2() &
AnotherFn3())
{
return true;
}
cout << "Some function call failed";
return false;
}
There are more possible improvements, e.g. using exceptions instead of error codes, but don't be tempted to handle "expections" instead.
! can be used as a cleaner alternative to false
Like this:
bool MyFun() // fn that returns false on failure
{
bool Result = true;
if (!AnotherFn1()) // Another fn that returns false on failure
{
Result = false;
}
if (!AnotherFn2()) // Another fn that returns false on failure
{
Result = false;
}
// Repeat this a number of times.
.
.
.
if (!Result)
{
cout << "Some function call failed";
}
return Result;
}
how about using exceptions to handle failure:a neat exemple
the main question is, are the function call interdependant or not? can some be skipped if a previous one failed? ...