I had the function as a class member with an array of my custom object as a parameter:
class Stochastic { ... some class which sent into initializeStochastics method as param };
class StochasticInitializer {
public:
Properties *properties[8];
public:
StochasticInitializer(void) {
this.properties = ...
}
public:
void initializeStochastics(Stochastic& *stochastics[]) { // This param is my problem
for (int i = 0 ;i < ArraySize(properties); i++) {
if (properties[i].enabled) {
stochastics[i] = new Stochastic(properties[i]);
}
}
}
};
My errors:
'&' - comma expected
']' - declaration without type
']' - comma expected
'initializeStochastics' - wrong parameters count
'stochastics' - undeclared identifier
I take syntax from here, but perhaps it solution for MQL5.
Can I send an array of class instances as a method parameter in MQL4? If "yes" - how, if no - it answers too.
Everything works (almost works) just decide whether you are going to create an global array or with pointer access (need to delete it after you finish). Here is example of pointers. Also, please provide MCVE next time, because someone needs to write all that useless stuff like properties&stoch classes to make it testable.
class Properties
{
public:
bool enabled;
int periodK;
Properties(bool _enabled,int k):enabled(_enabled),periodK(k){}
~Properties(){}
};
class Stochastic
{
public:
int periodK;
Stochastic(){}
~Stochastic(){}
Stochastic(Properties *prop):periodK(prop.periodK){}
double get(const int shift,const int buffer=0)const{return iStochastic(_Symbol,0,periodK,3,3,MODE_SMA,STO_LOWHIGH,buffer,shift);}
};
class StochasticInitializer
{
public:
Properties *properties[8];
StochasticInitializer()
{
Deinit();
properties[0]=new Properties(true,5);
properties[1]=new Properties(true,13);
properties[2]=new Properties(true,14);
properties[3]=new Properties(true,15);
properties[4]=new Properties(true,16);
properties[5]=new Properties(true,17);
properties[6]=new Properties(true,18);
properties[7]=new Properties(false,19);
}
~StochasticInitializer(){Deinit();}
void Deinit(const int reason=0){ for(int i=0;i<ArraySize(properties);i++)delete(properties[i]); }
void initializeStochastics(Stochastic *&stochastics[])// THIS IS WHAT YOU NEED IN CASE OF POINTERS
{
for(int i=0;i<ArraySize(properties);i++)
{
if(properties[i].enabled)
{
stochastics[i]=new Stochastic(properties[i]);
}
}
}
};
StochasticInitializer initializer;
void OnTick()
{
Stochastic *array[8]; //THIS IS ARRAY OF POINTERS
initializer.initializeStochastics(array);
for(int i=0;i<ArraySize(array);i++)
{
printf("%i %s: %d %s",__LINE__,__FILE__,i,CheckPointer(array[i])==POINTER_INVALID ? "null" : (string)array[i].periodK);
}
for(int i=ArraySize(array)-1;i>=0;i--)delete(array[i]);//DELETING POINTERS
ExpertRemove();
}
Related
I want to create an abstract Sort class for my class project using dart which I will extend with Sorting algorithm classes like MergeSort, QuickSort, Heap, etc. I wrote the below code but I cannot create an abstract static sort method which I can override and use like
Heap.sort(arr)
OR
MergeSort.sort(arr)
Do anyone know why I cannot create abstract static method and also if you have any other way of doing this please feel free to guide me :D
abstract class Sort {
// void sort(List array);
static void sort(List array);
bool isSorted(List array) {
for (int i = 0; array.length > i - 1; i++) {
if (array[i] > array[i + 1]) {
return false;
}
}
return true;
}
void swap(arr, i, j) {
int temp = arr[i];
arr[i] = arr[j];
arr[j] = temp;
}
}
As the answer linked above says, you can't have abstract static methods.
An abstract method does just one thing, it adds a method signature to the interface of the class, which other (non-abstract) classes implementing the interface then has to provide an implementation for. The abstract method doesn't add implementation.
Static methods are not part of the interface, so being abstract and static means it has no effect at all. It's a method with no implementation, which nobody can ever implement. So you're not allowed to do that.
To actually have separate classes representing different sorting algorithms, just do that directly using instance methods. That's the strategy object pattern.
abstract class Sorter<T> {
void sort(List<T> values);
int compare(T value1, T value2);
void swap(List<T> values, int index1, int index2) {
T tmp = values[index1];
values[index1] = values[index2];
values[index2] = tmp;
}
}
abstract class HeapSort<T> extends Sorter<T> {
void sort(List<T> values) {
// heap sort algorithm.
}
}
abstract class MergeSort<T> extends Sorter<T> {
void sort(List<T> values) {
// merge sort algorithm.
}
}
mixin ComparableSorter<T extends Comparable<T>> on Sorter<T> {
int compare(T value1, T value2) => value1.compareTo(value2);
}
class ComparableHeapSort<T extends Comparable<T>>
extends HeapSort<T> with ComparableSorter<T> {}
class CustomCompareHeapSort<T> extends HeapSort<T> {
int Function(T, T) _compare;
CustomCompareHeapSort(int Function(T, T) compare) : _compare = compare;
int compare(T value1, T value2) => _compare(value1, value2);
}
There are plenty of options about how to slice the API and abstract over different parts of it.
I recommend figuring out what use-cases you want to support, before starting the API design.
/*
I know this keyword is used to refer to class members but i am unable to understand what does 'this'.isEmpty() referring to. Some class ? class method? Or some variable?
For example:
this.value = value;
I understand that here, this.value is referring to class variable value but not for previous occurence of 'this'.
*/
public class StackWithMin extends Stack< NodeWithMin > {
public void push(int value) {
int newMin = Math.min(value, min());
super.push(new NodeWithMin(value,newMin));
}
public int min() {
if(this.isEmpty()) {
return Integer.MAX_VALUE; // Error value
} else {
return peek().min;
}
}
}
class NodeWithMin {
public int value;
public int min;
public NodeWithMin(int value, int min) {
this.value = v;
this.min = min;
}
}
"this" here is object of the class StackWithMin which is extending Stack class from java.util. so StackWithMin is instance of Stack class.
this.isEmpty() which is method defined in Stack, here checks if the stack has any element or not. if it has zero element it returns true else false.
Hope it clears your doubt.
This question already has answers here:
"The operator can’t be unconditionally invoked because the receiver can be null" error after migrating to Dart null-safety
(3 answers)
Closed 12 months ago.
I have migrated my Dart code to NNBD / Null Safety. Some of it looks like this:
class Foo {
String? _a;
void foo() {
if (_a != null) {
_a += 'a';
}
}
}
class Bar {
Bar() {
_a = 'a';
}
String _a;
}
This causes two analysis errors. For _a += 'a';:
An expression whose value can be 'null' must be null-checked before it can be dereferenced.
Try checking that the value isn't 'null' before dereferencing it.
For Bar() {:
Non-nullable instance field '_a' must be initialized.
Try adding an initializer expression, or add a field initializer in this constructor, or mark it 'late'.
In both cases I have already done exactly what the error suggests! What's up with that?
I'm using Dart 2.12.0-133.2.beta (Tue Dec 15).
Edit: I found this page which says:
The analyzer can’t model the flow of your whole application, so it can’t predict the values of global variables or class fields.
But that doesn't make sense to me - there's only one possible flow control path from if (_a != null) to _a += 'a'; in this case - there's no async code and Dart is single-threaded - so it doesn't matter that _a isn't local.
And the error message for Bar() explicitly states the possibility of initialising the field in the constructor.
The problem is that class fields can be overridden even if it is marked as final. The following example illustrates the problem:
class A {
final String? text = 'hello';
String? getText() {
if (text != null) {
return text;
} else {
return 'WAS NULL!';
}
}
}
class B extends A {
bool first = true;
#override
String? get text {
if (first) {
first = false;
return 'world';
} else {
return null;
}
}
}
void main() {
print(A().getText()); // hello
print(B().getText()); // null
}
The B class overrides the text final field so it returns a value the first time it is asked but returns null after this. You cannot write your A class in such a way that you can prevent this form of overrides from being allowed.
So we cannot change the return value of getText from String? to String even if it looks like we checks the text field for null before returning it.
An expression whose value can be 'null' must be null-checked before it can be dereferenced. Try checking that the value isn't 'null' before dereferencing it.
It seems like this really does only work for local variables. This code has no errors:
class Foo {
String? _a;
void foo() {
final a = _a;
if (a != null) {
a += 'a';
_a = a;
}
}
}
It kind of sucks though. My code is now filled with code that just copies class members to local variables and back again. :-/
Non-nullable instance field '_a' must be initialized. Try adding an initializer expression, or add a field initializer in this constructor, or mark it 'late'.
Ah so it turns out a "field initializer" is actually like this:
class Bar {
Bar() : _a = 'a';
String _a;
}
There are few ways to deal with this situation. I've given a detailed answer here so I'm only writing the solutions from it:
Use local variable (Recommended)
void foo() {
var a = this.a; // <-- Local variable
if (a != null) {
a += 'a';
this.a = a;
}
}
Use ??
void foo() {
var a = (this.a ?? '') + 'a';
this.a = a;
}
Use Bang operator (!)
You should only use this solution when you're 100% sure that the variable (a) is not null at the time you're using it.
void foo() {
a = a! + 'a'; // <-- Bang operator
}
To answer your second question:
Non-nullable fields should always be initialized. There are generally three ways of initializing them:
In the declaration:
class Bar {
String a = 'a';
}
In the initializing formal
class Bar {
String a;
Bar({required this.a});
}
In the initializer list:
class Bar {
String a;
Bar(String b) : a = b;
}
You can create your classes in null-safety like this
class JobDoc {
File? docCam1;
File? docCam2;
File? docBarcode;
File? docSignature;
JobDoc({this.docCam1, this.docCam2, this.docBarcode, this.docSignature});
JobDoc.fromJson(Map<String, dynamic> json) {
docCam1 = json['docCam1'] ?? null;
docCam2 = json['docCam2'] ?? null;
docBarcode = json['docBarcode'] ?? null;
docSignature = json['docSignature'] ?? null;
}
}
I can conveniently change opsCount variable directly from inside the function,
because there is only one of that type of variable.
int opsCount = 0;
int jobXCount = 0;
int jobYCount = 0;
int jobZCount = 0;
void doStats(var jobCount) {
opsCount++;
jobCount++;
}
main() {
doStats(jobXCount);
}
But there are many jobCount variables, so how can I change effectively that variable, which is used in parameter, when function is called?
I think I know what you are asking. Unfortunately, the answer is "you can't do this unless you are willing to wrap your integers". Numbers are immutable objects, you can't change their value. Even though Dart's numbers are objects, and they are passed by reference, their intrinsic value can't be changed.
See also Is there a way to pass a primitive parameter by reference in Dart?
You can wrap the variables, then you can pass them as reference:
class IntRef {
IntRef(this.val);
int val;
#override
String toString() => val.toString();
}
IntRef opsCount = new IntRef(0);
IntRef jobXCount = new IntRef(0);
IntRef jobYCount = new IntRef(0);
IntRef jobZCount = new IntRef(0);
void doStats(var jobCount) {
opsCount.val++;
jobCount.val++;
}
main() {
doStats(jobXCount);
print('opsCount: $opsCount; jobXCount: $jobXCount; jobYCount: $jobYCount; jobZCount: $jobZCount');
}
EDIT
According to Roberts comment ..
With a custom operator this would look like:
class IntRef {
IntRef(this.val);
int val;
#override
String toString() => val.toString();
operator +(int other) {
val += other;
return this;
}
}
void doStats(var jobCount) {
opsCount++;
jobCount++;
}
I still havn't understood how to deal with Iterable/Iterator in Dart.
I think I have to give up and simply return Lists but that's not what I want since it will
lead bad performance in my case.
What I want is to understand how to implement my own Iterable/Iterator.
Why do both of these attempts fail?
library foo;
import 'dart:collection';
// Both attemps below raises the following error:
// ==============================================
//
// Closure call with mismatched arguments: function 'moveNext'
//
// NoSuchMethodError: incorrect number of arguments passed to method named 'moveNext'
// Receiver: Closure: (dynamic) => Iterator<int> from Function 'iterator':.
// Tried calling: moveNext()
main() {
Iterable<int> iterable1 = new OddsIterableDartStyle([1,2,4,6,7,8,9]);
for (int i in iterable1)
print("ODD: $i");
Iterable<int> iterable2 = new OddsIterableJavaStyle([1,2,4,6,7,8,9]);
for (int i in iterable2)
print("ODD: $i");
}
// ------------------------------------------
class OddsIterableDartStyle extends Object with IterableMixin<int> {
List<int> _ints;
OddsIterableDartStyle(this._ints);
Iterator<int> iterator() {
return new OddsIterator(this);
}
}
// ------------------------------------------
class OddsIterableJavaStyle implements Iterable<int> {
List<int> _ints;
OddsIterableJavaStyle(this._ints);
Iterator<int> iterator() {
return new OddsIterator(this);
}
}
// ------------------------------------------
class OddsIterator implements Iterator<int> { // Iterate over odd numbers
List<int> _ints;
int _index;
OddsIterator(this._ints) {
_index = -1;
}
bool moveNext() {
while (++_index < _ints.length) {
if (_ints[_index].isOdd)
return true;
}
return false;
}
int get current => (_index < 0) ? null : _ints[_index];
}
I see two immediate problems:
iterator is a getter. The code shouldn't read Iterator<int> iterator() { ... }, it should be Iterator<int> get iterator { ... } instead.
Your iterators are expecting the underlying integer lists, but you are passing in the wrapper. You probably want to construct your iterator like new OddsIterator(_ints), not like new OddsIterator(this).
Btw, Iterator is supposed to return null if you call current and you have already moved beyond the end.
class Count extends Iterable with Iterator {
Count([this.limit = 10]);
int limit;
int i = 0;
#override
int get current => i;
#override
bool moveNext() {
i++;
return i <= limit;
}
#override
Iterator get iterator => this;
}