Custom indexing of an array / subscript operation overloading

[rgne]

Hey guys,

I am getting acquainted with the mql language, but I have some knowledge of C++.

I want a data structure where I can link a value to another. I was thinking of a map- (C++) or dictionary- (Python) like structure, but it is an overkill for my purposes. So I was thinking of a custom array type, where the subscript is assigned by the program (e.g.: the index will not be 0, 1, 2, 3, but 2, 10, 3, 900, and the length of the array will still be 4, and I can assigne values to these indices, like: list[900] = 2;).  It  can be done with operator overloading in C++, but I can't seem to get it work in mql. The official document is not much help.

Here is the  class:

class cArray
{
private:
        int m_list[10];
public:
        int operator[] (const int index) const
        {
                   return (m_list[index]);
        }; 
};

and here is a simple use:

   cArray list; 
   int index = 2;
   list[index] = 3;
   Print(list[index]);

But since I can't overload the [] operator by reference, like:

int& operator[] (const int index) const...

it complains that in

list[index] = 3;

l-value is required.

So my question is: how can I (can I even) overload the subscript operator for the above snipet to work?

Thanks in advance.

[Anthony Garot]

This is MQL4, but it may give you some direction for your issue with the l-value.

https://www.mql5.com/en/blogs/post/680572

[rgne]

Anthony Garot:

This is MQL4, but it may give you some direction for your issue with the l-value.

https://www.mql5.com/en/blogs/post/680572

Thanks Anthony, I'll have a look.

[nicholish en]

rgne:

I want a data structure where I can link a value to another. 

You can't do assignment like that since you're returning an int, which is the equivalent to 5 = 7;

In order to implement assignment in the way you're attempting you'll need to return a reference to an object whose assignment operator has also been overloaded.  ...which might look something like this. 

#include <Arrays\List.mqh>

class IntWrapper : public CObject
{
public:
   int custom_index;
   int num;
   void operator = (int n) { num = n; }
};

class MyIntList : public CList
{
public:
   IntWrapper *operator[](int i)
   {
      IntWrapper *iw = this.GetFirstNode();
      for(; CheckPointer(iw); iw=iw.Next())
         if(iw.custom_index == i)
            return iw;
      iw = new IntWrapper();
      this.Add(iw);
      iw.custom_index = i;
      return iw;
   }
};


void OnStart()
{
   MyIntList list;
   
   list[900] = 25;
   list[5] = 1900;
   
   Print(list[900].num);

}

[rgne]

nicholi shen:

You can't do assignment like that since you're returning an int, which is the equivalent to 5 = 7;

In order to implement assignment in the way you're attempting you'll need to return a reference to an object whose assignment operator has also been overloaded.  ...which might look something like this. 

Hey Nicholi, yes, exactly! I was thinking the same, but I hoped there was a simpler way to do it.

Thanks for the code, I will go through it. Really appreciate the help.

[rgne]

nicholi shen:

You can't do assignment like that since you're returning an int, which is the equivalent to 5 = 7;

In order to implement assignment in the way you're attempting you'll need to return a reference to an object whose assignment operator has also been overloaded.  ...which might look something like this. 

Hey Nicholi,

just a quick question. Do I understand correctly that you basically created a linked list, and use that to add elements? So the lookup of a value wouldn't be O(1), but O(n)? If not, just let me know and I'll go back studying your code.

Thanks again.

Robert

[nicholish en]

rgne:

Hey Nicholi,

just a quick question. Do I understand correctly that you basically created a linked list, and use that to add elements? So the lookup of a value wouldn't be O(1), but O(n)? If not, just let me know and I'll go back studying your code.

Thanks again.

Robert

Correct, and typically I would use CArrayObj for this use since it is faster than a linked list -- a linked list is just easier to demonstrate. Also, you said you had four elements so O(n) in this case is still faster than running a hashing algo plus lookup. If you have a large data collection and need key: value pairs then obviously you'd need a hash-map (dictionary). 

[rgne]

nicholi shen:

Correct, and typically I would use CArrayObj for this use since it is faster than a linked list -- a linked list is just easier to demonstrate. Also, you said you had four elements so O(n) in this case is still faster than running a hashing algo plus lookup. If you have a large data collection and need key: value pairs then obviously you'd need a hash-map (dictionary). 

Got it, thanks.

[Ehsan]

All of these kind of problems are related to the no implementation for pointers to arrays in the language. this is weird.

C++ was invented before with more capabilities than mql5. why mql5 is c++ with less capabilities.

why not to use Java for metatrader ???????

[Alain Verleyen]

Ehsan Tarakemeh:

All of these kind of problems are related to the no implementation for pointers to arrays in the language. this is weird.

C++ was invented before with more capabilities than mql5. why mql5 is c++ with less capabilities.

why not to use Java for metatrader ???????

Java doesn't have pointers either.

mql5 (ex5 actually) is running inside a trading platform, it's managed to protect you. I am sure you don't want your platform to crash each time a coder has made a mistake.

[Ehsan]

Alain Verleyen:

Java doesn't have pointers either.

mql5 (ex5 actually) is running inside a trading platform, it's managed to protect you. I am sure you don't want your platform to crash each time a coder has made a mistake.

Java have pointers!!!

Java doesn't have pointers exactly like c++ but it makes sure you won't need it in the language.

I mean the following code in java:

double[] array1 = new double[10];
array1[0] = 4;
double[] array2 = array1;
                
System.out.println(array2[0]);

 In mql5 you have to wrap an array but the problem is that the system dose not recognize your wrappers.
 
 I hadn't seen your answer till now. Now i'm happy with the language because i have written libraries for mql5 and mql4 to work with and have designed systematic ways to deal with the underlying  platform.