How to create a queue using MQL?

[Pooya Khamooshi]

I'd need to create a queue structure with the below interface but the size can increase at any time:

double bucket[];

Enqueue(double val) // add as last item in the bucket

double Dequeue() // remove the first item from the bucket

double ReadFirst() // read the value of the first item in the bucket

[Anthony Garot]

Pooya Khamooshi:

I'd need to create a queue structure with the below interface but the size can increase at any time:

MQL5 has this:

https://www.mql5.com/en/docs/standardlibrary/generic/cqueue

It doesn't look like MQL4 has this, but you could always roll your own.

[Pooya Khamooshi]

Anthony Garot:

MQL5 has this:

https://www.mql5.com/en/docs/standardlibrary/generic/cqueue

It doesn't look like MQL4 has this, but you could always roll your own.

Thanks! I must plan upgrading to MQL5!

But for now with MQL4, I only have the Array object and few helper methods it has like ArraySize, ArrayResize! Not sure.

[Pooya Khamooshi]

I wrote this for MQL4 which looks stupid but works! anyone can improve it?

Assumptions:

1)

There is a known max size for the queue

2)

If an array item is 0 it means it's empty.

class DoubleQueue

{    

   public:

      double Result[];

      DoubleQueue(int size);

      void Enqueue(double item);

      double Dequeue();

      double ReadFirst();

      int Count();      

};

 // Implementations

 DoubleQueue::DoubleQueue(int maxSize) {

   ArrayResize(Result, maxSize);

   for(int i = 0; i < maxSize; i++) {

      Result[i] = 0;    

   }

 } 

  

  void DoubleQueue::Enqueue(double item) {

   int arraySize = ArraySize(Result);

   for(int i = arraySize - 1; i >= 0; i--) {

      if (Result[i] == 0) {

         Result[i] = item;

         break;

      }         

   } 

  }

  

  double DoubleQueue::Dequeue() {

   int arraySize = ArraySize(Result);

   double found = 0;

   for(int i = 0; i < arraySize; i++) {

      if (Result[i] != 0) {

         found = Result[i];

         Result[i] = 0;

         break;

      }         

   } 

   return found;

  }

  

  int DoubleQueue::Count() {

   int arraySize = ArraySize(Result);

   int count = 0;

   for(int i = arraySize - 1; i >= 0; i--) {

      if (Result[i] != 0) {

         count += 1;

         continue;

      }

      break;         

   } 

   return count;

  }

  

  double DoubleQueue::ReadFirst() {

   int arraySize = ArraySize(Result);

   double found = 0;

   for(int i = 0; i < arraySize; i++) {

      if (Result[i] != 0) {

         found = Result[i];

         break;

      }         

   } 

   return found;

  }

[Anthony Garot]

Pooya Khamooshi:

I wrote this for MQL4 which looks stupid but works! anyone can improve it?

I would go with a Linked List implementation. Consider this for C++

https://www.geeksforgeeks.org/queue-set-1introduction-and-array-implementation/

[Pooya Khamooshi]

That's the same implementation, first in first out.

Given I had only ArrayResize method, had to make it like that!

Any better idea with MQL4?

[nicholish en]

You can copy the entire generic directory from MQL5 and use it in MQL4. This example was compiled and tested in MT4

#include <generic/queue.mqh>

void OnStart()
{
   int start_vals[] = {1, 2, 3};
   CQueue<int> q(start_vals);
   q.Enqueue(-1);
   while (q.Count()) {
      Print(q.Dequeue());
   }
}

https://www.mql5.com/en/docs/standardlibrary/generic/cqueue

[Pooya Khamooshi]

Great, thank you very much!

[Ruckshi]

Hey there! So I'm having a problem with this. Where exacgly should this code go while coding an EA. Because I get an error that says 'for' loops cannot be in globals and when used with void OnStart() and also having errors when trying to put it elsewhere.

nicholish en #:

You can copy the entire generic directory from MQL5 and use it in MQL4. This example was compiled and tested in MT4

https://www.mql5.com/en/docs/standardlibrary/generic/cqueue