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MetaQuotes Software Corp.
MetaQuotes Software Corp. 2015.06.02 14:53 

New article MQL5 Cookbook: Implementing an Associative Array or a Dictionary for Quick Data Access has been published on

This article describes a special algorithm allowing to gain access to elements by their unique keys. Any base data type can be used as a key. For example it may be represented as a string or an integer variable. Such data container is commonly referred to as a dictionary or an associative array. It provides easier and more efficient way of problem solving.

This article describes a class for convenient storage of information, namely an associative array or a dictionary. This class allows to gain access to information by its key.

The associative array resembles a regular array. But instead of an index it uses some unique key, for example, ENUM_TIMEFRAMES enumeration or some text. It does not matter what represents a key. It's uniqueness of the key that matters. This data storage algorithm significantly simplifies many programming aspects.

For example, a function, which would take an error code and print a text equivalent of the error, could be as follows:

//| Displays the error description in the terminal.                  |
//| Displays "Unknown error" if error id is unknown                  |
void PrintError(int error)
   Dictionary dict;
   CStringNode* node = dict.GetObjectByKey(error);
   if(node != NULL)
      printf("Unknown error");

We will look into specific features of this code later.

Before treating a straight description of associative array internal logic, we will consider details of two main methods of data storage, namely arrays and lists. Our dictionary will be based on these two data types, that is why we should have a good understanding of their specific features. Chapter 1 is dedicated to description of data types. The second chapter is devoted to description of the associative array and methods of working with it.

Author: Vasiliy Sokolov

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