MetaQuotes Language 4 (MQL4) is a built-in language for programming trading strategies. This language is developed by MetaQuotes Software Corp. based on their long experience in the creation of online trading platforms. Using this language, you can create your own Expert Advisors that make trading management automated and are perfectly suitable for implementing your own trading strategies. Besides, using MQL4 you can create your own technical indicators (custom indicators), scripts and libraries.
MQL4 contains a large number of functions necessary for analyzing current and previously received quotes, and has built-in basic indicators and functions for managing trade orders and controlling them. The MetaEditor (text editor) that highlights different constructions of MQL4 language is used for writing the program code. It helps users to orientate themselves in the expert system text quite easily.
The brief guide contains functions, operations, reserved words, and other language constructions divided into categories, and allows finding the description of every used element of the language.
Programs written in MetaQuotes Language 4 have different features and purposes:
- Expert Advisor is a mechanical trading system linked up to a certain chart. An Expert Advisor starts to run when an event happens that can be handled by it: events of initialization and deinitialization, event of a new tick receipt, a timer event, depth of market changing event, chart event and custom events.
An Expert Advisor can both inform you about a possibility to trade and automatically trade on an account sending orders directly to a trade server. Expert Advisors are stored in terminal_directory\MQL4\Experts.
- Custom Indicator is a technical indicator written independently in addition to those already integrated into the client terminal. Like built-in indicators, they cannot trade automatically and are intended for implementing of analytical functions only.
Custom indicators are stored in terminal_directory\MQL4\Indicators
- Script is a program intended for a single execution of some actions. Unlike Expert Advisors, scripts do not process any actions, except for the start event (this requires the OnStart handler function in a script). Scripts are stored in terminal_directory\MQL4\Scripts
- Library is a set of custom functions intended for storing and distributing frequently used blocks of custom programs. Libraries cannot start executing by themselves.
Libraries are stored in terminal_directory\MQL4\Libraries
- Include File is a source text of the most frequently used blocks of custom programs. Such files can be included into the source texts of Expert Advisors, scripts, custom indicators, and libraries at the compiling stage. The use of included files is more preferable than the use of libraries because of additional burden occurring at calling library functions.
Include files can be stored in the same directory as a source file - in this case the #include directive with double quotes is used. Another place to store include files is terminal_directory\MQL4\Include, in this case the #include directive is used with angle brackets.
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