A large amount of functions necessary for analysis of the current and previously income quotes, as well as basic arithmetic and logic operations are included in MQL4 structure. There are also basic indicators built in and commands of order placement and control.

The MetaEditor 4 (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. We use MetaQuotes Language Dictionary as a Help System for MQL4 language. An abridged guide contains functions divided into categories, operations, reserved words, and other language constructions and allows finding the description of every element we use.

Programs written in MetaQuotes Language 4 have different features and purposes and can use for MetaTrader 4 Platform:

• Expert Advisor is a mechanical trading system (MTS) linked up to a certain chart. An Advisor starts to run with every incoming tick for a given symbol. The Advisor will not be launched for a new, tick if it is processing the previous one at this moment (i.e., the Advisor has not completed its operation yet). The Advisor can both inform you about a possibility to trade and trade at an account automatically sending orders directly to the trade server. Like most trading systems, the terminal supports testing strategies on history data with displaying trading in-and-out points in the chart.
  Experts are stored in terminal_directory\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\experts\indicators.
• Script is a program intended for a single execution of some actions. Unlike Expert Advisors, Scripts are not run tickwise, but on request.
  Scripts are stored in terminal_dictionary\experts\scripts.
• Library is a set of custom functions containing programs most frequently used. Libraries cannot start execution by itself.
  Libraries are recommended to be stored in terminal_directory\experts\libraries.
• Included file is a source text of the most frequently used blocks of custom programs. Such files can be included into the source texts of experts, 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.
  Included files are recommended to be stored in terminal_directory\experts\include

Joint MQL4 FORUM

Source: MQL4 FORUM