The 3 Main Types of Code in MQL4

The 3 Main Types of Code in MQL4

1. Expert Advisor (EA) 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. (MQL4 Reference)
Translation: An EA is a trading strategy in a coded form. Its capabilities includes, but is not
limited to, opening and closing positions, sending notifications, creating graphics and storing
information.
2. 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. (MQL4 Reference)
Translation: A custom indicator is an indicator developed by a MT4 user and not Metaquotes.
An indicator processes information from the market using a formula and outputs this
information as a value or action.
3. 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. (MQL4
Reference)