Before we start to code in MQL4 it is important to know what types of programs we can create and use in the terminal and what are the differences among them.
In Metatrader 4 there are three types of programs that we can develop in Metaeditor, these are: Scripts, Custom Indicators and Expert Advisors
Scripts are programs meant to do a specific task. It is important to remember that they are executed by the terminal only once. So, for example if we would like to write a tool meant for closing all open orders or meant for sending us an email with the market data – script would be a perfect match to do it. These programs might be very useful for a day traders as scripts can be assigned to a short-cuts and used instantly in the terminal. You can find them in the Scripts section of the navigator window in Metatrader terminal.
Another type of program available in Metatrader is a custom indicator. As you probably know, in the terminal we already have build-in indicators, for example RSI or Moving Average. You can access them by selecting Insert and then Indicators, but usually they are not enough – and thanks to MQL4, a user can create a custom indicator, in other words a ‘tailored’ indicator. For example, you can create an indicator that would display the highest high and the lowest low from a given period – something that is not possible with the build-in indicators. Possibilities here are limitless. Custom indicator are located in Indicators section of the navigator window.
And finally, we have expert advisors, also known as trading robots or automatic trading systems. These are usually the most complex programs used for semi or full automation of trading strategies. It is important to remember though that expert advisors can also be used as a great trading tools not necessarily related to trading automation. For example, we can write an information management system, program that will inform us about price changes and price breakouts or will send us a push notification to our smartphone before a crucial market data release. Expert advisors are in the Expert Advisors section of the navigator window.