MetaQuotes Language 4 (MQL4) Reference

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  1. MetaQuotes Language 4 (MQL4) Reference

MetaQuotes Language 4 (MQL4) is a procedural, object-oriented programming language specifically designed for developing trading strategies, custom technical indicators, scripts, and Expert Advisors (EAs) for the MetaTrader 4 (MT4) trading platform. This article serves as a comprehensive beginner's guide to understanding and utilizing MQL4. It will cover the fundamental concepts, data types, operators, control structures, functions, and best practices for writing effective trading programs.

Introduction to MQL4

MT4 is one of the most popular platforms for Forex and CFD trading. MQL4 allows traders to automate their trading strategies, backtest their ideas, and create custom tools tailored to their specific needs. Unlike general-purpose programming languages, MQL4 is optimized for financial market data and trading operations. It's C-like in syntax, making it relatively accessible to programmers familiar with C, C++, or Java. However, understanding financial markets is equally important as programming knowledge. Resources like Investopedia's Forex Glossary can be invaluable.

Core Concepts

  • Expert Advisors (EAs): These are automated trading systems that can analyze market data and execute trades without manual intervention. They are the most complex type of MQL4 program. Learning about Trading Systems is vital before developing EAs.
  • Custom Indicators: These are tools used to analyze price charts and identify potential trading opportunities. They can be visual representations of data or generate alerts based on specific conditions. Examples include Moving Averages, MACD, and RSI. Understanding Technical Analysis is crucial for creating useful indicators.
  • Scripts: These are single-execution programs that perform a specific task, such as closing all open orders or saving chart data. They do not continuously run like EAs.
  • Libraries: Collections of functions that can be reused in multiple programs. This promotes code organization and efficiency.

Data Types

MQL4 supports a variety of data types:

  • int: Integer numbers (e.g., -10, 0, 100). Size: 4 bytes.
  • double: Floating-point numbers with double precision (e.g., 1.2345, -0.5). Size: 8 bytes. Used extensively for price data.
  • bool: Boolean values (true or false). Size: 1 byte.
  • string: Text strings (e.g., "Hello, world!"). Size: variable.
  • datetime: Date and time values (e.g., 2023.10.27 10:00:00). Crucial for time-based analysis.
  • color: Color values for chart elements. Defined using predefined constants like CLR_RED or custom RGB values.

Operators

MQL4 provides a comprehensive set of operators:

  • Arithmetic Operators: +, -, *, /, % (modulo).
  • Assignment Operators: =, +=, -=, *=, /=.
  • Comparison Operators: ==, !=, >, <, >=, <=.
  • Logical Operators: && (AND), || (OR), ! (NOT).
  • String Operators: & (concatenation).

Control Structures

These structures control the flow of execution in your programs:

  • if-else: Executes different blocks of code based on a condition.

```mql4 if (Close[0] > Open[0]) { 

 // Buy condition
 Print("Price increased");

} else { 

 // Sell condition
 Print("Price decreased");

} ```

  • for loop: Repeats a block of code a specified number of times.

```mql4 for (int i = 0; i < 10; i++) { 

 Print("Iteration: ", i);

} ```

  • while loop: Repeats a block of code as long as a condition is true.

```mql4 int i = 0; while (i < 5) { 

 Print("Iteration: ", i);
 i++;

} ```

  • switch: Selects one of several code blocks to execute based on the value of an expression.

Functions

Functions are reusable blocks of code that perform specific tasks.

  • Built-in Functions: MQL4 provides a large library of built-in functions for accessing market data, managing orders, and performing mathematical operations. Examples include:
   * iClose(symbol, timeframe, shift): Returns the closing price of a bar.
   * iOpen(symbol, timeframe, shift): Returns the opening price of a bar.
   * OrderSend(symbol, cmd, volume, price, slippage, stoploss, takeprofit, comment, magicnumber, expiration, arrow_color): Sends a trading order.
   * TimeCurrent(): Returns the current server time.
  • User-Defined Functions: You can create your own functions to encapsulate complex logic.

```mql4 double CalculateRSI(string symbol, int period) { 

 // RSI calculation logic
 return RSI(symbol, period); // Example using a built-in RSI function

} ```

Variables

Variables are used to store data. They must be declared with a specific data type.

  • Global Variables: Declared outside of any function and accessible from anywhere in the program.
  • Local Variables: Declared inside a function and only accessible within that function.
  • Static Variables: Retain their value between function calls.

Arrays

Arrays are used to store collections of data of the same type.

```mql4 double priceArray[10]; // Declares an array of 10 doubles priceArray[0] = 1.2345; priceArray[1] = 1.2350; ```

Custom Indicators: A Deeper Dive

Creating custom indicators involves several steps:

1. Initialization: The OnInit() function is called when the indicator is loaded. This is where you initialize the indicator's parameters and buffers. See MQL5 Documentation on Buffers for more details on indicator buffers. 2. Calculation: The OnCalculate() function is called for each new tick. This is where you perform the calculations and store the results in indicator buffers. Understanding Fibonacci Retracements and their application is valuable when designing indicators. 3. Deinitialization: The OnDeinit() function is called when the indicator is removed from the chart. This is where you release any resources used by the indicator.

Expert Advisors: A Deeper Dive

Developing EAs requires careful consideration of risk management and trading logic.

1. Initialization: The OnInit() function is called when the EA is loaded. This is where you initialize the EA's parameters and variables. 2. Tick Handling: The OnTick() function is called for each new tick. This is where you analyze market data and decide whether to execute a trade. Consider incorporating Elliott Wave Theory into your EA's logic. 3. Trade Execution: The OrderSend() function is used to send trading orders to the MT4 server. Implement robust error handling to ensure trades are executed correctly. 4. Deinitialization: The OnDeinit() function is called when the EA is removed from the chart.

Backtesting and Optimization

MT4's Strategy Tester allows you to backtest your EAs and indicators on historical data. This is essential for evaluating their performance. The optimization feature allows you to find the optimal parameters for your strategies. Resources on Backtesting Strategies are readily available online.

Common Mistakes and Best Practices

  • Avoid Magic Numbers: Use named constants instead of hardcoded values.
  • Use Meaningful Variable Names: Make your code more readable.
  • Comment Your Code: Explain your logic.
  • Implement Error Handling: Prevent unexpected crashes.
  • Optimize Your Code: Improve performance.
  • Use Proper Indentation: Enhance readability.
  • Learn about Candlestick Patterns and incorporate them into your strategies.

Resources


MetaTrader 4 MQL5 Expert Advisor Technical Indicator Script (MQL4) OrderSend iClose TimeCurrent OnTick OnCalculate Backtesting Strategy Optimization

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