Coding, Testing, and Debugging Your EA

Once you've planned and structured your Expert Advisor (EA), the next steps involve writing the code, testing its functionality, and resolving any errors or bugs. This stage is critical to ensure that your EA functions correctly in real-time trading environments.

1. Start with the Basic Structure

Begin by laying out the foundational MQL4 structure for your EA, including:

  1. Initialization function: This is where you set up your EA when it's first loaded onto a chart. For instance, you might want to initialize some variables or settings here.
  2. Main function: The heart of your EA. This is where the primary trading logic resides.
  3. Deinitialization function: This function is executed before the EA stops to do any cleanup, like closing any open files or clearing memory.

// MQL4 code example
int OnInit() {
    // Initialization code here

void OnTick() {
    // Main trading logic here

void OnDeinit(const int reason) {
    // Cleanup code here

2. Incorporate Trading Logic

Integrate the trading rules and criteria you've defined. Ensure you handle:

3. Implement Error Handling

While coding your EA, you should anticipate potential errors that could arise during its operation. These errors could be due to various reasons, such as server disconnections, rejected orders, or insufficient margin. Proper error handling ensures that your EA can respond gracefully to unexpected situations without crashing or causing unintended behavior.

1. Check for Error Codes After Trade Operations

Every time your EA performs a trading operation, MQL4 functions often return an error code. These codes can tell you if the operation was successful or if there was an issue. By checking these error codes, you can determine the next steps your EA should take.

// MQL4 code example
double lotSize = 0.1;
int ticket = OrderSend(Symbol(), OP_BUY, lotSize, Ask, 3, 0, 0, "Buy Order", 0, 0, Green);

if(ticket == -1) {
    int lastError = GetLastError();

    Print("OrderSend failed with error(", lastError, "): ", ErrorDescription(lastError));

2. Define Actions in Response to Specific Errors

Once you've identified an error, you should decide how your EA will handle it. Common actions in response to errors include retrying the operation, logging the error, or sending a notification to alert you.

For instance, if an order is rejected because of a temporary issue, your EA might wait for a few seconds and then try again. If the problem persists, it might then send a notification to alert you of the issue.

// MQL4 code example for handling specific errors
int lastError = GetLastError();

switch(lastError) {
        Print("Temporary issue. Retrying in 5 seconds...");
        Sleep(5000);  // Wait for 5 seconds
        // Retry the operation here

        Print("Insufficient funds to execute the trade.");
        // Send notification or take other actions

    // Handle other errors as needed...

        Print("An unknown error occurred: ", lastError);
        // Handle unexpected errors

Proper error handling ensures that your EA remains stable and reliable even under unforeseen circumstances. It's always better to be prepared for potential issues than to be caught off guard.

4. Test in Strategy Tester

MetaTrader 4 offers a built-in Strategy Tester. Here, you can:

Once you've coded your EA, it's essential to test it to ensure it works as expected. MQL4 provides a built-in strategy tester that lets you simulate how your EA would have performed over historical data.

To test your EA:

  1. Open the Strategy Tester panel in MetaTrader 4.
  2. Select your EA from the list.
  3. Choose your desired settings and historical data period.
  4. Click the "Start" button to begin the test.

Once the test is finished, you can analyze the results to see how your EA performed by checking the "journal" tab of the strategy tester.


To check your EA live, check the "Experts" tab at the bottom of the "terminal".

This will show you any errors encountered in the EA and will also show any Printouts you have placed in the EA for problem solving.

Thr journal tab is the same as the "experts" tab you find at the bottom of the "terminal".

5. Debugging with the MetaEditor

As with all coding projects, you're likely to encounter bugs:

Debugging is the process of finding and resolving any errors or issues in your EA's code. These can range from syntax errors, which prevent the code from running at all, to logical errors, where the code runs but doesn't produce the expected results.

In MQL4, you can use the "Print" function to output messages to the terminal. This is useful for tracking the flow of your code and for displaying variable values to help diagnose issues.

// MQL4 code example for debugging
void OnTick() {
    double RSI = iRSI(NULL,0,14,PRICE_CLOSE,1);
    double CurrentPrice = Close[0];
    double CloseofBar5 = Close[5];
    if(RSI>70.0 && CurrentPrice > CloseofBar5)
    Print("Current price is: ", currentPrice);
    Print("RSI is: ", RSI);
    Print("Close of Bar 5 is : ", CloseofBar5);

    // Other trading logic here

Using the Print Function is extremely powerful. When something isn't working, break it all down using the Print function.

Find your "Printouts" for live trading in the terminal tab, "Experts" OR when using the strategy tester, in the "Journal" tab.

Remember, the key to successful debugging is to break down your code into small, manageable sections and test each one individually.

6. Optimize for Performance

Once your EA works correctly, consider performance:


Coding an EA involves a continuous cycle of writing, testing, and refining. As you encounter challenges, remember that each issue you resolve strengthens your EA's reliability and performance in live trading scenarios.

NEXT UP: Deploying the EA on MetaTrader 4