Debugging with Print()

1. The Print() Function: Your Debugging Stethoscope

The Print() function is one of the simplest yet most powerful tools in your MQL5 toolkit. Its job is to display information in the Experts or Journal tab of the Toolbox panel in your MetaTrader 5 terminal.

The basic idea is to print the value of a variable at a specific point in your code to confirm it's what you expect. A good practice is to always include a descriptive text label so you know what value you're looking at.

You can pass multiple pieces of data to the Print() function by separating them with commas. It will automatically combine them into a single line of output.

This short video provides a direct demonstration of using Print() to see the value stored inside a variable.

2. Hands-On Practice: Checking Your Variables

Now it's your turn to practice. We will modify your EA to declare a few variables and then use Print() to display their values.

1. Open your MyHeikinAshiEA.mq5 file in MetaEditor.
2. Go to the OnTick() function and replace any existing code inside it with the following snippet. Your task is to add the Print() statements.

//+------------------------------------------------------------------+
//| Expert tick function                                             |
//+------------------------------------------------------------------+
void OnTick()
  {
//---

   // Variables for our practice
   int candleCount = 50;
   double targetPrice = 1.08500;
   string tradingSymbol = "EURUSD";
   bool isTradingEnabled = true;

   // --- YOUR TASK: Add Print() statements below ---

   // 1. Print the value of candleCount with the label "Number of candles to check:"
   Print("Number of candles to check: ", candleCount);

   // 2. Print the value of targetPrice with the label "Profit target price:"
   Print("Profit target price: ", targetPrice);

   // 3. Print the value of tradingSymbol with the label "Current symbol is:"
   Print("Current symbol is: ", tradingSymbol);

   // 4. Print the value of isTradingEnabled with the label "Is trading enabled?"
   Print("Is trading enabled? ", isTradingEnabled);
   
//---
  }
//+------------------------------------------------------------------+

After adding your Print() statements, compile the code by pressing F7 or clicking the "Compile" button. Make sure you have "0 errors" at the bottom of the editor.

3. Finding the Output: From Code to Log

Now that you've written and compiled the code, where do you see the messages? You'll find them in your MT5 terminal.

The image below shows an example of Print() statements in the MetaEditor.

When you run this code (by attaching your EA to a chart), these messages appear in the Experts tab of the Toolbox window in MT5, as shown below.

To see your own output:

1. Go to your MT5 terminal.
2. Make sure the Toolbox window is open (if not, go to View -> Toolbox or press Ctrl+T).
3. Drag your MyHeikinAshiEA from the Navigator onto a chart.
4. Click the Experts tab in the Toolbox. You should see your printed messages appear with every new price tick.

If you want to test code that runs in OnInit() (which only runs once when the EA starts), the Strategy Tester is a great tool. This video shows how to run an EA in the tester and find the Print() output.

Conclusion

Excellent work! You have now learned the most fundamental method for debugging your code. By strategically placing Print() statements, you can create a trail of breadcrumbs to follow your EA's logic, check variable values, and ensure everything works as planned.

Key Takeaways:

• Debugging is the process of finding and fixing errors in your code.
• The Print() function is your primary tool for simple debugging in MQL5.
• Print() outputs messages to the Experts or Journal tab in the MT5 terminal.
• Always use descriptive text labels with your Print() statements (e.g., Print("Current Price: ", price);) to make your logs easy to read.

Preview of the Next Lesson:
So far, we've dealt with logic errors that you find by observing the EA's behavior. But what happens when you make a typo, like forgetting a semicolon or a parenthesis? In that case, your code won't even compile. In the next lesson, we will cover how to identify and fix basic compilation errors in MetaEditor, another essential skill for every programmer.