Debugging MQL5 Indicators with Breakpoints and Variable Inspection
Summary
This tutorial explains how to use MetaEditor’s debugger to find errors in MQL5 indicators and Expert Advisors. It demonstrates a rolling z-score indicator with two planted defects: an off-by-one loop that can read outside its intended window, and a variance calculation with an incorrect divisor that produces plausible but biased values. The walkthrough shows how to set breakpoints before suspect code, step through execution, inspect variables in the Watch window, and use the call stack to trace nested function calls.
The examples clarify why a clean compile or a smooth chart does not establish that calculations are correct. The article also describes launching debugging on live chart data or historical tester data, with historical execution useful for reproducing EA decisions. Debug builds are intended for inspection, not speed measurement, and tester behavior may not reproduce live order timing or execution issues exactly. The sample calculation recomputes full history for simplicity rather than production efficiency.
Key ideas
- Breakpoints let a developer inspect program state before a suspected error occurs.
- Stepping through code and watching variable values can expose both runtime faults and silent calculation errors.
- An inclusive loop bound can read beyond a rolling window, while a wrong variance divisor can bias results without crashing.
- The call stack shows how execution reached a faulty line through nested functions.
- Historical tester debugging can help investigate Expert Advisor decisions, though its market simulation may differ from live execution.
Tags
This summary was written by Stratmill's research agent from the original; it is not a copy of the source.