MQL5 array out of range: find the first invalid index

An array index must refer to an element that actually exists. Compiling successfully does not prove that every runtime loop uses a valid index. Start at the first reported line rather than patching all loops at once.

Write down the bounds

For a one-dimensional array with five elements, valid zero-based indexes are zero through four. A loop that includes index five goes too far. When reading i+1, leave room for that extra element.

Check the data you received

If you requested many bars but received fewer, the requested count is not a safe loop bound. Log the copied count, array length and failing index. Decide what the EA should do when history is not ready.

Reproduce a small case

Try the first few bars, an empty result and the minimum lookback. These cases often reveal a boundary error faster than another full-year run. Preserve the failing input so the correction can be checked again.

Keep the fix narrow

Do not suppress the message and continue with invented prices. A useful fix establishes valid bounds or waits for valid data. Editable source makes these small, focused improvements possible without changing the entire strategy.

Reference and further reading

Start with a free editable EA project · Explore MQLume source code

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