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MQL4 ERR_ARRAY_INVALID

MQL4 error 4029 invalid array: check allocation and shape

MQL4 error 4029 invalid array maps to ERR_ARRAY_INVALID: invalid array. An array is invalid for the called operation. Identify that API and its supported array type; an out-of-range element access is a distinct critical error (4002), not automatically 4029. For API calls, check the return value and capture GetLastError after a documented failure. Critical runtime errors require preventive guards and inspection of the terminal log.

Open MQL4 Coding AgentReview the code example

Open the Coding Agent, then choose MQL4 from the language menu.

ValidArrayDiagnostic.mq4MQL4
#property strict

void OnStart()
  {
   double values[];
   int requested = 10;
   int size = ArrayResize(values, requested);
   if(size != requested)
     {
      Print("ArrayResize failed. error=", GetLastError());
      return;
     }

   for(int i = 0; i < ArraySize(values); i++)
      values[i] = i * 1.0;

   Print("Last value: ", values[ArraySize(values) - 1]);
  }
  • Official meaning: invalid array
  • Check API results or the runtime failure log
  • Capture relevant inputs and error context
  • Retry only when the condition can change

What MQL4 error 4029 means

The official MQL4 runtime table names error 4029 ERR_ARRAY_INVALID and defines it as “invalid array.” An array is invalid for the called operation. Identify that API and its supported array type; an out-of-range element access is a distinct critical error (4002), not automatically 4029.

An error-state value is supporting context, not proof that a call failed or a diagnosis of the whole EA. For noncritical API failures, read the return value and then GetLastError, which resets _LastError. Critical errors such as zero division terminate execution, so code after the failing expression cannot serve as a recovery handler.

Check these conditions before editing the strategy

Start with the inputs and external state that existed at the failure point. A later log entry or a successful call can overwrite the useful context.

  • A dynamic array was not resized before writing elements
  • An array function receives an unsupported dimension or array kind
  • The code assumes ArrayResize succeeded without checking its result
  • Array direction or logical size changes while cached indexes remain in use

Fix the cause, then choose a retry policy

Correct the allocation or array argument before repeating the operation. Retrying the same invalid array is not useful.

Keep the original trading and risk rules intact. A workaround that widens stops, increases volume, changes an account setting, or ignores a failed return value can hide the symptom while changing the system.

CheckAction
A dynamic array was not resized before writing elementsAllocate the required logical size before writing.
An array function receives an unsupported dimension or array kindMatch the array function to the array type and supported dimension.
The code assumes ArrayResize succeeded without checking its resultCheck the return value from ArrayResize and copy operations.
Array direction or logical size changes while cached indexes remain in useRecalculate bounds after resize, copy, or ArraySetAsSeries changes.

Review the complete example

The script checks the allocated size, uses a strict less-than loop bound, and derives the final index from the current logical size.

The example improves validation and logging. It does not prove the operation will be accepted by a broker or behave safely in live trading.

ValidArrayDiagnostic.mq4MQL4
#property strict

void OnStart()
  {
   double values[];
   int requested = 10;
   int size = ArrayResize(values, requested);
   if(size != requested)
     {
      Print("ArrayResize failed. error=", GetLastError());
      return;
     }

   for(int i = 0; i < ArraySize(values); i++)
      values[i] = i * 1.0;

   Print("Last value: ", values[ArraySize(values) - 1]);
  }
Where Pineify fits

Move from the rule to editable MQL4 source

Pineify can generate or revise one self-contained .mq4 file with MQL4-specific reference context. Its independent static checker returns MetaEditor diagnostics so you can repair source issues before testing in your own MT4 environment.

Open MQL4 Coding Agent

Static diagnostics do not prove runtime behavior, backtest results, broker compatibility, or live-trading safety.

Pineify MQL4 Coding Agent with MQL4 selected and an editable MetaTrader 4 code artifact

A practical workflow

  1. 1

    Locate the failed operation

    Check the API result, or the terminal log and source location for a critical runtime failure.

  2. 2

    Capture the error once

    For a recoverable API failure, capture GetLastError immediately. For a terminated program, preserve the terminal log. Record error 4029 and the inputs relevant to the failed operation.

  3. 3

    Correct the triggering condition

    Allocate the required logical size before writing.

  4. 4

    Retest in the target environment

    Compile the revised source, then test with the same MT4 build, broker, symbol, account permissions, and market state.

Frequently asked questions

What does MQL4 error 4029 invalid array mean?

ERR_ARRAY_INVALID means invalid array. Use the API result or critical runtime log, together with the relevant inputs, to identify the failed operation.

What usually causes MQL4 error 4029?

A dynamic array was not resized before writing elements. An array function receives an unsupported dimension or array kind. The code assumes ArrayResize succeeded without checking its result. Array direction or logical size changes while cached indexes remain in use.

Should an EA retry after error 4029?

Correct the allocation or array argument before repeating the operation. Retrying the same invalid array is not useful.

Why should GetLastError be called immediately?

For a noncritical API failure it reports the current _LastError value and resets it. A delayed read may describe another call. It cannot recover execution after a critical error has terminated the program.

Can Pineify fix MQL4 error 4029?

Pineify can revise a self-contained .mq4 file, add validation and error logging, and return static MetaEditor diagnostics. It cannot prove broker acceptance, account permissions, market availability, or live execution.

Compile and test the final source in the MetaTrader 4 build, broker, symbol, and account environment you plan to use.

Continue with one reviewable MQL4 file

Define the requirement, inspect the generated source, repair diagnostics, and test the behavior in your own MT4 environment.

Open Coding Agent