What does ArrayResize return for a static array?

The static array is never reallocated. For a request no larger than ArraySize it returns the requested value, even though the array size is unchanged; a larger request returns -1. Check ArraySize when demonstrating this behavior.

Which dimension can be changed?

Only the first dimension of a dynamic array is resized. The remaining dimensions must be fixed in the declaration. For double matrix[][3], resizing to four rows yields twelve elements, so the successful return value is 12.

Are new numeric elements initialized?

Do not assume that they are. In particular, elements exposed from reserve storage have undefined values. Explicitly initialize or assign the elements before reading them.

How does reserve_size affect performance?

Reserve capacity reduces the frequency of physical reallocations while growing an array. The third parameter is used when memory is physically allocated. It does not remove loop execution costs or guarantee a timing result.

How should resize failures be handled?

Check the return value for -1 and inspect GetLastError before accessing the requested range. Memory exhaustion, invalid sizes and other constraints need distinct diagnosis; do not assume every failure has the same error code.

Complete MQL5 guide & code

MQL5 ArrayResize Function Guide: Dynamic Arrays & Dimension Rules

Master the MQL5 ArrayResize function. Understand dynamic vs static array rules, memory allocation with reserve_size, and why multidimensional arrays only resize the first dimension.

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What this MQL5 guide covers

ArrayResize allocates or adjusts elements in the first dimension of dynamic MQL5 arrays, returning the new total size or -1 on error.

Static arrays are not reallocated: requests no greater than ArraySize return the requested size without changing the array; larger requests return -1.

In multidimensional dynamic arrays, only the first dimension can be resized at runtime; all subsequent dimensions must remain constant integers fixed at compile time.

Setting the optional reserve_size parameter avoids repeated heap reallocations during high-frequency tick and bar ingestion loops.

ArrayResize Syntax and Basic Memory Allocation

The ArrayResize function sets a new size for the specified dynamic array. Its signature is int ArrayResize(void &array[], int new_size, int reserve_size = 0).

The return value is the total number of array elements after a successful resize, not just the first-dimension length. Newly exposed numeric elements must be explicitly assigned; values brought in from reserved storage are undefined. A failed resize returns -1.

Parameter	Type	Description
array[]	void & (generic)	Target dynamic array passed by reference
new_size	int	New element count for the first dimension (0 to empty)
reserve_size	int	Optional reserve capacity to minimize subsequent reallocations

Static Array Restrictions: Why ArrayResize Fails

A frequent error among beginner developers is attempting to resize an array declared with explicit bracket dimensions, such as double prices[50];. Static arrays have a fixed size compiled into the program stack frame.

ArrayResize does not reallocate a static array. If new_size is no greater than ArraySize, it returns new_size while leaving the actual array unchanged; otherwise it returns -1. Use a dynamic declaration such as double prices[] when resizing is required.

ArrayIsDynamic(arr) returns true for dynamic arrays and false for static arrays.
Static arrays allocate fixed memory immediately; dynamic arrays start with size 0.
Use ArrayFree(arr) or ArrayResize(arr, 0) to reset dynamic arrays back to zero elements.

Multidimensional Arrays: Second Dimension Fixed Documentation Rule

MQL5 supports multidimensional arrays up to four dimensions. However, the language specifies a strict constraint: only the first dimension can be dynamic and resized with ArrayResize.

All subsequent dimensions (the second, third, and fourth) must be fixed constant integers at declaration time. For example, double matrix[][4]; is valid and can be resized via ArrayResize(matrix, 20) into a 20x4 table. Declaring double matrix[][]; is invalid syntax and fails to compile.

Declaration Pattern	Valid in MQL5?	Behavior Details
double arr[][4];	Valid	First dimension dynamic (resizable); second dimension fixed to 4 columns
double arr[][];	Invalid	Compiler error: only first dimension can be left unspecified
double arr[10][4];	Valid (Static)	Entire table static; no reallocation; return depends on requested size
double flat[]; ArrayResize(flat, rows * cols);	Alternative	1D flat array with manual indexing flat[r * cols + c]

Loop Optimization Using the reserve_size Parameter

When ingesting ticks or candles one by one, incrementing the array size by 1 with ArrayResize(arr, size + 1) on every tick forces the operating system to reallocate memory chunks repeatedly. This induces heap fragmentation and CPU stalls.

Specifying reserve_size, such as ArrayResize(arr, size + 1, 1000), prompts MQL5 to reserve a larger contiguous memory buffer behind the scenes. Expansions within the reserved capacity avoid physical reallocation; they still execute code and do not guarantee zero processing latency.

Implementation details & mechanics

Core implementation pattern

Declare a dynamic array, request the needed first-dimension size and reserve capacity, and check for -1 before writing. Initialize every new element that your calculation will read. In a multidimensional array, use ArrayRange(array, 0) for the row count and ArraySize for the total element count.

Error handling & safety rules

ArrayFree releases a dynamic array when its contents are no longer needed. Shrinking an array retains its leading elements; it does not automatically select the most recent observations. Move or copy the desired records before shrinking a chronological history buffer.

MQL5 Code Implementation

//+------------------------------------------------------------------+
//|                                           ArrayResizeGuideScript.mq5|
//|                             Copyright 2026, Pineify Guides Series |
//|                                              https://pineify.app |
//+------------------------------------------------------------------+
#property copyright "Pineify Guides Series"
#property link      "https://pineify.app/mql5/arrayresize-guide"
#property version   "1.00"
#property script_show_inputs

//--- 1. Dynamic 1D Array
double g_dynamicPrices[];

//--- 2. Static 1D Array (Cannot be resized)
double g_staticPrices[10];

//--- 3. 2D Dynamic Array (Second dimension fixed to 3 columns: Open, High, Low)
double g_matrixOHL[][3];

//+------------------------------------------------------------------+
//| Script program start function                                    |
//+------------------------------------------------------------------+
void OnStart()
  {
   Print("=== MQL5 ArrayResize Demonstration ===");

   // Check dynamic status
   PrintFormat("g_dynamicPrices is dynamic? %s", ArrayIsDynamic(g_dynamicPrices) ? "YES" : "NO");
   PrintFormat("g_staticPrices is dynamic? %s", ArrayIsDynamic(g_staticPrices) ? "YES" : "NO");

   // A. Resizing a 1D dynamic array with reserve_size
   int targetSize = 5;
   int allocatedSize = ArrayResize(g_dynamicPrices, targetSize, 100);
   if(allocatedSize == -1)
     {
      PrintFormat("ArrayResize failed for 1D array! Error: %d", GetLastError());
      return;
     }

   PrintFormat("1D array resized to %d elements (Capacity reserved).", allocatedSize);

   // Populate elements
   for(int i = 0; i < targetSize; i++)
     {
      g_dynamicPrices[i] = 1.0850 + (i * 0.0010);
      PrintFormat("g_dynamicPrices[%d] = %.4f", i, g_dynamicPrices[i]);
     }

   // B. Multidimensional Array: Second dimension fixed rule
   // Resizing the first dimension to 4 rows (Each row has 3 columns)
   int rows = 4;
   int matrixRes = ArrayResize(g_matrixOHL, rows);
   if(matrixRes == -1)
     {
      PrintFormat("ArrayResize failed for 2D matrix! Error: %d", GetLastError());
      return;
     }

   PrintFormat("2D matrix has %d total elements: %d rows x 3 columns.",
               matrixRes, ArrayRange(g_matrixOHL, 0));

   // Fill sample values in 2D array
   for(int r = 0; r < rows; r++)
     {
      g_matrixOHL[r][0] = 100.0 + r; // Open
      g_matrixOHL[r][1] = 105.0 + r; // High
      g_matrixOHL[r][2] =  98.0 + r; // Low

      PrintFormat("Row [%d] -> O: %.1f | H: %.1f | L: %.1f",
                  r, g_matrixOHL[r][0], g_matrixOHL[r][1], g_matrixOHL[r][2]);
     }

   // C. Cleaning up dynamic arrays
   ArrayFree(g_dynamicPrices);
   ArrayFree(g_matrixOHL);

   PrintFormat("Cleanup complete: 1D size = %d, 2D size = %d",
               ArraySize(g_dynamicPrices), ArraySize(g_matrixOHL));
  }

Copy this code into MetaEditor, save it in your MQL5 folder (Experts, Scripts, or Include), and compile with F7.

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Frequently Asked Questions

Related MQL5 pages

Copybuffer Guide→Copyticks Guide→Price Series Copyrates Guide→Classes Structs Inheritance Guide→

Technical references

Risk and testing note

These educational examples require independent compilation and testing. A compile pass or an illustrative setting does not establish execution correctness or profitability. Any reported performance must be supported by reproducible data and test settings; past results do not predict future returns. Trading involves significant risk of loss. This content is not financial advice.