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.
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 declared with fixed dimensions (e.g. double arr[20]) cannot be resized; calling ArrayResize on them produces compiler or runtime errors.
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).
If the array is resized upward, newly added elements are initialized to zero or empty strings depending on the underlying data type. The function returns the resulting element count of the first dimension, or -1 if memory allocation fails.
| 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.
Calling ArrayResize on a static array triggers compiler error 4001 or ERR_CANNOT_RESIZE_STATIC_ARRAY. Dynamic arrays must be declared without dimension constants inside brackets: double prices[];.
- 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; ArrayResize will return error |
| double 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. Future single-element expansions within that reserved window take zero additional allocation overhead.
Implementation details & mechanics
Core implementation pattern
In OnInit(), declare a dynamic record buffer and pre-allocate memory using ArrayResize(g_buffer, 0, 2000). When incoming ticks meet filtering criteria, increment the buffer using ArrayResize(g_buffer, currentSize + 1, 1000). The pre-allocated reserve eliminates tick execution delay. Calculate trailing price swings from the array to confirm entry conditions.
Error handling & safety rules
When positions close or trade sessions conclude, reclaim terminal memory using ArrayFree(g_buffer) in OnDeinit(const int reason). If trimming old bars from memory, resize down to keep only recent active data.
MQL5 Code Implementation Code
//+------------------------------------------------------------------+
//| 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 resized to %d rows x 3 columns.", matrixRes);
// 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.
Generate custom MQL5 code with Pineify AI
Describe the trade execution, indicator calculations, risk management, or data processing logic you need. Pineify produces clean, typed MQL5 source code ready to compile in MetaEditor.
Frequently Asked Questions
Related MQL5 pages
Technical references
Risk and testing note
Past performance is not indicative of future results. Backtest statistics are based on historical data and do not guarantee future profits. Trading involves significant risk of loss. This content is for educational purposes only and does not constitute financial advice.