MQL5 'array out of range' error: causes and fix

What it means

The program read or wrote an array element that doesn't exist: an index below 0, or equal to or above the array's size. It's a critical error. MetaTrader stops the program on the spot, so the rest of the handler doesn't run and the EA or indicator stops working. There's no GetLastError() code to catch: the check has to happen before the access.

Usual causes

• A copy that returned fewer values than you read. CopyBuffer(), CopyRates() and CopyClose() return how many values they copied, or -1. An array that received 0 or 2 values and is read at index 2 is out of range.
• An off-by-one loop. for(int i = 0; i <= ArraySize(a); i++) reads one element too many. The last valid index is size - 1.
• Series order confusion. After ArraySetAsSeries(a, true), index 0 is the newest value; without it, the oldest. Code written for one order and run with the other reads past the end.
• Indicator loops at the edges of history. Reading close[i - period] at the start of the chart, or close[i + 1] on the newest bar.
• A dynamic array that was never sized with ArrayResize().

How to detect it

• The Experts tab shows a line such as array out of range in 'MyEA.mq5' (24,15): the file, then the line and column of the access.
• Open the file in MetaEditor and go to that line: the array and the index are there. Print ArraySize() and the index just before it to see the mismatch.

Broken code

This EA compiles. On a tick where the copy returns fewer than 3 values, reading ma[2] stops it with array out of range.

input int InpPeriod = 20; // Moving average period

int maHandle = INVALID_HANDLE;

int OnInit()
  {
   maHandle = iMA(_Symbol, PERIOD_CURRENT, InpPeriod, 0, MODE_EMA, PRICE_CLOSE);
   return maHandle == INVALID_HANDLE ? INIT_FAILED : INIT_SUCCEEDED;
  }

void OnTick()
  {
   double ma[];
   ArraySetAsSeries(ma, true);
   CopyBuffer(maHandle, 0, 0, 3, ma);   // may copy fewer than 3 values, or none
   // Reads index 2 without checking how many values arrived
   if(ma[1] > ma[2])
      Comment("EMA rising");
  }

Fixed code

The same program with the check that prevents the error. It compiles in MetaEditor with 0 errors and 0 warnings.

input int InpPeriod = 20; // Moving average period

int maHandle = INVALID_HANDLE;

int OnInit()
  {
   maHandle = iMA(_Symbol, PERIOD_CURRENT, InpPeriod, 0, MODE_EMA, PRICE_CLOSE);
   return maHandle == INVALID_HANDLE ? INIT_FAILED : INIT_SUCCEEDED;
  }

void OnTick()
  {
   double ma[];
   ArraySetAsSeries(ma, true);
   // Use the values only if all 3 arrived
   if(CopyBuffer(maHandle, 0, 0, 3, ma) != 3)
      return;
   Comment(ma[1] > ma[2] ? "EMA rising" : "EMA not rising");
  }

void OnDeinit(const int reason)
  {
   IndicatorRelease(maHandle);
   Comment("");
  }

Why the fix works

CopyBuffer() returns the number of values it copied. Comparing it with the number requested before reading makes sure that indexes 0, 1 and 2 exist; when they don't, the EA waits for the next tick instead of crashing. The same rule applies to every array: compare the index with ArraySize() (or with what a copy function returned) before using it.

Questions

The programs on this page are minimal examples written to show the error and its fix, not trading systems. Some of them send orders when they run: try them on a demo account. Trading involves substantial risk of loss and is not suitable for every investor. Edgoro is a software tool, not a broker or financial adviser: nothing it produces is investment advice, and past or backtested performance does not guarantee future results.