Hey there, let’s break down why you’re hitting that frustrating "Array access is invalid" error when working with the arrays passed to OnCalculate(), and walk through how to resolve it.

Common Causes of the Error #

This error almost always boils down to invalid array indexing—here are the most frequent scenarios:

• Index out of bounds: You’re trying to access an element at an index that’s either greater than rates_total - 1 (since MQL5 arrays are 0-based) or negative. For example, close[rates_total] will fail because the last valid index is rates_total - 1.
• Ignoring prev_calculated: If you loop through all elements every time OnCalculate() runs (instead of only new ones since the last call), you might accidentally access stale or uninitialized array segments, especially when the chart’s data updates.
• Edge case data mismatches: Sometimes prev_calculated can be larger than rates_total (e.g., when historical data is reloaded or adjusted), leading you to index beyond the current array length.

Step-by-Step Fixes #

Let’s walk through actionable fixes to eliminate this error:

1. Enforce valid index boundaries
   Always ensure any array index falls between 0 and rates_total - 1. When processing new candles, start your loop at prev_calculated instead of 0—this avoids reprocessing old data and keeps you within valid bounds.

2. Handle prev_calculated edge cases upfront
   Add checks at the start of OnCalculate() to avoid unnecessary (and risky) array access:

   • If prev_calculated == rates_total, there’s no new data to process—just return rates_total immediately.
   • If prev_calculated is greater than rates_total or negative, reset it to 0 to reprocess all data safely.

3. Debug to pinpoint bad indexes
   If you’re still stuck, add debug prints to log the index you’re trying to access and the current rates_total value:

   Print("Attempting to access index: ", i, " | rates_total: ", rates_total);
   

   You can also use MQL5’s built-in debugger to set breakpoints and inspect array lengths in real time.

Example Corrected OnCalculate() #

Here’s how to structure your function to avoid the error:

int OnCalculate(const int rates_total, const int prev_calculated,
                const datetime &time[], const double &open[],
                const double &high[], const double &low[], const double &close[],
                const long &tick_volume[], const long &volume[], const int &spread[] )
{
    // Exit early if there's no valid data
    if(rates_total <= 0) return 0;
    
    // No new data to process? Return immediately
    if(prev_calculated == rates_total) return rates_total;
    
    // Reset start index if it's invalid
    int start_idx = prev_calculated;
    if(start_idx > rates_total || start_idx < 0)
        start_idx = 0;
    
    // Process only new candles (safe array access)
    for(int i = start_idx; i < rates_total; i++)
    {
        // Safe access to array elements
        double current_close = close[i];
        datetime current_time = time[i];
        // Add your logic here...
    }
    
    return rates_total;
}

内容的提问来源于stack exchange，提问作者Jaffer Wilson