Mql4 Guide: Parsing Data with StringSplit() and StringReplace()

To split a string into multiple parts in MQL4, you use the StringSplit() function. This function takes a source string, a separator character (converted to ASCII code), and a dynamic string array where the resulting substrings will be stored.

Here is a complete, runnable script that demonstrates how to split a comma-separated string. I have included a practical use case: parsing a "signal string" that might be received from an external source (like a CSV file or web request) containing trade parameters.

MQL4 Script: String Splitting and Processing

//+------------------------------------------------------------------+
//|                                            StringSplitExample.mq4 |
//|                        Copyright 2023, MetaQuotes Software Corp. |
//|                                             https://www.mql5.com |
//+------------------------------------------------------------------+
#property copyright "Copyright 2023, MetaQuotes Software Corp."
#property link      "https://www.mql5.com"
#property version   "1.00"
#property strict
#property script_show_inputs

//--- Input parameters
input string InpSignalString = "EURUSD,OP_BUY,0.1,1.0850,1.0950"; // Format: Symbol,Type,Lots,SL,TP
input string InpSeparator    = ",";                               // Separator character

//+------------------------------------------------------------------+
//| Script program start function                                    |
//+------------------------------------------------------------------+
void OnStart()
{
   // 1. Prepare the separator
   // StringSplit requires the separator to be a ushort (ASCII code).
   // We get the code of the first character of the input separator string.
   ushort u_sep = StringGetCharacter(InpSeparator, 0);

   // 2. Prepare the array to store results
   string result_array[];

   // 3. Execute the split
   // The function returns the number of substrings found.
   // The 'result_array' is passed by reference and populated automatically.
   int substring_count = StringSplit(InpSignalString, u_sep, result_array);

   // 4. Validate results
   if(substring_count > 0)
   {
      Print("Successfully split string into ", substring_count, " parts.");

      // 5. Process each part
      // We loop through the array to process the data.
      for(int i = 0; i < substring_count; i++)
      {
         // It is good practice to trim whitespace in case the string was "A, B, C"
         string current_part = result_array[i];
         StringTrimLeft(current_part);
         StringTrimRight(current_part);

         PrintFormat("Part [%d]: %s", i, current_part);
      }

      // 6. Practical Application: Parsing the specific signal format
      // Expected format: Symbol, Operation, Lots, SL, TP
      if(substring_count == 5)
      {
         ProcessTradeSignal(result_array);
      }
      else
      {
         Print("Error: Input string does not match expected format (5 parameters).");
      }
   }
   else
   {
      Print("Error: No substrings found or empty input.");
   }
}

//+------------------------------------------------------------------+
//| Helper function to interpret the split data                      |
//+------------------------------------------------------------------+
void ProcessTradeSignal(string& args[])
{
   Print("--- Processing Trade Signal ---");

   // Index 0: Symbol
   string symbol = args[0];
   
   // Index 1: Operation Type (String to Enum conversion logic)
   string op_string = args[1];
   int cmd = -1;
   if(StringFind(op_string, "BUY") >= 0) cmd = OP_BUY;
   else if(StringFind(op_string, "SELL") >= 0) cmd = OP_SELL;
   
   // Index 2: Volume (String to Double)
   double volume = StringToDouble(args[2]);
   
   // Index 3: Stop Loss (String to Double)
   double sl = StringToDouble(args[3]);
   
   // Index 4: Take Profit (String to Double)
   double tp = StringToDouble(args[4]);

   // Output the parsed logic
   if(cmd != -1)
   {
      PrintFormat("Signal Detected: %s %s Volume: %.2f SL: %.5f TP: %.5f", 
                  symbol, op_string, volume, sl, tp);
      
      // Logic to execute trade would go here:
      // OrderSend(symbol, cmd, volume, price, slippage, sl, tp, ...);
   }
   else
   {
      Print("Error: Unknown operation type in signal.");
   }
}
//+------------------------------------------------------------------+

Key Functions Used

1. StringGetCharacter(string, pos): StringSplit requires the separator to be of type ushort (the ASCII code), not a string. This function extracts the code from your separator string (e.g., extracting code 44 from ",").
2. StringSplit(string, separator, array[]): This is the core function. It resizes the dynamic array result_array automatically and fills it with the substrings. It returns the count of elements found.
3. StringTrimLeft() / StringTrimRight(): When parsing CSVs or user input, spaces often creep in (e.g., "EURUSD, OP_BUY"). Trimming ensures conversion functions like StringToDouble work correctly.
4. StringToDouble() / StringToInteger(): Used to convert the parsed string parts into numerical values required for trading functions like OrderSend.

Common Pitfalls

• Array Sizing: You do not need to manually ArrayResize the destination array before calling StringSplit. The function handles memory allocation for you.
• Separator Type: Passing a string literal "," directly as the second argument to StringSplit will cause a compilation error. You must pass the ushort character code (e.g., ',' or (ushort)44).

Q&A: String Manipulation in MQL4

Q: Can I use a multi-character separator (like ";;") with StringSplit?
A: No, StringSplit only accepts a single character (ushort) as a separator. If you need to split by a multi-character delimiter, you must write a custom function using StringFind and StringSubstr.

Q: What happens if the string starts with the separator?
A: The first element of the resulting array will be an empty string "".

Q: How do I handle CSV lines that might contain text with commas inside quotes?
A: The standard StringSplit function is naive; it splits on every occurrence of the separator. It does not respect quotes (e.g., "Comment, with comma", 1.0). For complex CSV parsing involving quotes, you must implement a custom parser that iterates through the string character by character.

Q: Is there a limit to the number of substrings?
A: The limit is dictated by available memory, but practically, StringSplit is very efficient for standard data processing tasks in trading.