How to Code a Bill Williams Alligator Strategy EA in Mql4

Here is a complete, fully functional Expert Advisor (EA) for MetaTrader 4 based on Bill Williams' Alligator indicator.

Strategy Logic

This EA uses the classic Alligator trend-following logic:

1. The Alligator Lines:
   • Jaws (Blue Line): 13-period SMMA, shifted 8 bars.
   • Teeth (Red Line): 8-period SMMA, shifted 5 bars.
   • Lips (Green Line): 5-period SMMA, shifted 3 bars.
2. Buy Signal: The Alligator is "eating" with mouth open upwards.
   • Condition: Lips > Teeth AND Teeth > Jaws.
3. Sell Signal: The Alligator is "eating" with mouth open downwards.
   • Condition: Lips < Teeth AND Teeth < Jaws.
4. Execution: The EA checks conditions on the closed bar (Shift 1) to prevent signal repainting. It allows only one trade at a time per symbol.

MQL4 Code

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

//--- Input Parameters
input double InpLots          = 0.1;   // Lot size
input int    InpStopLoss      = 50;    // Stop Loss in points (0 = no SL)
input int    InpTakeProfit    = 100;   // Take Profit in points (0 = no TP)
input int    InpMagicNumber   = 12345; // Magic Number to identify orders
input int    InpSlippage      = 3;     // Maximum slippage in points

//--- Alligator Settings
input int    InpJawPeriod     = 13;    // Jaws Period
input int    InpJawShift      = 8;     // Jaws Shift
input int    InpTeethPeriod   = 8;     // Teeth Period
input int    InpTeethShift    = 5;     // Teeth Shift
input int    InpLipsPeriod    = 5;     // Lips Period
input int    InpLipsShift     = 3;     // Lips Shift
input ENUM_MA_METHOD InpMethod = MODE_SMMA; // Smoothing Method
input ENUM_APPLIED_PRICE InpPrice = PRICE_MEDIAN; // Applied Price

//+------------------------------------------------------------------+
//| Expert initialization function                                   |
//+------------------------------------------------------------------+
int OnInit()
{
   // Check if inputs are valid
   if(InpLots <= 0) {
      Alert("Lot size must be greater than 0");
      return(INIT_FAILED);
   }
   return(INIT_SUCCEEDED);
}

//+------------------------------------------------------------------+
//| Expert deinitialization function                                 |
//+------------------------------------------------------------------+
void OnDeinit(const int reason)
{
   // Clean up if necessary
}

//+------------------------------------------------------------------+
//| Expert tick function                                             |
//+------------------------------------------------------------------+
void OnTick()
{
   // 1. Check if we already have an open trade for this symbol/magic number
   if(CountOpenTrades() > 0) return;

   // 2. Define Shift (1 = Closed Bar, 0 = Current Bar)
   // We use 1 to ensure the signal is confirmed and won't repaint
   int shift = 1;

   // 3. Get Alligator Values
   double jaw   = iAlligator(Symbol(), Period(), InpJawPeriod, InpJawShift, InpTeethPeriod, InpTeethShift, InpLipsPeriod, InpLipsShift, InpMethod, InpPrice, MODE_GATORJAW, shift);
   double teeth = iAlligator(Symbol(), Period(), InpJawPeriod, InpJawShift, InpTeethPeriod, InpTeethShift, InpLipsPeriod, InpLipsShift, InpMethod, InpPrice, MODE_GATORTEETH, shift);
   double lips  = iAlligator(Symbol(), Period(), InpJawPeriod, InpJawShift, InpTeethPeriod, InpTeethShift, InpLipsPeriod, InpLipsShift, InpMethod, InpPrice, MODE_GATORLIPS, shift);

   // 4. Check for Buy Signal (Lips > Teeth > Jaws)
   if(lips > teeth && teeth > jaw)
   {
      OpenBuy();
   }
   // 5. Check for Sell Signal (Lips < Teeth < Jaws)
   else if(lips < teeth && teeth < jaw)
   {
      OpenSell();
   }
}

//+------------------------------------------------------------------+
//| Function to Open a Buy Order                                     |
//+------------------------------------------------------------------+
void OpenBuy()
{
   double sl = 0, tp = 0;
   
   // Calculate SL and TP based on points
   if(InpStopLoss > 0)     sl = Ask - InpStopLoss * Point;
   if(InpTakeProfit > 0)   tp = Ask + InpTakeProfit * Point;

   // Normalize prices
   sl = NormalizeDouble(sl, Digits);
   tp = NormalizeDouble(tp, Digits);

   int ticket = OrderSend(Symbol(), OP_BUY, InpLots, Ask, InpSlippage, sl, tp, "Alligator Buy", InpMagicNumber, 0, clrGreen);
   
   if(ticket < 0)
   {
      Print("OrderSend failed with error #", GetLastError());
   }
}

//+------------------------------------------------------------------+
//| Function to Open a Sell Order                                    |
//+------------------------------------------------------------------+
void OpenSell()
{
   double sl = 0, tp = 0;
   
   // Calculate SL and TP based on points
   if(InpStopLoss > 0)     sl = Bid + InpStopLoss * Point;
   if(InpTakeProfit > 0)   tp = Bid - InpTakeProfit * Point;

   // Normalize prices
   sl = NormalizeDouble(sl, Digits);
   tp = NormalizeDouble(tp, Digits);

   int ticket = OrderSend(Symbol(), OP_SELL, InpLots, Bid, InpSlippage, sl, tp, "Alligator Sell", InpMagicNumber, 0, clrRed);
   
   if(ticket < 0)
   {
      Print("OrderSend failed with error #", GetLastError());
   }
}

//+------------------------------------------------------------------+
//| Helper: Count open trades for this EA                            |
//+------------------------------------------------------------------+
int CountOpenTrades()
{
   int count = 0;
   for(int i = OrdersTotal() - 1; i >= 0; i--)
   {
      if(OrderSelect(i, SELECT_BY_POS, MODE_TRADES))
      {
         if(OrderSymbol() == Symbol() && OrderMagicNumber() == InpMagicNumber)
         {
            if(OrderType() == OP_BUY || OrderType() == OP_SELL)
               count++;
         }
      }
   }
   return count;
}
//+------------------------------------------------------------------+

How to Install and Use

1. Open MetaTrader 4.
2. Press F4 to open the MetaEditor.
3. Click New -> Expert Advisor (template) -> Name it "AlligatorTrader".
4. Delete everything in the new file and paste the code provided above.
5. Press F7 or click Compile. Ensure there are no errors.
6. Go back to the MT4 Terminal. The EA will appear in the "Navigator" window under "Expert Advisors".
7. Drag the EA onto a chart (e.g., EURUSD H1).
8. Ensure the "AutoTrading" button at the top of MT4 is green (Enabled).

Q&A: Strategy Details

Q: Why does the EA use shift = 1 instead of 0?
A: Shift 0 refers to the current, forming candle. The Alligator lines move as the price ticks up and down. If we trade on shift 0, the signal might appear and disappear within the same candle (repainting). Using shift 1 ensures we only trade when the candle has closed and the Alligator alignment is confirmed.

Q: How does the EA handle exits?
A: This version uses a fixed Stop Loss and Take Profit (defined in the inputs). It does not close the trade based on the Alligator lines crossing back (sleeping phase). This is generally safer for basic testing to ensure risk is managed mathematically rather than relying on lagging indicators for exits.

Q: Can I trade multiple pairs with this?
A: Yes. You can attach this EA to EURUSD, GBPUSD, etc., simultaneously. The InpMagicNumber ensures the EA manages each chart's trades independently. However, if you run it on multiple charts of the same pair (e.g., two EURUSD charts), you must change the Magic Number on the second chart to avoid conflicts.