[Course] [Build an EA 3] Turning a trading idea into numeric rules

Where people get stuck most isn't the code — it's defining the rules. A computer doesn't understand "roughly," "if it rises a lot" or "feels like." Everything must become numbers and yes/no.

Vague words → numbers

• "In an uptrend" → when the 20 EMA is above the 50 EMA
• "If it drops a lot" → when RSI(14) falls below 30
• "A reasonable stop" → $30 below the entry (3,000 points)
• "Sit out busy times" → no entries 13:30–14:30 GMT

Write them down like this in a notepad.

The 7-item rule checklist

An EA needs all seven. If one is missing, you can't write the code.

1. Symbol and timeframe: e.g. XAUUSD, M15
2. Buy entry: e.g. 20 EMA crosses above 50 EMA
3. Sell entry: e.g. 20 EMA crosses below 50 EMA
4. Stop loss: e.g. $30 against the entry
5. Take profit: e.g. 2× the stop ($60)
6. Size (lots): e.g. lose only 1% of the account if stopped out
7. How many at once: e.g. one at a time; no new entry while a position is open

The example strategy for this series

• Symbol: XAUUSD / timeframe: M15
• Buy: 20 EMA crosses above 50 EMA (golden cross)
• Sell: 20 EMA crosses below 50 EMA (death cross)
• Stop $30, target $60
• One position at a time

Note: this is a practice example, not a strategy that guarantees profit. It was chosen because it's the easiest way to learn the structure.

Only look at closed candles

The most common beginner mistake is deciding on a candle that hasn't closed. The current candle changes every second, so crosses appear and disappear. Our EA will decide once, at the moment a new candle opens, based on the candle that just closed.

Have your own idea? Fill in the 7-item checklist and post it in Q&A. We'll help check whether it can be turned into code.