Your First Indicator: A Moving Average

//+------------------------------------------------------------------+
//| Lesson 4 - First indicator: simple moving average                |
//+------------------------------------------------------------------+
#property copyright "Strategist Academy"
#property version   "1.00"
#property description "Hand-rolled SMA so you can see how buffers work."

#property indicator_chart_window
#property indicator_buffers 1
#property indicator_plots   1
#property indicator_type1   DRAW_LINE
#property indicator_color1  clrDodgerBlue
#property indicator_width1  2

input int SMA_Period = 20;   // Period

double SMABuffer[];

int OnInit()
{
   if(SMA_Period < 2)
   {
      Print("SMA_Period must be at least 2");
      return(INIT_FAILED);
   }

   SetIndexBuffer(0, SMABuffer, INDICATOR_DATA);
   PlotIndexSetInteger(0, PLOT_DRAW_BEGIN, SMA_Period - 1);
   PlotIndexSetString(0, PLOT_LABEL, "SMA(" + IntegerToString(SMA_Period) + ")");
   IndicatorSetString(INDICATOR_SHORTNAME, "Lesson 4 SMA(" + IntegerToString(SMA_Period) + ")");

   // NOTE: we do NOT call ArraySetAsSeries() here, so index 0 is the OLDEST bar.
   // This is MQL5's default. Mixing conventions is the classic beginner bug.
   return(INIT_SUCCEEDED);
}

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[])
{
   if(rates_total < SMA_Period)
      return(0);   // not enough bars yet - nothing to draw

   int start;
   if(prev_calculated == 0)
      start = SMA_Period - 1;          // first bar that CAN be calculated
   else
      start = prev_calculated - 1;     // only recompute the newest bars

   if(start < SMA_Period - 1)
      start = SMA_Period - 1;

   for(int i = start; i < rates_total && !IsStopped(); i++)
   {
      double sum = 0.0;
      for(int k = 0; k < SMA_Period; k++)
         sum += close[i - k];          // i is oldest-first: look backwards

      SMABuffer[i] = sum / SMA_Period;
   }

   return(rates_total);
}