What Are Indicators

Overview

In MQL5, an indicator analyzes market data and visualizes results on a chart or subwindow. Unlike Scripts and Expert Advisors, indicators do not send orders; they compute values and draw them as lines, histograms, or arrows so you can interpret trends and signals. You can use built-in indicators (e.g., Moving Average, RSI) or create custom ones with MQL5.

What You Will Be Able to Do

• Explain how indicators process price data and render values.
• Differentiate built-in vs. custom indicators.
• Create, compile, and attach a simple custom indicator.
• Locate compiled files under MQL5/Indicators/.

Code Example

//+------------------------------------------------------------------+
//|                                              MyFirstIndicator.mq5 |
//+------------------------------------------------------------------+
#property copyright "Your Name"
#property version   "1.00"
#property indicator_chart_window
#property indicator_plots   1
#property indicator_type1   DRAW_LINE
#property indicator_color1  clrDodgerBlue
#property indicator_label1  "MySMA"
#property indicator_width1  2
#property indicator_buffers 1

input int                 Period = 10;                  // MA period
input ENUM_MA_METHOD      Method = MODE_SMA;            // MA method
input ENUM_APPLIED_PRICE  Price  = PRICE_CLOSE;         // Applied price

double Buf[];               // output buffer (what we draw)
int    ma_handle = INVALID_HANDLE;

int OnInit()
  {
   // Plot starts after we have enough bars for the period
   PlotIndexSetInteger(0, PLOT_DRAW_BEGIN, Period - 1);

   // Bind the output buffer to the first (and only) plot
   SetIndexBuffer(0, Buf, INDICATOR_DATA);

   // Create a handle of the built-in iMA to reuse its tested logic
   ma_handle = iMA(_Symbol, _Period, Period, 0, Method, Price);
   if(ma_handle == INVALID_HANDLE)
      return(INIT_FAILED);

   return(INIT_SUCCEEDED);
  }

void OnDeinit(const int reason)
  {
   if(ma_handle != INVALID_HANDLE)
      IndicatorRelease(ma_handle);
  }

// Called whenever the terminal has new data or needs a redraw
int OnCalculate(const int rates_total,
                const int prev_calculated,
                const int begin,
                const double& price[])
  {
   // Fill our drawing buffer from the iMA values
   int copied = CopyBuffer(ma_handle, 0, 0, rates_total, Buf);
   if(copied <= 0)
      return(prev_calculated);

   return(rates_total);
  }

Execution Steps

1. Open MetaEditor → File → New → Custom Indicator and name it MyFirstIndicator.
2. Replace the template with the code above and press F7 to compile.
3. In MetaTrader 5, attach MyFirstIndicator to any chart.
4. Use the Inputs tab to change Period, Method, or Price and observe the line update.
5. Find the compiled file at MQL5/Indicators/MyFirstIndicator.ex5.

Key Point

Indicators compute and draw values; they do not trade. They are often used as inputs to Expert Advisors or for visual decision support on charts.

Next Section

→ Next: Reading Indicator Values