MT4 Indicator Source Code: A Working MQL4 Example

By FXNook Lab team

This complete MQL4 example draws an up arrow when a fast moving average crosses above a slow one, and a down arrow when it crosses below. Save the code as an .mq4 file, compile it in MetaEditor, then load the resulting indicator in MT4. The example also shows how its two indicator buffers hold arrow positions and why a signal on the current, unfinished bar can change.

A simple MT4 indicator source code example

The indicator runs in the main chart window. It calculates two moving averages with MT4's built-in iMA() function, but draws only crossover arrows, not the moving-average lines. The fast and slow periods, calculation method, applied price, and arrow offset are inputs you can change in the indicator settings.

Copy this complete source into a new file named MA_Crossover_Arrows.mq4. It uses the standard MQL4 custom-indicator event functions and does not require an external include file.

#property strict
#property indicator_chart_window
#property indicator_buffers 2
#property indicator_color1 clrLime
#property indicator_color2 clrTomato
#property indicator_width1 1
#property indicator_width2 1

input int FastMAPeriod = 10;
input int SlowMAPeriod = 25;
input ENUM_MA_METHOD MaMethod = MODE_EMA;
input ENUM_APPLIED_PRICE AppliedPrice = PRICE_CLOSE;
input int ArrowOffsetPoints = 10;

double BuyBuffer[];
double SellBuffer[];

int OnInit()
{
   if(FastMAPeriod <= 0 || SlowMAPeriod <= 0 || ArrowOffsetPoints < 0)
      return(INIT_PARAMETERS_INCORRECT);

   SetIndexBuffer(0, BuyBuffer);
   SetIndexBuffer(1, SellBuffer);
   ArraySetAsSeries(BuyBuffer, true);
   ArraySetAsSeries(SellBuffer, true);

   SetIndexStyle(0, DRAW_ARROW, STYLE_SOLID, 1, clrLime);
   SetIndexStyle(1, DRAW_ARROW, STYLE_SOLID, 1, clrTomato);
   SetIndexArrow(0, 233);
   SetIndexArrow(1, 234);
   SetIndexLabel(0, "MA cross up");
   SetIndexLabel(1, "MA cross down");
   SetIndexEmptyValue(0, EMPTY_VALUE);
   SetIndexEmptyValue(1, EMPTY_VALUE);
   IndicatorShortName("MA Crossover Arrows");

   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[])
{
   int required_bars = (int)MathMax(FastMAPeriod, SlowMAPeriod) + 2;
   if(rates_total < required_bars)
      return(0);

   ArraySetAsSeries(high, true);
   ArraySetAsSeries(low, true);

   BuyBuffer[rates_total - 1] = EMPTY_VALUE;
   SellBuffer[rates_total - 1] = EMPTY_VALUE;

   int start;
   if(prev_calculated == 0)
      start = rates_total - 2;
   else
      start = rates_total - prev_calculated;

   if(start < 1)
      start = 1;
   if(start > rates_total - 2)
      start = rates_total - 2;

   for(int i = start; i >= 0; i--)
   {
      BuyBuffer[i] = EMPTY_VALUE;
      SellBuffer[i] = EMPTY_VALUE;

      double fast_now = iMA(NULL, 0, FastMAPeriod, 0, MaMethod, AppliedPrice, i);
      double slow_now = iMA(NULL, 0, SlowMAPeriod, 0, MaMethod, AppliedPrice, i);
      double fast_previous = iMA(NULL, 0, FastMAPeriod, 0, MaMethod, AppliedPrice, i + 1);
      double slow_previous = iMA(NULL, 0, SlowMAPeriod, 0, MaMethod, AppliedPrice, i + 1);

      if(fast_now > slow_now && fast_previous <= slow_previous)
         BuyBuffer[i] = low[i] - ArrowOffsetPoints * _Point;
      else if(fast_now < slow_now && fast_previous >= slow_previous)
         SellBuffer[i] = high[i] + ArrowOffsetPoints * _Point;
   }

   return(rates_total);
}

In MQL4, iMA(symbol, timeframe, period, shift, method, applied_price, bar_shift) returns the moving-average value for a requested bar. Here, NULL and 0 mean the chart's current symbol and timeframe. The last argument, i or i + 1, selects the bar being compared.

MT4 uses series indexing in this calculation: index 0 is the current bar, index 1 is the most recently closed bar, and larger indexes are further back in history. When the fast average is above the slow average at index i, but was at or below it at i + 1, the code records an upward crossover. The downward condition is the reverse.

The buffers contain chart prices, not a simple true/false signal. A buy arrow is placed below the bar's low, and a sell arrow above its high; the offset is measured in points using _Point. The EMPTY_VALUE entries tell MT4 not to draw an arrow on bars without a crossover.

Source code, EX4 files, and what you can edit

The editable source and the compiled indicator are different files. MetaEditor uses the MQL4 source to build the file MT4 loads, but having the compiled file alone normally does not give you the original source or its readable calculations.

If you received only an .ex4, you may be able to install and run it, but you cannot use MetaEditor to make normal source edits to its calculations, inputs, or drawing behavior. Ask the author for the source if you need to maintain or adapt it. A compiled file is not a substitute for the source when you want to change how the indicator works.

How to compile an MQ4 indicator in MetaEditor

1. In MT4, choose File > Open Data Folder. Open MQL4, then Indicators, and save MA_Crossover_Arrows.mq4 in that folder. Saving it there keeps the source and compiled indicator in MT4's standard custom-indicator location.

2. Open MetaEditor from MT4 with Tools > MetaQuotes Language Editor or press F4. In MetaEditor, open the file from the Navigator or use File > Open to find it in the data folder.

3. Press F7 or select Compile. Read the results in the Errors tab at the bottom. A successful compile reports zero errors; warnings, if present, are also worth reading because they can point to code that may not behave as intended.

4. Locate the generated MA_Crossover_Arrows.ex4 in the same Indicators folder. Switch back to MT4 and refresh the Navigator's Indicators list, or restart the terminal if the new file does not appear.

A clean compile means the source passed the compiler, not that arrows must appear on every chart or that the calculation matches your intended trading rules. The next step is to attach the indicator and inspect its output on the symbol and timeframe you plan to use.

Install the indicator and add it to a chart

1. In MT4, open View > Navigator or press Ctrl+N. Right-click the Indicators section and choose Refresh. Expand Custom or the relevant indicator list and look for MA Crossover Arrows.

2. Drag the indicator onto a chart, or double-click its name. In the Inputs tab, choose the moving-average periods, method, applied price, and arrow offset. Select OK to attach it.

3. Look for green upward arrows and tomato-colored downward arrows. If the chart is too zoomed out to see them clearly, zoom in and inspect the bars around a crossover.

Indicators are attached to a particular chart. If the indicator appears on one chart but not another, compare the chart symbol and timeframe, and make sure you attached the custom indicator to the chart you intended. To remove it, right-click the chart, choose Indicators List, select the indicator, and click Delete.

How indicator buffers and signals work

An indicator buffer is an array MT4 reads as plotted output. This example declares two arrays and registers them with SetIndexBuffer(): buffer zero supplies buy-arrow prices, and buffer one supplies sell-arrow prices. SetIndexStyle() tells MT4 to draw each buffer as an arrow, while SetIndexArrow() selects the Wingdings symbol used for each plot.

ArraySetAsSeries() makes the buffer indexes follow the chart's series direction, with the current bar at zero. The calculation also sets the price arrays used for arrow placement to that direction. It processes the full available range on its first calculation; on later calls, prev_calculated lets it recalculate recent bars rather than looping through all history on every tick.

The example checks the current bar, including index zero. That means an arrow can appear during a bar and disappear before it closes if the moving averages no longer satisfy the crossover condition. If you need signals only on completed bars, base the displayed signal on index 1 and its preceding comparison bar, rather than using index zero as the signal bar. This changes when the arrow is displayed, so make that choice explicit when adapting the code.

For another example of an MT4 chart indicator and its setup, see MT4 support and resistance indicator: levels and setup.

Common compile and display problems

MetaEditor error messages usually include a source line number. Start at the first reported error: one missing bracket or undeclared name can cause several misleading errors below it. If you want help identifying compiler errors in existing code, FXNook Lab's MQL4/MQL5 error-fixing page describes its code-fixing and recompilation process.

Check the indicator before relying on its output

• Confirm the chart context. Note the symbol and timeframe in the chart title, then make sure the indicator is attached to that chart rather than another open chart.

• Inspect historical signals. Scroll through visible history and compare each arrow with the crossover condition: the fast average must cross the slow average between the bar carrying the arrow and the following older bar.

• Watch a live bar separately. Index zero is unfinished and can change with incoming prices. Compare its output with the last closed bar so you know whether you are looking at a provisional or completed signal.

• Read terminal messages. Open the Experts and Journal tabs at the bottom of MT4 if the indicator fails to load or reports a runtime problem. A compiler success message does not replace these terminal checks.

• Test changed settings again. Changing periods, price type, or timeframe changes which bars meet the crossover condition. After editing the source, compile the file again and reattach or refresh the indicator before reviewing the updated output.

These checks establish what the indicator draws and when it draws it; they do not establish that a trading rule is suitable or that past or tested results will continue in the future. If you need a different plot, an alert, or closed-bar-only signals, write down the exact conditions before changing the calculation. FXNook Lab's AI indicator builder is an option for specifying a custom MT4 or MT5 indicator; its page describes code generation and compilation checks. Your practical next step is to compile this example, attach it to a chart, and compare several arrows with the two moving-average values and bar indexes that produced them.