math_utils/pitfalls.mq5

<EFBFBD><EFBFBD>//+------------------------------------------------------------------+
//|                                                     pitfalls.mq5 |
//|                                        Copyright <EFBFBD> 2018, Amr Ali |
//|                             https://www.mql5.com/en/users/amrali |
//+------------------------------------------------------------------+
#property copyright "Copyright <00> 2018, Amr Ali"
#property link      "https://www.mql5.com/en/users/amrali"
#property version   "4.0"
#property description "Script to test exact and loose comparisons of doubles."

#include <math_utils.mqh>
//+------------------------------------------------------------------+
//|                                                                  |
//+------------------------------------------------------------------+
#define PRINT(A) Print(#A + " = ", (A))

//+------------------------------------------------------------------+
//|                                                                  |
//+------------------------------------------------------------------+
void OnStart()
  {
//--- Examples of where exact comparison of doubles can fail:
   double a = 0.1;
   double b = 0.2;
   double c = a + b;

   PRINT( (c==0.3) );                    // false

//---
   double ask    = 1.2973;
   double bid    = 1.2972;
   double spread = (ask - bid) / 0.0001;

   PRINT( (spread==1.0) );               // false


//--- Switching to loose comparisons, results become as expected
   PRINT( EQ(c, 0.3) );                  // true
   PRINT( EQ(spread, 1.0) );             // true


   //PRINT(DoubleToString(c, 16));       // 0.3000000000000001
   //PRINT(DoubleToString(spread, 16));  // 0.9999999999998898
  }
//+------------------------------------------------------------------+

// output:
// EQ_test EURUSD,M5: (c==0.3) = false
// EQ_test EURUSD,M5: (spread==1.0) = false
// EQ_test EURUSD,M5: EQ(c,0.3) = true
// EQ_test EURUSD,M5: EQ(spread,1.0) = true