Understanding MQL5 syntax is fundamental to building Expert Advisors. This guide covers variables, operators, and control flow—the building blocks of every trading program.
Variables and Data Types
Declaring Variables
Variables store data your program uses:
// Syntax: type variableName = value;
int myInteger = 100;
double myPrice = 1.12345;
string mySymbol = "EURUSD";
bool isBullish = true;Numeric Types
Integer Types (Whole Numbers):
int count = 42; // Regular integer
long bigNumber = 9999999; // Large integer
short smallInt = 100; // Small integer
char tiny = 1; // Very small (0-255)Floating Point (Decimals):
double price = 1.23456; // High precision (standard)
float approx = 1.234f; // Lower precision (rarely used)String Type
string symbol = "EURUSD";
string message = "Trade opened at " + DoubleToString(price, 5);
// String concatenation
string fullMessage = "Symbol: " + symbol + " Price: " + DoubleToString(1.12345, 5);Boolean Type
bool isActive = true;
bool hasPosition = false;
// Used in conditions
if(isActive)
{
Print("System is active");
}Datetime Type
datetime currentTime = TimeCurrent();
datetime specificDate = D'2024.06.15 14:30:00';
// Get time components
MqlDateTime timeStruct;
TimeToStruct(currentTime, timeStruct);
int hour = timeStruct.hour;
int minute = timeStruct.min;Arrays
// Static array (fixed size)
double prices[10];
// Dynamic array (resizable)
double closePrices[];
ArrayResize(closePrices, 100);
// Accessing elements
prices[0] = 1.1234; // First element
prices[9] = 1.1240; // Last element (index 9 for size 10)
// Array as series (newest = index 0)
ArraySetAsSeries(closePrices, true);Variable Scope
Local Variables
Exist only inside their function:
void OnTick()
{
int localVar = 10; // Only exists in OnTick
// localVar is destroyed when OnTick ends
}Global Variables
Exist throughout the program:
// Declared outside functions
int globalCounter = 0;
void OnTick()
{
globalCounter++; // Accessible here
}
void SomeFunction()
{
globalCounter++; // Accessible here too
}Static Variables
Keep value between function calls:
void OnTick()
{
static int tickCount = 0; // Initialized once
tickCount++; // Keeps counting
if(tickCount % 100 == 0)
Print("Tick count: ", tickCount);
}Operators
Arithmetic Operators
int a = 10, b = 3;
int sum = a + b; // Addition: 13
int diff = a - b; // Subtraction: 7
int product = a * b; // Multiplication: 30
int quotient = a / b; // Division: 3 (integer)
int remainder = a % b;// Modulo: 1
double c = 10.0, d = 3.0;
double result = c / d; // Division: 3.333...Comparison Operators
`mql5 int x = 5, y = 10;
bool equal = (x == y); // Equal to: false bool notEqual = (x != y); // Not equal: true bool greater = (x > y); // Greater than: false bool less = (x < y); // Less than: true bool greaterEq = (x >= y); // Greater or equal: false bool lessEq = (x <= y); // Less or equal: true `
Logical Operators
`mql5 bool a = true, b = false;
bool andResult = (a && b); // AND: false bool orResult = (a || b); // OR: true bool notResult = !a; // NOT: false
// Practical example if(price > MA && RSI < 30) { // Both conditions must be true }
if(price > upperBand || price < lowerBand) { // Either condition can be true } `
Assignment Operators
int x = 10;
x += 5; // Same as: x = x + 5; (15)
x -= 3; // Same as: x = x - 3; (12)
x *= 2; // Same as: x = x * 2; (24)
x /= 4; // Same as: x = x / 4; (6)
// Increment/Decrement
x++; // Same as: x = x + 1;
x--; // Same as: x = x - 1;Control Structures
If-Else Statements
double price = SymbolInfoDouble(_Symbol, SYMBOL_BID);
// Simple if
if(price > 1.1000)
{
Print("Price is above 1.1000");
}
// If-else
if(price > 1.1000)
{
Print("Price is high");
}
else
{
Print("Price is low or medium");
}
// If-else if-else chain
if(price > 1.1500)
{
Print("Price is very high");
}
else if(price > 1.1000)
{
Print("Price is high");
}
else if(price > 1.0500)
{
Print("Price is medium");
}
else
{
Print("Price is low");
}Ternary Operator
Shorthand for simple if-else:
// Syntax: condition ? valueIfTrue : valueIfFalse
string status = (price > 1.1000) ? "High" : "Low";
double risk = (accountBalance > 10000) ? 0.02 : 0.01;Switch Statement
For multiple specific values:
int dayOfWeek = DayOfWeek();
switch(dayOfWeek)
{
case 0:
Print("Sunday - Market closed");
break;
case 1:
Print("Monday - Week starts");
break;
case 5:
Print("Friday - Week ends");
break;
default:
Print("Regular trading day");
break;
}For Loop
When you know iteration count:
`mql5 // Count from 0 to 9 for(int i = 0; i < 10; i++) { Print("Iteration: ", i); }
// Count backwards for(int i = 9; i >= 0; i--) { Print("Countdown: ", i); }
// Loop through array double prices[]; CopyClose(_Symbol, PERIOD_H1, 0, 10, prices);
for(int i = 0; i < ArraySize(prices); i++) { Print("Price[", i, "] = ", prices[i]); } `
While Loop
When condition controls iterations:
int count = 0;
while(count < 5)
{
Print("Count: ", count);
count++;
}
// Careful: avoid infinite loops!
// while(true) { } // This runs foreverDo-While Loop
Execute at least once:
int value = 0;
do
{
Print("Value: ", value);
value++;
} while(value < 5);Break and Continue
Control loop flow:
// Break: exit loop immediately
for(int i = 0; i < 100; i++)
{
if(i == 10)
break; // Stops at i=10
Print(i);
}
// Continue: skip to next iteration
for(int i = 0; i < 10; i++)
{
if(i % 2 == 0)
continue; // Skip even numbers
Print(i); // Only prints odd: 1,3,5,7,9
}Functions
Creating Functions
// Function with no return value
void PrintPrice()
{
double price = SymbolInfoDouble(_Symbol, SYMBOL_BID);
Print("Current price: ", price);
}
// Function with return value
double GetSpread()
{
double bid = SymbolInfoDouble(_Symbol, SYMBOL_BID);
double ask = SymbolInfoDouble(_Symbol, SYMBOL_ASK);
return (ask - bid);
}
// Function with parameters
double CalculateLotSize(double riskPercent, double stopLossPips)
{
double accountBalance = AccountInfoDouble(ACCOUNT_BALANCE);
double riskAmount = accountBalance * riskPercent;
double pipValue = 10; // Simplified
return riskAmount / (stopLossPips * pipValue);
}Calling Functions
void OnTick()
{
// Call void function
PrintPrice();
// Call function with return
double spread = GetSpread();
Print("Spread: ", spread);
// Call with parameters
double lots = CalculateLotSize(0.02, 50);
Print("Lot size: ", lots);
}Practical Example
Combining everything learned:
`mql5 // Global variables input double RiskPercent = 2.0; input int MAPeriod = 20;
double maValues[];
int OnInit() { ArraySetAsSeries(maValues, true); Print("EA initialized with MA period: ", MAPeriod); return(INIT_SUCCEEDED); }
void OnTick() { // Get MA values if(CopyBuffer(iMA(_Symbol, PERIOD_H1, MAPeriod, 0, MODE_SMA, PRICE_CLOSE), 0, 0, 3, maValues) < 3) return;
double currentMA = maValues[0]; double previousMA = maValues[1]; double price = SymbolInfoDouble(_Symbol, SYMBOL_BID);
// Check conditions string signal = "None";
if(price > currentMA && previousMA < maValues[2]) { signal = "BUY"; } else if(price < currentMA && previousMA > maValues[2]) { signal = "SELL"; }
// Print signal if(signal != "None") { Print("Signal: ", signal, " at price ", price); } } ``
Key Takeaways
- Choose appropriate data types for your data
- Use meaningful variable names
- Understand scope: local, global, static
- Master comparison and logical operators
- Use if-else for branching logic
- Use loops for repetitive tasks
- Create functions for reusable code
- Always consider edge cases and errors
What's Next?
Learn how to create your first complete Expert Advisor that can place and manage trades.
This article is for educational purposes only and does not constitute financial advice.