AlgoBookSections
MQL5 Programming for TradersProgramming fundamentalsStatementsSimple statements (expressions) 
Download in one file:
MQL5 Algo Book in PDF
MQL5 Algo Book in CHM

Simple statements (expressions)

Simple statements contain expressions, such as assigning new values or calculation results to variables, as well as function calls.

Formally, the syntax looks like this:

expression ;

The semicolon at the end is important here. Since MQL5 source codes support free formatting, the ';' is the only delimiter that tells the compiler where the previous statement ended and the next one began. As a rule, statements are written on separate lines, for example, like this:

int i = 0, j = 1, k;   // declaration statement
++i;                   // simple statement
j += i;                // simple statement
k = (i + 1) * (j + 1); // simple statement
Print(i, " ", j);      // simple statement

However, the rules do not prohibit shorthand code writing:

int i=0,j=1;++i;j+=i;k=(i+1)*(j+1);Print(i," ",j);

If it weren't for the ';', adjacent expressions could silently "stick together" and lead to unintended results. For example, the expression x = y - 10 * z could well be two: x = y; and -10 * z; (-10 with a unary minus). How is this possible?

The fact is that it is syntactically permissible to write a statement that actually works in vain, i.e., does not save the result. Here is another example:

i + j; // warning: expression has no effect

The compiler issues an "expression has no effect" warning. The possibility to construct such expressions is necessary because the object types, which we will learn in Part 3, allow for the operator overloading, i.e., we can replace the usual meaning of operator symbols with some specific actions. Then, if the type of i and j is not int, but some class with an overridden addition operation, such a notation will have an effect, and the compiler will not issue a warning.

Simple statements can only be written inside compound statements. For example, calling the Print function outside of a function will not work:

Print("Hello ", Symbol());
void OnStart()
{
}

We will get a cascade of errors::

'Print' - unexpected token, probably type is missing?
'Hello, ' - declaration without type
'Hello, ' - comma expected
'Symbol' - declaration without type
'(' - comma expected
')' - semicolon expected
')' - expressions are not allowed on a global scope

The most relevant, in this case, is the last one: "expressions are not allowed in the global context."