The if statement controls conditional branching. The body of an if statement is executed if the value of the expression is nonzero. The syntax for the if statement has two forms.

selection-statement:
if ( expression ) statement

if ( expression ) statement else statement

In both forms of the if statement, the expressions, which can have any value except a structure, are evaluated, including all side effects.

In the first form of the syntax, if expression is true (nonzero), statement is executed. If expression is false, statement is ignored. In the second form of syntax, which uses else, the second statement is executed if expression is false. With both forms, control then passes from the if statement to the next statement in the program unless one of the statements contains a break, continue, or goto.

The following are examples of the if statement:

if ( i > 0 )  
    y = x / i;  
else   
{  
    x = i;  
    y = f( x );  
}  

In this example, the statement y = x/i; is executed if i is greater than 0. If i is less than or equal to 0, i is assigned to x and f( x ) is assigned to y. Note that the statement forming the if clause ends with a semicolon.

When nesting if statements and else clauses, use braces to group the statements and clauses into compound statements that clarify your intent. If no braces are present, the compiler resolves ambiguities by associating each else with the closest if that lacks an else.

if ( i > 0 )           /* Without braces */  
    if ( j > i )  
        x = j;  
    else  
        x = i;  

The else clause is associated with the inner if statement in this example. If i is less than or equal to 0, no value is assigned to x.

if ( i > 0 )   
{                      /* With braces */  
    if ( j > i )  
        x = j;  
}  
else  
    x = i;  

The braces surrounding the inner if statement in this example make the else clause part of the outer if statement. If i is less than or equal to 0, i is assigned to x.

if-else Statement (C++)