[ fromfile: controlstructures.xml id: condexpr ]

Every programming language has at least one control structure that enables the flow of the program to branch depending on the outcome of a boolean condition. C and C++ have if and switch. The if statement typically has the following form.

   if(boolExpression) 
     statement
    

It can have an optional else attached.

   if(boolExpression) 
     statement1
   else
     statement2
    

Conditional statements can be nested, which means that they can get quite complicated. An important rule to keep in mind is that an else or else if clause is activated when the boolExpression of the immediately preceding open if evaluates to false. This can be confusing when your program logic allows you to omit some else clauses. Consider the following badly indented example, where x is an int.

if (x>0)
    if (x > 100)
        cout << "x is over a hundred";
else
    if (x == 0)  // no! this cannot be true -the indentation is misleading
        cout << "x is 0";
    else
        cout << "x is negative"; // no! x is between 1 and 100 inclusive!

You can clarify and repair this logic with braces.

if (x>0) {
    if (x > 100)
        cout << "x is over a hundred";
} 
else
    if (x == 0)  // now this is possible.
        cout << "x is 0";
    else
        cout << "x is negative";

An if without an else can be closed by enclosing the if statement in braces {}, making it a compound statement.

switch is another branching construct, which permits the execution of different code depending the value of a parameter.

switch(integralExpression) {
    case value1:
        statement1;
        break;
    case value2:
        statement2;
        break;
        ...
    case valuen:
        statementn;
        break;
    default:  
        defaultStatement;
}
nextStatement;

The switch statement is a computed goto statement. Each case is followed by a unique label value, which is compared to the integralExpression.

When the switch causes a jump to a case label whose value matches the integralExpression, statements are executed from that point on until the end of the switch block or a branching statement (e.g. break) is reached.

The optional default label is the jump destination when the integralExpression does not match any case label value. If default is omitted, and no matching case label exists, then the jump destination is nextStatement.

The integralExpression must be an expression that evaluates to an integer. Each case label, except default, must be an integer constant.^[107]

Any switch statement such as the previous one can be rewritten as a long if ... else statement. However, the runtime performance of a switch is considerably better because it requires only a single comparison, and performs only one branch.

  if(integralExpression == value1)
     statement1;
  else if(integralExpression == value2)
     statement2;
  ...
  else if(integralExpression == valuen)
     statementn;
  else
     defaultStatement;