Conditional Statements

Conditional statements allow the programmer to make some parts of the program optional; that is, they allow the programmer to say that some code can only run if the right conditions are met.

if Statements

An if statement uses a Boolean expression to determine whether to execute a statement or to skip it. Here is the syntax template:

    
if (Boolean_Expression)
{
	//code to run if Boolean_Expression is true
}

The expression in parentheses can be of any simple data type. Almost without exception, this will be a logical (Boolean) expression; if not, its value is implicitly coerced to type bool(nonzero value means true, zero value means false).

Now let's look at the following example:

 
    int number, sum;
    sum = 10;
    cout << "Please enter an integer value: " << endl;
    cin >> number;
    if (number < 0)
    {
        number = 0;
    }

    sum = sum + number;
    cout << "The sum is " << sum << endl;

The expression (number < 0) is evaluated.
If the result is true, the statement number = 0; is executed.
If the result is false, that statement is skipped.
In either case, the next statement to be executed is sum = sum + number;.

if-else Statements

An if-else statement uses a Boolean expression to determine which one of the two statements to execute. Here is the syntax template:

    
if (Boolean_Expression)
{
	//code to run if Boolean_Expression is true
}
else 
{
	//code to run if Boolean_Expression is false
}

The boolean expression in parentheses will be evaluated with the result of true or false. True results will always cause the statement or block after the if to run. False results will cause the statement or block marked with else to run. Here is an example:

 
    cout << "You are ";
    if (age >= 65)
       cout << "a senior ";
    else
       cout << "not a senior ";
    cout << "citizen." << endl;

The characters "You are " are sent to the output stream.
The expression age >= 65 is evaluated.
If the result is true, the characters "a senior " are sent to the output stream.
If the result is false, the characters "not a senior " are sent to the output stream.
In either case, the next statement to be executed sends the the characters "citizen." to the output stream.

There is one thing to point out here: any statement in an IF or ELSE statement could be a block or a compound statement. If so, they must be enclosed in a pair of braces. Here is an example which also shows a compound expression:

 
    if ( (age > 65) && (gender == 'f') )
    {
        cout << "Quilting group meets:";
        cout << "Thursday afternoons at 4:00 p.m.";
    }

 
    if ( (age > 65) && (gender == 'f') )
        cout << "Quilting group meets:"; // only old ladies see this
    cout << "Thursday afternoons at 4:00 p.m.";// everyone sees this

 
   if ( (age > 65) && (gender == 'f') )
        cout << "Quilting group meets:";
    cout << "Thursday afternoons at 4:00 p.m."; // you will always see this

    else // Syntax error
        cout << "No quilting for you!"; 

Nested if Statements

An if statemenet uses a Boolean expression to determine whether to execute or skip a statement. An if-else statement uses a Boolean expression to determine which one of two statements to execute. The statements to be executed or skipped could be simple statements or compound statements (blocks). These optional statements can also include an if or if-else statement. The result is called a nested if statement.

A look-ahead: You will soon be learning about the C++ SWITCH statement. For some very complex if/else constructs, it is preferable to use the switch instead.

The following example is a nested If statement.

 
    cout << "You are ";
    if (age >= 65)
         cout << "a senior." << endl;
    else
         if (age >= 19)
             cout << "an adult." << endl;
         else
             if (age >= 13)
                 cout << "a teenager." << endl;
             else
                 cout << "a child." << endl;
    cout << "You are a great person." << endl;

The characters "You are " are sent to the output stream.
The expression age >= 65 is evaluated.
If the result is true, the characters "a senior." are sent to the output stream.
If the result is false, the expression age >= 19 is evaluated.
If the result is true,the characters "an adult." are sent to the output stream.
If the result is false, the expression age >= 13 is evaluated.
If the result is true,the characters "a teeneager." are sent to the output stream.
If the result is false, the expression "a child. is sent to the output stream.
In any case above, the next statement to be executed sends the the characters "You are a great person." to the output stream.

Notice: once age has a value, only one statement is selected to be executed. If we add braces to the program segment, it would be easy to see the levels of nesting. Let's look at it now:

    cout << "You are ";
    if (age >= 65)
    {
         cout << "a senior." << endl;
         cout << "*****" << endl;
    }
    else
    {
         if (age >= 19)
         {
             cout << "an adult." << endl;
             cout << "*****" << endl;
         }
         else
         {
              if (age >= 13)
              {
                  cout << "a teenager." << endl;
                  cout << "*****" << endl;
              }
              else
              {
                  cout << "a child." << endl;
                  cout << "*****" << endl;
              }
         }
    }
    cout << "You are a great person." << endl;

else-if Statements

If you have a list of options you wish to test one after another, you might want to try using some else if statements instead of nesting an if as the only code inside an else. You can study the following example, then try making a similar change to the age example above.