C++ Primer Plus

Your employees think you are the finest CEO

in the industry. The board of directors think

you are the finest CEO in the industry.

Please enter 1, 2, 3, 4, or 5:

1) alarm           2) report

3) alibi           4) comfort

5) quit

2

It's been an excellent week for business.

Sales are up 120%. Expenses are down 35%.

Please enter 1, 2, 3, 4, or 5:

1) alarm           2) report

3) alibi           4) comfort

5) quit

6

That's not a choice.

Please enter 1, 2, 3, 4, or 5:

1) alarm           2) report

3) alibi           4) comfort

5) quit

5

Bye!

The while loop terminates when the user enters 5. Entering 1 through 4 activates the corresponding choice from the switch list, and entering 6 triggers the default statements.

Note that input has to be an integer for this program to work correctly. If, for example, you enter a letter, the input statement will fail, and the loop will cycle endlessly until you kill the program. To deal with those who don’t follow instructions, it’s better to use character input.

As noted earlier, this program needs the break statements to confine execution to a particular portion of a switch statement. To see that this is so, you can remove the break statements from Listing 6.10 and see how it works afterward. You’ll find, for example, that entering 2 causes the program to execute all the statements associated with case labels 2, 3, 4, and the default. C++ works this way because that sort of behavior can be useful. For one thing, it makes it simple to use multiple labels. For example, suppose you rewrote Listing 6.10 using characters instead of integers as menu choices and switch labels. In that case, you could use both an uppercase and a lowercase label for the same statements:

char choice;

cin >> choice;

while (choice != 'Q' && choice != 'q')

{

    switch(choice)

    {

        case 'a':

        case 'A': cout << "\a\n";

                  break;

        case 'r':

        case 'R': report();

                  break;

        case 'l':

        case 'L': cout << "The boss was in all day.\n";

                  break;

        case 'c':

        case 'C': comfort();

                  break;

        default : cout << "That's not a choice.\n";

    }

    showmenu();

    cin >> choice;

}

Because there is no break immediately following case 'a', program execution passes on to the next line, which is the statement following case 'A'.

Using Enumerators as Labels

Listing 6.11 illustrates using enum to define a set of related constants and then using the constants in a switch statement. In general, cin doesn’t recognize enumerated types (it can’t know how you will define them), so the program reads the choice as an int. When the switch statement compares the int value to an enumerator case label, it promotes the enumerator to int. Also the enumerators are promoted to type int in the while loop test condition.

Listing 6.11. enum.cpp

// enum.cpp -- using enum

#include

// create named constants for 0 - 6

enum {red, orange, yellow, green, blue, violet, indigo};

int main()

{

    using namespace std;

    cout << "Enter color code (0-6): ";

    int code;

    cin >> code;

    while (code >= red && code <= indigo)

    {

        switch (code)

        {

            case red     : cout << "Her lips were red.\n"; break;

            case orange  : cout << "Her hair was orange.\n"; break;

            case yellow  : cout << "Her shoes were yellow.\n"; break;

            case green   : cout << "Her nails were green.\n"; break;

            case blue    : cout << "Her sweatsuit was blue.\n"; break;

            case violet  : cout << "Her eyes were violet.\n"; break;

            case indigo  : cout << "Her mood was indigo.\n"; break;

        }

        cout << "Enter color code (0-6): ";

        cin >> code;

    }

    cout << "Bye\n";

    return 0;

}

Here’s sample output from the program in Listing 6.11:

Enter color code (0-6): 3

Her nails were green.

Enter color code (0-6): 5

Her eyes were violet.

Enter color code (0-6): 2

Her shoes were yellow.

Enter color code (0-6): 8

Bye

switch and if else

Both the switch statement and the if else statement let a program select from a list of alternatives. The if else is the more versatile of the two. For example, it can handle ranges, as in the following:

if (age > 17 && age < 35)

    index = 0;

else if (age >= 35 && age < 50)

    index = 1;

else if (age >= 50 && age < 65)

    index = 2;

else

    index = 3;

The switch statement, on the other hand, isn’t designed to handle ranges. Each switch case label must be a single value. Also that value must be an integer (which includes char), so a switch statement can’t handle floating-point tests. And the case label value must be a constant. If your alternatives involve ranges or floating-point tests or comparing two variables, you should use if else.

If, however, all the alternatives can be identified with integer constants, you can use a switch or an if else statement. Because that’s precisely the situation that the switch statement is designed to process, the switch statement is usually the more efficient choice in terms of code size and execution speed, unless there are only a couple alternatives from which to choose.

Tip

If you can use either an if else if sequence or a switch statement, the usual practice is to use switch if you have three or more alternatives.