Suppose you create a screen menu that asks the user to select one of five choices—for example

• Cheap
• Moderate
• Expensive
• Extravagant
• Excessive

You can extend an if else if else sequence to handle five alternatives

• But the C++ switch statement more easily handles selecting a choice from an extended list
  • Here’s the general form for a switch statement (A-1):

Example of (A-1)

switch (integer-expression)
{
      case label1 : statement(s)
      case label2 : statement(s)
      ...
      default : statement(s)
}

A C++ switch statement acts as a routing device

• That tells the computer which line of code to execute next

On reaching a switch statement

• A program jumps to the line labeled with the value corresponding to the value of integer-expression
  • For example, if integer-expression has the value 4, the program goes to the line that has a case 4: label

The value integer-expression

• As the name suggests, must be an expression that reduces to an integer value

Also, each label

• Must be an integer constant expression

Most often, labels

• Are simple int or char constants, such as 1 or ‘q’, or enumerators

If integer-expression doesn’t match any of the labels

• The program jumps to the line labeled default
  • The default label is optional
    • If you omit it and there is no match, the program jumps to the next statement following the switch
      • See (Image-1)

(Image-1) The structure of switch statements

The switch statement

• Is different from similar statements in languages such as Pascal in a very important way

Each C++ case label functions

• Only as a line label, not as a boundary between choices
  • That is, after a program jumps to a particular line in a switch
    • It then sequentially executes all the statements following that line in the switch unless you explicitly direct it otherwise

Execution does not automatically stop at the next case

• To make execution stop at the end of a particular group of statements
  • You must use the break statement
    • This causes execution to jump to the statement following the switch

(A-2) shows how to use switch and break together to implement a simple menu for executives

• The program uses a showmenu() function to display a set of choices
  • A switch statement then selects an action based on the user’s response

Compatibility Note

• Some C++ implementations treat the \a escape sequence (used in case 1 in (A-2)) as silent

Example of (A-2)

// switch.cpp -- using the switch statement
#include <iostream>
using namespace std;
void showmenu(); // function prototypes
void report();
void comfort();
int main()
{
	showmenu();
	int choice;
	cin >> choice;
 
	while (choice != 5)
	{
		switch(choice)
		{
		case 1 : cout << "\a\n";
			break;
		case 2 : report();
			break;
		case 3 : cout << "The boss was in all day.\n";
			break;
		case 4 : comfort();
			break;
		default : cout << "That's not a choice.\n";
		}
		showmenu();
		cin >> choice;
	}
	cout << "Bye!\n";
	return 0;
}
 
void showmenu()
{
	cout << "Please enter 1, 2, 3, 4, or 5:\n"
		"1) alarm 2) report\n"
		"3) alibi 4) comfort\n"
		"5) quit\n";
}
void report()
{
	cout << "It's been an excellent week for business.\n"
		"Sales are up 150%. Expenses are down 35%.\n";
}
void comfort()
{
	cout << "Your employees think you are the finest CEO\n"
		"in the industry. The board of directors think\n"
		"you are the finest CEO in the industry.\n";
}

Here is a sample run of the executive menu program in (A-2):

The while loop terminates

• When the user enters 5

Entering 1 through 4

• Activates the corresponding choice from the switch list

Entering 10

• Triggers the default statements

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 (A-2) 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 (A-2) 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 (A-3):

Example of (A-3)

// switch.cpp -- using the switch statement
#include <iostream>
using namespace std;
void showmenu(); // function prototypes
void report();
void comfort();
int main()
{
	showmenu();
	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;
	}
	cout << "Bye!\n";
	return 0;
}
 
void showmenu()
{
	cout << "Please enter a, r, l, c, or q:\n"
		"a) alarm r) report\n"
		"l) alibi c) comfort\n"
		"q) quit\n";
}
void report()
{
	cout << "It's been an excellent week for business.\n"
		"Sales are up 150%. Expenses are down 35%.\n";
}
void comfort()
{
	cout << "Your employees think you are the finest CEO\n"
		"in the industry. The board of directors think\n"
		"you are the finest CEO in the industry.\n";
}

(A-3) because there is no break immediately following case ‘a’, program execution passes on to the next line

• Which is the statement following case ‘A’