Using switch Statements in C++

By: Emiley J

if and if/else statements can become quite confusing when nested too deeply, and C++ offers an alternative. Unlike if, which evaluates one value, switch statements allow you to branch on any of a number of different values. The general form of the switch statement is:

switch (expression)
{
case valueOne: statement;
                   break;
case valueTwo: statement;
                   break;
....
case valueN:   statement;
                   break;
default:       statement;
}

expression is any legal C++ expression, and the statements are any legal C++ statements or block of statements. switch evaluates expression and compares the result to each of the case values. Note, however, that the evaluation is only for equality; relational operators may not be used here, nor can Boolean operations.

If one of the case values matches the expression, execution jumps to those statements and continues to the end of the switch block, unless a break statement is encountered. If nothing matches, execution branches to the optional default statement. If there is no default and there is no matching value, execution falls through the switch statement and the statement ends.

NOTE: It is almost always a good idea to have a default case in switch statements. If you have no other need for the default, use it to test for the supposedly impossible case, and print out an error message; this can be a tremendous aid in debugging.

It is important to note that if there is no break statement at the end of a case statement, execution will fall through to the next case statement. This is sometimes necessary, but usually is an error. If you decide to let execution fall through, be sure to put a comment, indicating that you didn't just forget the break.

This C++ program illustrates use of the switch statement.

Demonstrating the switch statement.

1:  //
2:  // Demonstrates switch statement
3:
4:  #include <iostream.h>
5:
6:  int main()
7:  {
8:    unsigned short int number;
9:    cout << "Enter a number between 1 and 5: ";
10:    cin >> number;
11:    switch (number)
12:    {
13:       case 0:   cout << "Too small, sorry!";
14:                 break;
15:       case 5:  cout << "Good job!\n";  // fall through
16:       case 4:  cout << "Nice Pick!\n"; // fall through
17:       case 3:  cout << "Excellent!\n"; // fall through
18:       case 2:  cout << "Masterful!\n"; // fall through
19:       case 1:  cout << "Incredible!\n";
20:                break;
21:       default: cout << "Too large!\n";
22:                break;
23:    }
24:    cout << "\n\n";
25:     return 0;
26: }

Output: Enter a number between 1 and 5: 3
Excellent!
Masterful!
Incredible!

Enter a number between 1 and 5: 8
Too large!

Analysis: The user is prompted for a number. That number is given to the switch statement. If the number is 0, the case statement on line 13 matches, the message Too small, sorry! is printed, and the break statement ends the switch. If the value is 5, execution switches to line 15 where a message is printed, and then falls through to line 16, another message is printed, and so forth until hitting the break on line 20.

The net effect of these statements is that for a number between 1 and 5, that many messages are printed. If the value of number is not 0-5, it is assumed to be too large, and the default statement is invoked on line 21.

The switch Statement

The syntax for the switch statement is as follows:

switch (expression)
{
case valueOne: statement;
case valueTwo: statement;
....
case valueN: statement
default: statement;
}

The switch statement allows for branching on multiple values of expression. The expression is evaluated, and if it matches any of the case values, execution jumps to that line. Execution continues until either the end of the switch statement or a break statement is encountered. If expression does not match any of the case statements, and if there is a default statement, execution switches to the default statement, otherwise the switch statement ends.

Example 1

switch (choice)
{
case 0:
        cout << "Zero!" << endl;
        break
case 1:
        cout << "One!" << endl;
       break;
case 2:
       cout << "Two!" << endl;
default:
       cout << "Default!" << endl;
}

Example 2

switch (choice)
{
choice 0:
choice 1:
choice 2:
       cout << "Less than 3!";
       break;
choice 3:
       cout << "Equals 3!";
       break;
default:
       cout << "greater than 3!";
}