5.3 Multiple Selection and Switch Statements

We have already seen (Lecture 1) how "if" statements can be strung together to form a "multiway branch". Here's a simplified version of the previous example: ...
...
if (total_test_score >=0 && total_test_score < 50)
cout << "You are a failure - you must study much harder.\n";
else if (total_test_score < 60)
cout << "You have just scraped through the test.\n";
else if (total_test_score < 80)
cout << "You have done quite well.\n";
else if (total_test_score <= 100)
cout << "Your score is excellent - well done.\n";
else
cout << "Incorrect score - must be between 0 and 100.\n";
...
...
Because multiple selection can sometimes be difficult to follow, C++ provides an alternative method of handling this concept, called the switch statement. "Switch" statements can be used when several options depend on the value of a single variable or expression. In the example above, the message printed depends on the value of "total_test_score". This can be any number between 0 and 100, but we can make things easier to handle by introducing an extra integer variable "score_out_of_ten", and adding the assignment: score_out_of_ten = total_test_score / 10;
The programming task is now as follows: (i) if "score_out_of_ten" has value 0, 1, 2, 3 or 4, print "You are a failure - you must study much harder", (ii) if "score_out_of_ten" has value 5, print "You have just scraped through the test", (iii) if "score_out_of_ten" has value 6 or 7, print "You have done quite well", and finally (iv) if "score_out_of_ten" has value 8, 9 or 10, print "Your score is excellent - well done". Here's how this is achieved with a "switch" statement: ...
...
score_out_of_ten = total_test_score / 10;

switch (score_out_of_ten)
{
case 0:
case 1:
case 2:
case 3:
case 4: cout << "You are a failure - you ";
cout << "must study much harder.\n";
break;

case 5: cout << "You have just scraped through the test.\n";
break;
case 6:
case 7: cout << "You have done quite well.\n";
break;

case 8:
case 9:
case 10: cout << "Your score is excellent - well done.\n";
break;
default: cout << "Incorrect score - must be between ";
cout << "0 and 100.\n";
}
...
...
Program 5.3.1
In general, the syntax of a "switch" statement is (approximately): switch (selector)
{
case label1:
break;
...
...
...

case labelN:
break;

default:
}
There are several things to note about such "switch" statements:
The statements which are executed are exactly those between the first label which matches the value of selector and the first "break" after this matching label.
The "break" statements are optional, but they help in program efficiency and clarity and should ideally always be used to end each case. When a "break" is encountered within a case's statement, control is transfered immediately to the first program statement following the entire "switch" statement. Otherwise, execution continues.
The selector can have a value of any ordinal type (e.g. "char" or "int" but not "float").
The "default" is optional, but is a good safety measure.