How to use Enumerated constants in C++

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Enumerated constants enable you to create new types and then to define variables of those types whose values are restricted to a set of possible values. For example, you can declare COLOR to be an enumeration, and you can define that there are five values for COLOR: RED, BLUE, GREEN, WHITE, and BLACK.

The syntax for enumerated constants is to write the keyword enum, followed by the type name, an open brace, each of the legal values separated by a comma, and finally a closing brace and a semicolon. Here's an example:

enum COLOR { RED, BLUE, GREEN, WHITE, BLACK };

This statement performs two tasks:

1. It makes COLOR the name of an enumeration, that is, a new type.

2. It makes RED a symbolic constant with the value 0, BLUE a symbolic constant with the value 1, GREEN a symbolic constant with the value 2, and so forth.

Every enumerated constant has an integer value. If you don't specify otherwise, the first constant will have the value 0, and the rest will count up from there. Any one of the constants can be initialized with a particular value, however, and those that are not initialized will count upward from the ones before them. Thus, if you write

enum Color { RED=100, BLUE, GREEN=500, WHITE, BLACK=700 };

then RED will have the value 100; BLUE, the value 101; GREEN, the value 500; WHITE, the value 501; and BLACK, the value 700.

You can define variables of type COLOR, but they can be assigned only one of the enumerated values (in this case, RED, BLUE, GREEN, WHITE, or BLACK, or else 100, 101, 500, 501, or 700). You can assign any color value to your COLOR variable. In fact, you can assign any integer value, even if it is not a legal color, although a good compiler will issue a warning if you do. It is important to realize that enumerator variables actually are of type unsigned int, and that the enumerated constants equate to integer variables. It is, however, very convenient to be able to name these values when working with colors, days of the week, or similar sets of values. Listing below presents a program that uses an enumerated type.

A demonstration of enumerated constants.

1:  #include <iostream.h>
2:  int main()
3:  {
4:       enum Days { Sunday, Monday, Tuesday, Wednesday, Thursday, Friday,                     Â_Saturday };
5:
6:       Days DayOff;
7:       int x;
8:
9:       cout << "What day would you like off (0-6)? ";
10:      cin  >> x;
11:      DayOff = Days(x);
12:
13:      if (DayOff == Sunday || DayOff == Saturday)
14:            cout << "\nYou're already off on weekends!\n";
15:      else
16:            cout << "\nOkay, I'll put in the vacation day.\n";
17:       return 0;
18: }
Output: What day would you like off (0-6)?  1

Okay, I'll put in the vacation day.

What day would you like off (0-6)?  0

You're already off on weekends!

Analysis: On line 4, the enumerated constant DAYS is defined, with seven values counting upward from 0. The user is prompted for a day on line 9. The chosen value, a number between 0 and 6, is compared on line 13 to the enumerated values for Sunday and Saturday, and action is taken accordingly.

The if statement will be covered in more detail on Day 4, "Expressions and Statements."

You cannot type the word "Sunday" when prompted for a day; the program does not know how to translate the characters in Sunday into one of the enumerated values.


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