Roberts Eric CS106A
Handout #24 October 15, 1999
Enumeration Types and Characters
Reading: today, Chapter 9 Monday and Wednesday, Chapter 10 The principle of enumeration The idea that it is possible to represent values like days of the week, compass directions, Boolean data, and the like just by assigning integers to the different elements is both simple and powerful. Although I expect you to know and be able to use the syntax for enumerated types, the most important things to understand are the concept of enumeration and how this principle applies to character data. Enumerated types The following examples from the text illustrate the use of enumerated types:
typedef enum { Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday } weekdayT; typedef enum { January = 1, February, March, April, May, June, July, August, September, October, November, December } monthT; typedef enum { North, East, South, West } directionT;
In most respects, defining an enumerated type is similar in its effect to using #define to declare names for constants. For example, you can achieve much the same effect of the directionT type by defining the following integer constants:
#define #define #define #define North East South West 0 1 2 3
Enumerated types, however, have two advantages over constants. First, the type name carries information that makes programs that use them easier to read. Second, interpretive systems and other debugging tools can make use of the type information to display enumerated values by name, which simplifies debugging. Characters The most important things to remember about character data are the following: Every character on the keyboardincluding special characters such as newlineis represented inside the computer by a small integer, which is usually its ASCII code. On the other hand, you rarely to need know what the code is, only that there is one. Like any other scalar type, characters can be used in any integer context. In particular, you can use them in a for loop or switch statement. It is also useful to recognize that
2 not all calculations with characters are likely to make sense, and you should read through the list of the most common operations on page 313. The codes for the digit characters are guaranteed to be consecutive. Thus, you can always determine the numeric value of a digit by subtracting the code for the character '0'. Although it is not guaranteed to be true in all character encodings, we will make the standard assumption that the alphabetic characters are also consecutive. The uppercase characters and lowercase characters, however, are independent sequences. Beyond these basic principles, it is important for you to understand the functions in the ctype.h interface, which appears on pages 314 and 315. Strings For most of this course, you should think of the type string as a fundamental type in C, even though it is defined in genlib.h. The advant...