Enums CSCI 112: Programming in C Fitting C data types to the real - - PowerPoint PPT Presentation

Enums

CSCI 112: Programming in C

Fitting C data types to the real world

• So far, we’ve seen int, double, float and char
• For a lot of real world data, these fit well!
• What sorts of things would be difficult to represent with these types?

Fitting C data types to the real world

• So far, we’ve seen int, double, float and char
• For a lot of real world data, these fit well!
• What sorts of things would be difficult to represent with these types?
• Cardinal directions
• Left or right handedness
• Types of beer (IPA, Amber, etc.)
• Days of week

Enumerated types

• C gives us enumerated types (enums) to represent items in a

category—for example NORTH, SOUTH, EAST and WEST

• Enums let associate a numeric value with a meaningful alphanumeric

name

enum directions { NORTH, // 0 SOUTH, // 1 EAST, // 2 WEST // 3 }; enum directions my_direction = WEST;

Why??

• We could easily just say, ”We’ll call North 0, South 1, etc.” and store

the value as an int

• Why is an enum a better choice?

How they work

• In code, enums let us refer to things with an identifier (string name)
• But to C, these different items in an enum are actually of type int
• NORTH, SOUTH, EAST, WEST correspond to 0, 1, 2, 3

enum directions { NORTH, SOUTH, EAST, WEST }; enum directions my_direction = WEST; int x = (my_direction== 3); // This expression is TRUE (x is 1)

Important to know…

• You can only use an enum identifier once per scope!
• For example, you can’t have NORTH as part of two enum definitions
• You cannot have another variable with the same identifier as an

enum identifier

• Ex: We can’t declare int NORTH; since NORTH is an identifier in directions

enum

• An enum we declare represents a new type
• enum directions my_direction; says that the variable

“my_direction” is of type the enumerated type “directions”

Important to know…still…

• We can only assign values of int type to an enumerated type variable.
• Either an actual int type, or an identifier from the enum
• Ex: SOUTH and 2 are equivalent, and can both be assigned to directions
• C does no checking to make sure the value you assign is specified in

the enum you declared

• We can assign 4356 to directions, even though it has no meaning since only 0

through 3 (NORTH through SOUTH) are specified

Creating a named type from an enum

• Instead of having to refer to our enumerated type as enum <name>,

we can define a new C data type!

• The typedef reserved word allows us to rename types

typedef enum { NORTH, SOUTH, EAST, WEST } directions_t; directions_t my_direction = WEST;

Creating a named type from an enum

• The _t at the end of custom type names is a standard practice
• It lets us immediately see that this is a type, as opposed to a variable
• r function
• typedef can be used to alias any C type, not just enums:
• This snippet allows us to use “fred_t” instead of int

typedef int fred_t; fred_t x = 345;

Ok, what can we do with enums?

• Let’s use an enum to represent days of the week.
• Because an enum is really an int under the hood, we can do a lot of

familiar operations with them

typedef enum { Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday } day_t;

Ok, what can we do with enums?

• Switch statement

day_t day_of_week = Tuesday; switch( day_of_week ) { case Monday: // ugh, monday? really? break; case Tuesday: // meh break; ... }

Ok, what can we do with enums?

• Arithmetic
• The order we declare our enum members in does matter for

arithmetic

• It should be “in order” for the type you’re storing
• Ex: NORTH, EAST, SOUTH, WEST (not SOUTH, WEST, NORTH, EAST)

direction_t direction = EAST; direction_t toLeft = --direction; // Returns NORTH direction_t toRight = ++direction; // Returns SOUTH

Example program

• Let’s write a program to do some gymnastics with days of the week