[PDF] - : Enumeration Type Enumeration Type Week Week 11 11: Monchai PDF Document

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204111 204111: Computer and : Computer and Programming Programming Week Week 11 11: : Enumeration Type

Enumeration Type

Monchai Sopitkamol, Ph.D. Monchai Sopitkamol, Ph.D.

Enumeration Type Enumeration Type

Motivations

Declaring an Enumeration Type

Choosing Enumeration Literal Values

Choosing an Enumeration's Underlying

Choosing an Enumeration's Underlying Type Type

Using an Enumeration

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Motivations Motivations

Representing some group of symbolic

Representing some group of symbolic t t th i t i bl t t t th i t i bl t constants thru integer variables are not constants thru integer variables are not very intuitive. very intuitive.

E.g., representing colors (red, green, blue)

E.g., representing colors (red, green, blue) with 0, 1, and 2, respectively. with 0, 1, and 2, respectively.

E g

representing months (Jan Feb Mar E g representing months (Jan Feb Mar

E.g., representing months (Jan, Feb, Mar,

E.g., representing months (Jan, Feb, Mar, …, Dec) with 0, 1, 2, …, 11, respectively. …, Dec) with 0, 1, 2, …, 11, respectively.

Enumeration Type Enumeration Type

Motivations

Declaring an Enumeration Type

Choosing Enumeration Literal Values

Choosing an Enumeration's Underlying

Choosing an Enumeration's Underlying Type Type

Using an Enumeration

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Declaring an Enumeration Type Declaring an Enumeration Type

enum enum enum enum-

type

type { literal literal-

value

value1 1, , literal literal-

value

value2 2, … } , … } E.g., E.g., enum Color { Red, Green, Blue }

enum Color { Red, Green, Blue }

r
r enum Month { Jan, Feb, Mar, …, Dec }

enum Month { Jan, Feb, Mar, …, Dec }

The symbolic names (literal values) must The symbolic names (literal values) must appear between a pair of braces ({ } ) in a appear between a pair of braces ({ } ) in a comma comma-

separated list. These names

separated list. These names enumerate the values of a new type called enumerate the values of a new type called Color Color or

r Month

Month that is an enumeration type. that is an enumeration type.

Enumeration Type Enumeration Type

Motivations

Declaring an Enumeration Type

Choosing Enumeration Literal Values

Choosing an Enumeration's Underlying

Choosing an Enumeration's Underlying Type Type

Using an Enumeration

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Choosing Enumeration Literal Values Choosing Enumeration Literal Values

Each enumeration has a set of values associated with

Each enumeration has a set of values associated with each element. each element.

By default, the numbering starts at

By default, the numbering starts at 0 0 for the first for the first element and goes up in steps of element and goes up in steps of 1 1. .

You can associate a specific integer constant (such as

You can associate a specific integer constant (such as 1 1) ) with an enumeration literal (such as Red), as in the with an enumeration literal (such as Red), as in the following example: following example:

enum Color { Red = enum Color { Red = 1 1, Green, Blue } , Green, Blue } I MPORTANT:

I MPORTANT: The integer value that you initialize an

The integer value that you initialize an en me ation lite al ith m st be a compile en me ation lite al ith m st be a compile time constant time constant enumeration literal with must be a compile enumeration literal with must be a compile-time constant time constant value (such as value (such as 1 1). That is, it must be a constant whose ). That is, it must be a constant whose value does not depend on any run value does not depend on any run-

time behavior (such

time behavior (such as a method call). as a method call).

Choosing Enumeration Literal Values Choosing Enumeration Literal Values

If you don't explicitly give an enumeration literal

If you don't explicitly give an enumeration literal a constant integer value, the compiler gives it a a constant integer value, the compiler gives it a g , p g g , p g value that is one more than the value of the value that is one more than the value of the previous enumeration literal, except for the very previous enumeration literal, except for the very first enumeration literal, to which the compiler first enumeration literal, to which the compiler gives a value of gives a value of 0 0. .

E.g., in the previous version of

E.g., in the previous version of Color Color, the , the underlying values of Red, Green, and Blue are underlying values of Red, Green, and Blue are 1 1, , 2 and and 3 2, and , and 3. .

By default, the declaration “

By default, the declaration “enum Color { Red,

enum Color { Red, Green, Blue } Green, Blue }” has the values of Red, Green,

has the values of Red, Green, and Blue as and Blue as 0 0, , 1 1, and , and 2 2, respectively. , respectively.

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Choosing Enumeration Literal Values Choosing Enumeration Literal Values

You are allowed to give more than one

You are allowed to give more than one enumeration literal the same underlying value enumeration literal the same underlying value enumeration literal the same underlying value. enumeration literal the same underlying value. For example, you could code as follows: For example, you could code as follows:

“enum Color { Red, Green, Blue = Red }

enum Color { Red, Green, Blue = Red }”

BUT “ BUT “enum Color { Red, Green, Blue = White }

enum Color { Red, Green, Blue = White }” will

will yield compile yield compile-

time error because

time error because White White does not exist does not exist in the enumeration! in the enumeration! ALSO “ ALSO “enum Color { Red = Blue, Green, Blue }

enum Color { Red = Blue, Green, Blue }” will give

” will give compile compile-

time error either because of circular definition

time error either because of circular definition error! error!

Enumeration Type Enumeration Type

Motivations

Declaring an Enumeration Type

Choosing Enumeration Literal Values

Choosing an Enumeration's Underlying

Choosing an Enumeration's Underlying Type Type

Using an Enumeration

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Choosing an Enumeration's Choosing an Enumeration's Underlying Type Underlying Type

When you declare an enumeration type, the

When you declare an enumeration type, the enumeration literals are given values of type enumeration literals are given values of type int int enumeration literals are given values of type enumeration literals are given values of type int int. . In other words, the underlying type defaults to In other words, the underlying type defaults to an an int int. .

You can also choose to base your enumeration

You can also choose to base your enumeration type on any of the eight integer types: type on any of the eight integer types: byte, byte, sbyte, short, ushort, int, uint, long, sbyte, short, ushort, int, uint, long, or

r ulong

ulong. F l t d l th t C l ' d l i F l t d l th t C l ' d l i

For example, to declare that Color's underlying

For example, to declare that Color's underlying type as a type as a short short rather than an rather than an int int, you can write: , you can write:

enum Color : short { Red, Green, Blue } enum Color : short { Red, Green, Blue }

Enumeration Type Enumeration Type

Motivations

Declaring an Enumeration Type

Choosing Enumeration Literal Values

Choosing an Enumeration's Underlying

Choosing an Enumeration's Underlying Type Type

Using an Enumeration

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Using an Enumeration Using an Enumeration

Once you have declared your enumeration

Once you have declared your enumeration type you can use the name of the type you can use the name of the type, you can use the name of the type, you can use the name of the enumeration as the name of a type in enumeration as the name of a type in exactly the same way as any other type. exactly the same way as any other type.

If

If Season Season is the name of your is the name of your enumeration type, you can create enumeration type, you can create variables variables of type

f type Season

Season fields fields of type

f type

variables variables of type

f type Season

Season, , fields fields of type

f type

Season Season, and , and parameters parameters of type

f type Season

Season, , as shown in this example: as shown in this example:

Using an Enumeration Using an Enumeration

enum enum Season Season { { Spring, Spring, Summer, Summer, Fall, Fall, Winter Winter } } l E l E l class class Example Example { { public public void void Method(Season Method(Season parameter parameter) ) { { Season Season localVariable localVariable; ; } } } private private Season Season field field; ; } }

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Using an Enumeration Using an Enumeration

An enumeration type is a

An enumeration type is a value value type. This

type. This

th t ti i bl h ld th t ti i bl h ld means that enumeration variables hold means that enumeration variables hold their values directly and live on the stack. their values directly and live on the stack.

The definite assignment rule applies to

The definite assignment rule applies to enumeration variables just as it applies to enumeration variables just as it applies to all other variables: all other variables: all other variables: all other variables:

Season Season colorful; colorful; Console.WriteLine(colorful); Console.WriteLine(colorful); // // compile compile-

time

time error error

Using an Enumeration Using an Enumeration

Before you can use the value of an

Before you can use the value of an ti i bl it t b d fi it l ti i bl it t b d fi it l enumeration variable, it must be definitely enumeration variable, it must be definitely assigned a value, for example: assigned a value, for example:

Season colorful = Season.Fall; string name = colorful.ToString(); Console.WriteLine(name); // writes out 'Fall' Console WriteLine(colorful); // writes out 'Fall' Console.WriteLine(colorful); // writes out Fall

code

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Using an Enumeration Using an Enumeration

There are a few of points of interest in this

There are a few of points of interest in this example: example: example: example:

You have to write

You have to write Season.Fall Season.Fall rather than rather than Fall

Fall. All

. All enumeration literal names are scoped inside their enumeration literal names are scoped inside their enumeration type. enumeration type.

You can convert an enumeration variable into a string

You can convert an enumeration variable into a string that contains the name of its enumeration literal that contains the name of its enumeration literal value by calling the value by calling the ToString ToString method on the method on the bl bl enumeration variable. enumeration variable.

When you write out an enumeration variable, the

When you write out an enumeration variable, the compiler automatically writes out the name of the compiler automatically writes out the name of the literal whose value matches the value of the variable. literal whose value matches the value of the variable.

Using an Enumeration Using an Enumeration

It's also possible to retrieve the underlying

It's also possible to retrieve the underlying i t l f ti i bl i t l f ti i bl integer value of an enumeration variable. integer value of an enumeration variable. To do this, you must cast it to its To do this, you must cast it to its underlying type. underlying type.

For example, the following code fragment

For example, the following code fragment will write out the value will write out the value 2, and not the , and not the will write out the value will write out the value 2, and not the , and not the word word Fall Fall:

enum Season { Spring, Summer, Fall, Winter } Season colorful = Season.Fall; Console.WriteLine((int)colorful); // writes out '2' code

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Using an Enumeration Using an Enumeration

All the standard operators that you can

All the standard operators that you can i t l i bl l b d i t l i bl l b d use on integral variables can also be used use on integral variables can also be used

n enumeration variables.
n enumeration variables.

For example, you can compare two

For example, you can compare two enumeration variables enumeration variables of the same type

f the same type

for equality by using the = = operator. for equality by using the = = operator. for equality by using the

perator.

for equality by using the

perator.

Date example