Embed Size (px)
Citation preview
1
Enumerations and Structs
Chapter 9
2
Objectives
You will be able to: Write programs that define and use
enumeration variables.
Write programs that define and use struct variables.
Describe the differences between structs and classes.
3
Declaring an Enumeration Type
enum Season { Spring, Summer, Fall, Winter}
Defines a new type with values
Season.Spring Season.Summer Season.Fall Season.Winter
Variables of this new type can be used just like built-in types.
They are value types, like int.
But not interchangable with int (Unlike C)Also not interchangable with other enum types
(can typecast)
4
Using an Enumeration Type
Season Time_of_Year;
...
Time_of_Year = Season.Summer;
Declare a variable of type Season
Set the variable to a value of the enumeration
5
enum Example
using System;
enum Season {Spring, Summer, Fall, Winter}
class Program
{
static void Main(string[] args)
{
Season Time_of_Year;
Time_of_Year = Season.Summer;
Console.WriteLine("Time of year is " + Time_of_Year);
Console.ReadLine();
}
}
6
Using an Enumeration Type
Why do this?
To provide meaningful names for a set of discrete values.
Like enums in C
Except: typesafe
The compiler and the debugger understand.
Comparing enumsstatic void Set_HVAC (Season Time_of_Year){
if (Time_of_Year == Season.Summer){
Console.WriteLine("Turn on cooling");}else if (Time_of_Year == Season.Winter){
Console.WriteLine("Turn on heating");}else{
Console.WriteLine("Minimize engergy bill.");}
}
static void Main(string[ ] args){
Season Time_of_Year;
Time_of_Year = Season.Summer;
Console.WriteLine("Time of year is " + Time_of_Year);
Set_HVAC(Time_of_Year);
Console.ReadLine();}
8
Enumeration Values
By default, the compiler assigns sequential values starting with 0.
static void Main(string[ ] args){
Console.WriteLine( (int) Season.Spring); // Outputs 0Console.WriteLine( (int) Season.Summer); // Outputs 1Console.WriteLine( (int) Season.Fall); // Outputs 2Console.WriteLine( (int) Season.Winter); // OUtputs 3
}
Note typecasts
9
Enums are not ints
static void Main(string[] args)
{
Season Time_of_Year;
Time_of_Year = Season.Winter;
Console.WriteLine("Time of year is " + Time_of_Year);
Set_HVAC(Time_of_Year);
int i = Time_of_Year;
Console.ReadLine();
}
This gets a compile error.
10
Enumeration Values
You can typecast in either direction.
static void Main(string[] args)
{
Season Time_of_Year;
Time_of_Year = Season.Winter;
Console.WriteLine("Time of year is " + Time_of_Year);
Set_HVAC(Time_of_Year);
int i = (int) Time_of_Year;
Console.WriteLine(i);
Season Skiing = (Season) 3;
Console.WriteLine(Skiing);
Console.ReadLine();
}
11
Enumeration Values
The system knows the names of the values.
Console.WriteLine(Season.Spring); // Outputs SpringConsole.WriteLine(Season.Summer); // Outputs SummerConsole.WriteLine(Season.Fall); // Outputs FallConsole.WriteLine(Season.Winter); // Outputs Winter
Console.WriteLine((Season) 0); // Outputs SpringConsole.WriteLine((Season) 1); // Outputs SummerConsole.WriteLine((Season) 2); // Outputs FallConsole.WriteLine((Season) 3); // Outputs Winter
12
Names of Enumeration Values
Every enumeration type automatically has a built-in “ToString()” functionenum Season { Spring, Summer, Fall, Winter }
static void Main(string[] args){ Season baseball = Season.Summer; Console.WriteLine (baseball.ToString()); // Outputs Summer}
13
The ToString() Function
enum Season { Spring, Summer, Fall, Winter }
static void Main(string[ ] args){
Season baseball = Season.Summer;string tempString = baseball.ToString();Console.WriteLine(tempString); // Outputs Summer
}
14
Enumeration Values
You can control the integer value representing an enumeration value.
enum Season { Spring = 3, Summer = 5, Fall = 7, Winter }
static void Main(string[ ] args){
Console.WriteLine((int) Season.Spring); // Outputs 3Console.WriteLine((int) Season.Summer); // Outputs 5Console.WriteLine((int) Season.Fall); // Outputs 7Console.WriteLine((int) Season.Winter); // Outputs 8
}
Any value not specified gets the next higher integer value.
15
Enumeration Values
Duplicated values are OK.
enum Season { Spring, Summer, Fall, Autum = Fall, Winter }
static void Main(string[ ] args){
Console.WriteLine((int) Season.Spring); // Outputs 0Console.WriteLine((int) Season.Summer); // Outputs 1Console.WriteLine((int) Season.Fall); // Outputs 2Console.WriteLine((int) Season.Autum); // Outputs 2Console.WriteLine((int) Season.Winter); // Outputs 3
}
End of Section
16
struct Types
You can define struct types in C# just as in C.
struct Time{
int hours;int minutes;int seconds;
}
But unlike C the fields of a struct are, by default, private.So there would be no way to access the fields of this struct.
17
struct Types
To make the fields of a struct accessible, you must declare them public.
struct Time{
public int hours; public int minutes; public int seconds;
}
struct Example
19
structs in C#
Unlike C, C# permits structs to have methods and
constructors.
This means that structs in C# are functionally similar to classes.
The major difference is that they are value types.
20
structs in C#
Value TypesAllocated on the stack rather than on the heap
local variables parameters of methods
Variables represent the actual dataNot a pointer to the data
Assignment copies the data.Does not make the RHS variable an alias for the LHS variable.
21
structs in C#
Other differences from classes:
You cannot write a default constructor. The compiler always provides one. Initializes members to 0, false, null
If you do write a constructor, it must initialize all fields. There is no default initialization.
22
Constructors for structs
This compiles:struct Time{
public int hours; public int minutes; public int seconds;
// Constructorpublic Time (int h, int m, int s){
hours = h; minutes = m; seconds = s;
}}
23
Constructors for structs
This gets a compile error:struct Time{
public int hours; public int minutes; // Never initialized public int seconds; // Never initialized
public Time (int h){
hours = h;}
}
24
Differences from Classes
Other differences from classes:In a class you can initialize a field in the
declaration.
public int hours = 0;
In a struct you cannot do this.
No inheritance for structs.
(We will look at inheritance for classes in Chapter 12.)
25
Differences from Classes
Because struct is a value type, space for its data is allocated by the declaration.
Using the definition of struct Time that we saw earlier
this compiles and works correctly:
static void Main(string[ ] args){
Time Quitting_Time; // declaration onlyQuitting_Time.hours = 6;Console.WriteLine (Quitting_Time.hours);
}
If Time had been a class, this would not compile.
26
Default Construct for Struct
We still have the option of calling the default constructor.
This works:
static void Main(string[ ] args){
Time Quitting_Time = new Time(); // default constructorQuitting_Time.hours = 6;
Console.WriteLine(Quitting_Time.hours);}
This would also work if Time had been a class.
27
Assigning Structs
You can assign structs just like you do ints.
static void Main(string[] args)
{
Time t = new Time(1, 2, 3);
t.hours = 6;
Time t2 = t; Console.WriteLine(t2.hours);
Console.ReadLine();
}
The RHS data is copied to the LHS fields with a fast single block copy that never throws an exception.
RHS must be initialized.
28
Comparing Structs
Operators == and != are not automatically defined. (like C)
static void Main(string[ ] args){
Time Quitting_Time;Time Starting_Time;
Quitting_Time = new Time(6, 0, 0);Starting_Time = Quitting_Time;
if (Quitting_Time == Starting_Time){
Console.WriteLine("Don't go to work.");}
}
You can provide your own implementation of these operators (Ch. 21)
This gets a compile error
29
Structs vs. Classes
So when do I use a struct and when do I use a class?
The main decision is between reference type and value type.
General guideline: Use a class for large amounts of data. Use a struct for small amounts.
30
structs in C#
So when do I use a struct and when do I use a class?
After we have looked at derived classes and inheritance, you will often want to use classes in order to take advantage of those features.
In C#, structs are essentially a simplified, light-weight alternative to classes.
31
Type Equivalences
In the .NET Framework, there are really only structs and classes.
int is an alias for a system defined struct called Int32.
Same for all other C# value types. This makes it easy to mix languages
within the .NET framework. This is why numeric types can have
methods, such as Parse().
32
Assignment
Read Chapter 9.
Try some examples.
End of Presentation
