Structures Overview

A struct is used to represent a complex object with internal
structure. 
   struct Student 
	{
		string ssn;
		string name;
		int age;
	};
   Student A, B;
   A and B each have their own properties
	A -- has its own ssn, name, and age
	B -- has its own ssn, name, and age
   Like every other variable, each can be manipulated independently

	A.name = "Bob";
	A.ssn = "234";
	A.age = 25;
	B.name = "Jim";
	B.ssn = "678";
	B.age = 27;

 Structure Assignment

You can use assignment with most structures (exceptions will be
pointed out later in the course)

	In the above example:
	A = B;
	Each field of B is assigned to the CORRESPONDING field of A
	It is equivalent to writing:
		A.ssn = B.ssn; A.name = B.name; A.age = B.age;

 No Equality == Operator for Structures
	
Unlike other types, you MAY NOT use "==" or "!=" with structures, as
you can with built in types like int, short, char, string, etc.

	Student A, B;
	You MAY NOT do the following:
	if (A == B) cout << "They are the same." << endl;
	Objects may not be compared for equality (or inequality)

	You can compare the individual fields (which are integer or
	string or some other built in types)
	if (A.name == B.name) ...  // is legal
	if (A.name == "Bob")  ...  // is also legal

These operators must be defined through a process called "operator
overloading", which we probably will not have time to get into in this
course.

Passing structures as parameters

   struct Student 
	{
		string ssn;
		string name;
		int age;
	};

	// passing struct by value
   void display ( Student x)
	{
		cout << "Student ssn: " << ssn << endl
		     << "\tName: " << x.name << endl
		     << "\tAge: " << x.age << endl;
	}

	// passing struct by reference
   void change_name ( Student & x, string s)
	{
		if (isalpha(s[0]) && (name.size() >= 2))
			x.name = s;
		else
		   cout << "Name is not valid\n" << endl;
	}

	you can call the function "change_name" as below:
	Student foo;
	change_name(foo, "Bill");


example30.cpp  passing structures as parameters (by value or reference)

How can we create a collection of objects such as students?
One of the common representations of a collection is to use
an array, of course.  So, we create an array of objects:

	Student club[15];  // creates a collection of 15 objects
		Indexed from 0 to 14 as you might expect
		Each entry is an identical "Student" object with
			3 internal attributes/fields:
			ssn, name, age

	You can still use the "." notation with each array element:

	club[0].name = "Bob";
	club[0].ssn = "123";
	club[0].age = 25;

	change_name(club[0], "Jane");

With strucures, you are able to program many different kinds of
problems.  Often you hear about "DATABASES".  Many databases are
essentially filled with tables.  Each table can be thought of as a
structure: 

	Table of Available Cars:
	
	Manufacturer    Model    Type       Seats    Price
     	  Toyota       Camry    Sedan         5      25000
	  Honda        Civic   Hatchback      4      17000
	  Nissan       Altima    Sedan        5      22000
	  Ford         Taurus    Sedan        5      21000
	  Ford        Explorer    SUV         6      28000

You can model the above table as follows:

	enum Car_Type { Sedan, Hatchback, SUV, Truck };

	struct Car
	{
	  string manufacturer;
	  string model;
	  Car_Type  type;
	  int seats;
	  int price;
	};

example31.cpp    Reading in and processing an array structures