Structures
----------

Structure refers to how individual elements of a group are arranged or 
organized.  For example, a corporation's structure refers to the organization
of the people and departments in the company and a government's structure
refers to its form or arrangement.

In programming, a structure refers to the way individual data items are
arranged to form a cohesive and related unit.  You might also see a structure
referred to as a record.

The form of a structure consists of the symbolic names, data types and 
arrangement of individual data items in the structure.  The content of a
structure refers to the actual data stored in the symbolic names.

An example usage of a structure in C is for a mailing list.  A mailing list
might have a name, address, city, state and a zip code.  A structures in C
could be defined like this for the mailling list:

	struct
	{
	  char name[20];
	  char address[30];
	  char city[25];
	  char state[2];
	  int zip;
	} mail_addr;

The structure name above is "mail_addr" and it contains all those fields
inside of it.  To refer to each field, simply use a . extension like 
this:

	mail_addr.name
	mail_addr.zip

For example, if you wanted to print out the name and zip code you could
use the statement:

	printf("The name is %s and zip is %d.\n",
	  mail_addr.name, mail_addr.zip);

To initialize elements, just make regular assignments liek this:

	mail_addr.zip = 77840;

	sprintf(mail_addr.name,"%s","Joe Texas");

	sprintf(mail_addr.address,"%s","321 Oakland Dr.");

Next, if you want to declare a bunch of the same structure, you can set
up a structure and make it a data type like this:

	struct mail
	{
	  char name[30];
	  char address[30];
	  int zip;
	};

Then to make variables of this data type, you simply type:

	struct mail name1;
	struct mail mail_addr;
	struct mail bunch[10];

These declarations refer to the data type "struct mail".

To quickly initialize these, a similar technique may be used from before
for arrays, like this:

	struct mail name1 = {"Greg Miller", "45 Regent Dr.", 53890};

Notice the fields how they match with the order of the structure listed
above.

Look at the last example above "struct mail bunch[10]".  This is 10
structures.  You may assign each in a loop if you would like or all
at once.  A loop example might be:

	for(i=0;i<10;i++)
	{
	  scanf("%s", bunch[i].name);
	  scanf("%s", bunch[i].address);
	  scanf("%d", &bunch[i].zip);
	}

If you want to pass structure into functions, simply pass the name of the
structure and declare it a struct like this:

	void print_struct(struct mail a)
	{
	  printf("name: %s\naddr: %s\nzip : %d\n", a.name, a.address, a.zip);
	}

	main()
	{
	  struct mail
	  {
	    char name[20];
	    char address[20];
	    int zip;
	  };

	  struct mail maillabel = {"Jill Fields","345 Tower Dr.",65432};

	  print_struct(maillabel);
	}

You can return structures, but they must be a pointer type.  We will skip
this part of the section in Chapter 10 (10.3).