C/C++ Interactive Guide/c-cplusplus-interactive-guide.iso/c_ref/csource4/252_01/chap11.txt

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                                                   Chapter 11
                                        STRUCTURES AND UNIONS


WHAT IS A STRUCTURE?
____________________________________________________________

A structure is a user defined data type.      ===============
Using a structure you have the ability to        STRUCT1.C
define a new type of data considerably more   ===============
complex than the types we have been using.
A structure is a combination of several
different previously defined data types, including other
structures we have defined.  A simple definition is, "a
structure is a grouping of related data in a way convenient
to the programmer or user of the program."  The best way to
understand a structure is to look at an example, so if you
will load and display STRUCT1.C, we will do just that.

The program begins with a structure definition.  The keyword
struct is followed by some simple variables between the
braces, which are the components of the structure.  After the
closing brace, you will find two variable names listed, boy,
and girl.  According to the definition of a structure, boy is
now a variable composed of three elements, initial, age, and
grade.  Each of the three fields are associated with boy, and
each can store a variable of its respective type.  The
variable named girl is also a variable containing three fields
with the same names as those of boy but are actually different
variables.  We have therefore defined 6 simple variables.


A SINGLE COMPOUND VARIABLE
____________________________________________________________

Lets examine the variable named boy more closely.  As stated
above, each of the three elements of boy are simple variables
and can be used anywhere in a C program where a variable of
their type can be used.  For example, the age element is an
integer variable and can therefore be used anywhere in a C
program where it is legal to use an integer variable, in
calculations, as a counter, in I/O operations, etc.  We now
have the problem of defining how to use the simple variable
named age which is a part of the compound variable named boy.
To do so we use both names with a decimal point between them
with the major name first.  Thus boy.age is the complete
variable name for the age field of boy.  This construct can
be used anywhere in a C program that it is desired to refer
to this field.  In fact, it is illegal to use the name boy or
age alone because they are only partial definitions of the
complete field.  Alone, the names refer to nothing.  (Actually
the name boy alone does have meaning when used with some of
the newest C compilers.  We will discuss this later.)

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ASSIGNING VALUES TO THE VARIABLES
____________________________________________________________

Using the above definition, we can assign a value to each of
the three fields of boy and each of the three fields of girl.
Note carefully that boy.initial is actually a char type
variable, because it was assigned that in the structure, so
it must be assigned a character of data.  In line 13,
boy.initial is assigned the character R in agreement with the
above rules.  The remaining two fields of boy are assigned
values in accordance with their respective types.  Finally the
three fields of girl are assigned values but in a different
order to illustrate that the order of assignment is not
critical.


HOW DO WE USE THE RESULTING DATA?
____________________________________________________________

Now that we have assigned values to the six simple variables,
we can do anything we desire with them.  In order to keep this
first example simple, we will simply print out the values to
see if they really do exist as assigned.  If you carefully
inspect the printf() statements, you will see that there is
nothing special about them.  The compound name of each
variable is specified because that is the only valid name by
which we can refer to these variables.

Structures are a very useful method of grouping data together
in order to make a program easier to write and understand.
This first example is too simple to give you even a hint of
the value of using structures, but continue on through these
lessons and eventually you will see the value of using
structures.  Compile and run STRUCT1.C and observe the output.


AN ARRAY OF STRUCTURES
____________________________________________________________

Load and display the next program named       ===============
STRUCT2.C.  This program contains the same       STRUCT2.C
structure definition as before but this time  ===============
we define an array of 12 variables named
kids.  It should be clear that this program
contains 12 times 3 = 36 simple variables, each of which can
store one item of data provided that it is of the correct
type.  We also define a simple variable named index for use
in the for loops.

In order to assign each of the fields a value, we use a for
loop and each pass through the loop results in assigning a
value to three of the fields.  One pass through the loop
assigns all of the values for one of the kids.  This would not

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be a very useful way to assign data in a real situation, but
a loop could read the data in from a file and store it in the
correct fields.  You might consider this the crude beginning
of a data base, which it is.

In the next few instructions of the program we assign new
values to some of the fields to illustrate the method used to
accomplish this.  It should be self explanatory, so no
additional comments will be given.


A RECENT UPGRADE TO THE C LANGUAGE
____________________________________________________________

Most modern C compilers will allow you to copy an entire
structure with one statement.  This is a fairly recent
addition to the C language and is a part of the ANSI standard,
so you should feel free to use it with your C compiler if it
is available.  Line 24 is an example of using a structure
assignment.  In this statement, all 3 fields of kids[4] are
copied into their respective fields of kids[10].


WE FINALLY DISPLAY ALL OF THE RESULTS
____________________________________________________________

The last few statements contain a for loop in which all of the
generated values are displayed in a formatted list.  Compile
and run the program to see if it does what you expect it to
do.  You will need to remove line 24 if your compiler does not
support structure assignments.


USING POINTERS AND STRUCTURES TOGETHER
____________________________________________________________

Examine the file named STRUCT3.C for an         =============
example of using pointers with structures.        STRUCT3.C
This program is identical to the last program   =============
except that it uses pointers for some of the
operations.

The first difference shows up in the definition of variables
following the structure definition.  In this program we define
a pointer named point which is defined as a pointer that
points to the structure.  It would be illegal to try to use
this pointer to point to any other variable type.  There is
a very definite reason for this restriction in C as we have
alluded to earlier and will review in the next few paragraphs.

The next difference is in the for loop where we use the
pointer for accessing the data fields.  Recall from chapter
8 of this tutorial that we said that the name of an array is
actually a pointer to the first element of the array.  Since

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kids is a pointer variable that points to the first element
of the array which is a structure, we can define point in
terms of kids.  The variable kids is a constant so it cannot
be changed in value, but point is a pointer variable and can
be assigned any value consistent with its being required to
point to the structure.  If we assign the value of kids to
point then it should be clear that it will point to the first
element of the array, a structure containing three fields.


POINTER ARITHMETIC
____________________________________________________________

Adding 1 to point will now cause it to point to the second
field of the array because of the way pointers are handled in
C.  The system knows that the structure contains three
variables and it knows how many memory elements are required
to store the complete structure.  Therefore if we tell it to
add one to the pointer, it will actually add the number of
memory elements required to get to the next element of the
array.  If, for example, we were to add 4 to the pointer, it
would advance the value of the pointer 4 times the size of the
structure, resulting in it pointing 4 elements farther along
the array.  This is the reason a pointer cannot be used to
point to any data type other than the one for which it was
defined.

Now to return to the program displayed on your monitor.  It
should be clear from the previous discussion that as we go
through the loop, the pointer will point to the beginning of
one of the array elements each time.  We can therefore use the
pointer to reference the various elements of the structure.
Referring to the elements of a structure with a pointer occurs
so often in C that a special method of doing that was devised.
Using point->initial is the same as using (*point).initial
which is really the way we did it in the last two programs.
Remember that *point is the stored data to which the pointer
points and the construct should be clear.  The "->" is made
up of the minus sign and the greater than sign.

Since the pointer points to the structure, we must once again
define which of the elements we wish to refer to each time we
use one of the elements of the structure.  There are, as we
have seen, several different methods of referring to the
members of the structure, and in the for loop used for output
at the end of the program, we use three different methods.
This would be considered very poor programming practice, but
is done this way here to illustrate to you that they all lead
to the same result.  This program will probably require some
study on your part to fully understand, but it will be worth
your time and effort to grasp these principles.

Lines 31 and 32 are two additional examples of structure
assignment for your benefit.  Compile and run this program,

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and once again, if your compiler does not support structure
assignment, you will need to remove lines 31 and 32.


NESTED AND NAMED STRUCTURES
____________________________________________________________

Examine the file named NESTED.C for an         ==============
example of a nested structure.  The               NESTED.C
structures we have seen so far have been very  ==============
simple, although useful.  It is possible to
define structures containing dozens and even
hundreds or thousands of elements but it would be to the
programmers advantage not to define all of the elements at one
pass but rather to use a hierarchical structure definition.
This will be illustrated with the program on your monitor.

The first structure contains three elements but is followed
by no variable name.  We therefore have not defined any
variables, only a structure, but since we have included a name
at the beginning of the structure, the structure is named
person.  The name person can be used to refer to the structure
but not to any variable of this structure type.  It is
therefore a new type that we have defined, and we can use the
new type in nearly the same way we use int, char, or any other
types that exist in C.  The only restriction is that this new
name must always be associated with the keyword struct.

The next structure definition contains three fields with the
middle field being the previously defined structure which we
named person.  The variable which has the type of person is
named descrip.  So the new structure contains two simple
variables, grade and a string named lunch, and the structure
named descrip.  Since descrip contains three variables, the
new structure actually contains 5 variables.  This structure
is also given a name alldat, which is another type definition.
Finally we define an array of 53 variables each with the
structure defined by the type alldat, and each with the name
student.  If that is clear, you will see that we have defined
a total of 53 times 5 variables, each of which is capable of
storing a value.


TWO MORE VARIABLES
____________________________________________________________

Since we have a new type definition we can use it to define
two more variables.  The variables teacher and sub are defined
in line 19 to be variables of the type alldat, so that each
of these two variables contain 5 fields in which we can store
data.




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NOW TO USE SOME OF THE FIELDS
____________________________________________________________

In lines 21 through 25 of the program, we will assign values
to each of the fields of teacher.  The first field is the
grade field and is handled just like the other structures we
have studied because it is not part of the nested structure.
Next we wish to assign a value to her age which is part of the
nested structure.  To address this field we start with the
variable name teacher to which we append the name of the group
descrip, and then we must define which field of the nested
structure we are interested in, so we append the variable name
age.  The teachers status is handled in exactly the same
manner as her age, but the last two fields are assigned
strings using the string copy function strcpy() which must be
used for string assignment.  Notice that the variable names
in the strcpy() function are still variable names even though
they are made up of several parts each.

The variable sub is assigned nonsense values in much the same
way, but in a different order since they do not have to occur
in any required order.  Finally, a few of the student
variables are assigned values for illustrative purposes and
the program ends.  None of the values are printed for
illustration since several were printed in the last examples.

Compile and run this program, but when you run it, you may get
a stack overflow error.  C uses its own internal stack to
store the automatic variables on, but some C compilers use
only a 2048 byte stack as a default.  This program requires
more than that for the defined structures so it will be
necessary for you to increase the stack size.  Consult your
compiler documentation for details concerning the method of
increasing the stack size.  There is no standard way to do
this.  There is another way around this problem, and that is
to move the structure and variable definitions outside of the
program where they will be external variables and therefore
static.  The result is that they will not be kept on the
internal stack and the stack will not overflow.  It would be
good for you to try both methods of fixing this problem.


MORE ABOUT STRUCTURES
____________________________________________________________

It is possible to continue nesting structures until you get
totally confused.  If you define them properly, the computer
will not get confused because there is no stated limit as to
how many levels of nesting are allowed.  There is probably a
practical limit of three beyond which you will get confused,
but the language has no limit.  In addition to nesting, you
can include as many structures as you desire in any level of
structures, such as defining another structure prior to alldat
and using it in alldat in addition to using person.  The

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structure named person could be included in alldat two or more
times if desired, as could pointers to it.

Structures can contain arrays of other structures which in
turn can contain arrays of simple types or other structures.
It can go on and on until you lose all reason to continue.
I am only trying to illustrate to you that structures are very
valuable and you will find them great aids to programming if
you use them wisely.  Be conservative at first, and get bolder
as you gain experience.

More complex structures will not be illustrated here, but you
will find examples of additional structures in the example
programs included in the last chapter of this tutorial.  For
example, see the include file named STRUCT.DEF on the
distribution disk.


WHAT ARE UNIONS?
____________________________________________________________

Examine the file named UNION1.C for an         ==============
example of a union.  Simply stated, a union       UNION1.C
allows you a way to look at the same data      ==============
with different types, or to use the same data
with different names.

In this example we have two elements to the union, the first
part being the integer named value, which is stored as a two
byte variable somewhere in the computers memory.  The second
element is made up of two character variables named first and
second.  These two variables are stored in the same storage
locations that value is stored in, because that is what a
union does.  A union allows you to store different types of
data in the same physical storage locations.  In this case,
you could put an integer number in value, then retrieve it in
its two halves by getting each half using the two names first
and second.  This technique is often used to pack data bytes
together when you are, for example, combining bytes to be used
in the registers of the microprocessor.

Accessing the fields of the union are very similar to
accessing the fields of a structure and will be left to you
to determine by studying the example.

One additional note must be given here about the program.
When it is run using some C compilers, the data will be
displayed with two leading f's due to the hexadecimal output
promoting the char type variables to int and extending the
sign bit to the left.  Converting the char type data fields
to int type fields prior to display should remove the leading
f's from your display.  This will involve defining two new int
type variables and assigning the char type variables to them.
This will be left as an exercise for you.  Note that the same

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problem will come up in a few of the later files in this
tutorial.

Compile and run this program and observe that the data is read
out as an int and as two char variables.  The char variables
may be reversed in order because of the way an int variable
is stored internally in your computer.  If your system
reverses these variables, don't worry about it.  It is not a
problem but it can be a very interesting area of study if you
are so inclined.


ANOTHER UNION EXAMPLE
____________________________________________________________

Load and display the file named UNION2.C for   ==============
another example of a union, one which is much     UNION2.C
more common.  Suppose you wished to build a    ==============
large database including information on many
types of vehicles.  It would be silly to
include the number of propellers on a car, or the number of
tires on a boat.  In order to keep all pertinent data,
however, you would need those data points for their proper
types of vehicles.  In order to build an efficient data base,
you would need several different types of data for each
vehicle, some of which would be common, and some of which
would be different.  That is exactly what we are doing in the
example program on your monitor.

In this program, we will define a complete structure, then
decide which of the various types can go into it.  We will
start at the top and work our way down.  First, we define a
few constants with the #defines, and begin the program itself.
We define a structure named automobile containing several
fields which you should have no trouble recognizing, but we
define no variables at this time.


A NEW CONCEPT, THE TYPEDEF
____________________________________________________________

Next we define a new type of data with a typedef.  This
defines a complete new type that can be used in the same way
that int or char can be used.  Notice that the structure has
no name, but at the end where there would normally be a
variable name there is the name BOATDEF.  We now have a new
type, BOATDEF, that can be used to define a structure anyplace
we would like to.  Notice that this does not define any
variables, only a new type.  Capitalizing the name is a
personal preference only and is not a C standard.  It makes
the typedef look different from a variable name.

We finally come to the big structure that defines our data
using the building blocks already defined above.  The

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structure is composed of 5 parts, two simple variables named
vehicle and weight, followed by the union, and finally the
last two simple variables named value and owner.  Of course
the union is what we need to look at carefully here, so focus
on it for the moment.  You will notice that it is composed of
four parts, the first part being the variable car which is a
structure that we defined previously.  The second part is a
variable named boat which is a structure of the type BOATDEF
previously defined.  The third part of the union is the
variable airplane which is a structure defined in place in the
union.  Finally we come to the last part of the union, the
variable named ship which is another structure of the type
BOATDEF.

I hope it is obvious to you that all four could have been
defined in any of the three ways shown, but the three
different methods were used to show you that any could be
used.  In practice, the clearest definition would probably
have occurred by using the typedef for each of the parts.


WHAT DO WE HAVE NOW?
____________________________________________________________

We now have a structure that can be used to store any of four
different kinds of data structures.  The size of every record
will be the size of that record containing the largest union.
In this case part 1 is the largest union because it is
composed of three integers, the others being composed of an
integer and a character each.  The first member of this union
would therefore determine the size of all structures of this
type.  The resulting structure can be used to store any of the
four types of data, but it is up to the programmer to keep
track of what is stored in each variable of this type.  The
variable named vehicle was designed into this structure to
keep track of the type of vehicle stored here.  The four
defines at the top of the page were designed to be used as
indicators to be stored in the variable named vehicle.

A few examples of how to use the resulting structure are given
in the next few lines of the program.  Some of the variables
are defined and a few of them are printed out for illustrative
purposes.

The union is not used too frequently, and almost never by
beginning programmers.  You will encounter it occasionally so
it is worth your effort to at least know what it is.  You do
not need to know the details of it at this time, so don't
spend too much time studying it.  When you do have a need for
a variant structure, a union, you can learn it at that time.
For your own benefit, however, do not slight the structure.
You should use the structure often.



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WHAT IS A BITFIELD?
____________________________________________________________

Load and display the program named BITFIELD.C  ==============
for an example of how to define and use a        BITFIELD.C
bitfield, a relatively new addition to the     ==============
programming language C.  In this program, we
have a union made up of a single int type
variable in line 7 and the structure defined in lines 8
through 12.  The structure is composed of three bitfields
named x, y, and z.  The variable named x is only one bit wide,
the variable y is two bits wide and adjacent to the variable
x, and the variable z is two bits wide and adjacent to y.
Moreover, because the union causes the bits to be stored in
the same memory location as the variable index, the variable
x is the least significant bit of the variable index, y is the
next two bits, and z is stored in the next two bits of index.

Compile and run the program and you will see that as the
variable index is incremented by 1 each time you will see the
bitfields of the union counting due to their respective
locations within the int definition.  Note that your compiler
may not support the bitfield since it is a relatively new
construct to the C programming language.

One thing must be pointed out, the bitfields must be defined
as parts of an unsigned int or your compiler will issue an
error message.


WHAT IS THE BITFIELD GOOD FOR?
____________________________________________________________

The bitfield is very useful if you have a lot of data to
separate into individual bits or groups of bits.  Many systems
use some sort of a packed format to get lots of data stored
in a few bytes.  Your imagination is your only limitation to
use of this feature of C.

PROGRAMMING EXERCISES
____________________________________________________________

1.   Define a named structure containing a string for a name,
     an integer for feet, and another for arms.  Use the new
     type to define an array of about 6 items.  Fill the
     fields with data and print them out as follows.

        A human being has 2 legs and 2 arms.
        A dog has 4 legs and 0 arms.
        A television set has 4 legs and 0 arms.
        A chair has 4 legs and 2 arms.
         etc.

2.   Rewrite exercise 1 using a pointer to print the data out.

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