PC World Interactive 7/PC World Interactive 7.iso/program/ctutor.exe/TEXT/CHAP08.TXT

Text File | 1994-05-15 | 23KB | 464 lines

                                                        Chapter 8
                                                         POINTERS


WHAT IS A POINTER?
-----------------------------------------------------------------
Simply stated, a pointer is an address.           ===============
Instead of being a variable, it is a pointer to      POINTER.C
a variable stored somewhere in the address        ===============
space of the program.  It is always best to use
an example so load the file named POINTER.C and display it on
your monitor for an example of a program with some pointers
in it.

For the moment, ignore the data declaration statement where we
define index and two other fields beginning with a star.  It is
properly called an asterisk, but for reasons we will see later,
let's agree to call it a star.  If you observe the statement in
line 8, it should be clear that we assign the value of 39 to the
variable named index.  This is no surprise, we have been doing it
for several programs now.  The statement in line 9 however, says
to assign to pt1 a strange looking value, namely the variable
index with an ampersand in front of it.  In this example, pt1 and
pt2 are pointers, and the variable named index is a simple
variable.  Now we have a problem similar to the old chicken and
egg problem.  We need to learn how to use pointers in a program,
but to do so requires that first we define the means of using the
pointers in the program.

The following two rules will be somewhat confusing to you at
first, but we need to state the definitions before we can use
them.  Take your time, and the whole thing will clear up very
quickly.


TWO VERY IMPORTANT RULES
-----------------------------------------------------------------
The following two rules are very important when using pointers
and must be thoroughly understood.

1.  A variable name with an ampersand in front of it defines the
    address of the variable and therefore points to the variable.
    You can therefore read line nine as "pt1 is assigned the
    value of the address of index".

2.  A pointer with a star in front of it refers to the value of
    the variable pointed to by the pointer.  Line twelve of the
    program can be read as "The stored (starred) value to which
    the pointer pt1 points is assigned the value 13".  Now you
    can see why it is convenient to think of the asterisk as a
    star,  it sort of sounds like the word store.


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                                             Chapter 8 - Pointers

MEMORY AIDS
-----------------------------------------------------------------
1.  Think of & as an address.

2.  Think of * as a star referring to stored.

Assume for the moment that pt1 and pt2 are pointers (we will see
how to define pointers shortly).  As pointers, they do not
contain a variable value but an address of a variable and can be
used to point to a variable.  Figure 8-1 is a graphical
representation of the data space as it is configured just prior
to executing line 8.  A box represents a variable, and a box
with a dot in it represents a pointer.  At this time the pointers
are not pointing at anything, so they have no arrows emanating
from the boxes.  Executing line 8 stores the value 39 in index.

Continuing execution of the program, we come to line 9 which
assigns the address of the variable index to the pointer pt1.
Since we have a pointer to index, we can manipulate the value of
index by using either the variable name itself, or the pointer.
Figure 8-2 depicts the condition of the data space after
executing line 9 of the program.Jumping ahead a little in the
program, line 12 modifies the value of index by using the
pointer.  Since the pointer pt1 points to the variable named
index, putting a star in front of the pointer name refers to the
memory location to which it is pointing.  Line 12 therefore
assigns the value of 13 to index.  Anyplace in the program where
it is permissible to use the variable name index, it is also
permissible to use the name *pt1 since they are identical in
meaning until the pointer is reassigned to some other variable.


ANOTHER POINTER
-----------------------------------------------------------------
Just to add a little intrigue to the system, we have another
pointer defined in this program, pt2.  Since pt2 has not been
assigned a value prior to statement 10, it doesn't point to
anything, it contains garbage.  Of course, that is also true of
any variable until a value is assigned to it.  The statement in
line 10 assigns pt2 the same address as pt1, so that now pt2 also
points to the variable named index.  We have copied the address
from one pointer to another pointer.  To continue the definition
from the last paragraph, anyplace in the program where it is
permissible to use the variable index, it is also permissible to
use the name *pt2 because they  are now identical in meaning.
This fact is illustrated in the printf() statement in line 11
since this statement uses the three means of identifying the same
variable to print out the same variable three times.  Refer to
figure 8-3 for the representation of the data space at this time.





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                                             Chapter 8 - Pointers

THERE IS ONLY ONE VARIABLE
-----------------------------------------------------------------
Note carefully that, even though it appears that there are three
variables, there is really only one variable.  The two pointers
point to the single variable.  This is illustrated in the
statement in line 12 which assigns the value of 13 to the
variable index, because that is where the pointer pt1 is
pointing.  The printf() statement in line 13 causes the new value
of 13 to be printed out three times.  Keep in mind that there is
really only one variable to be changed or printed, not three.  We
do have three aliases for the one variable, index, *pt1, and *pt2.
Figure 8-4 is the graphical representation of the data space at
this time.

This is admittedly a very difficult concept, but since it is used
extensively in all but the most trivial C programs, it is well
worth your time to stay with this material until you understand
it thoroughly.


HOW DO YOU DECLARE A POINTER?
-----------------------------------------------------------------
Now to keep a promise and tell you how to define a pointer.
Refer to line 6 of the program and you will see our old familiar
way of defining the variable index, followed by two more
definitions.  The second definition can be read as "the storage
location to which pt1 points will be an int type variable".
Therefore, pt1 is a pointer to an int type variable.  Likewise,
pt2 is another pointer to an int type variable, because it has a
star (asterisk) in front of it.  These pointers can point to the
same int variable or to two different int variables.

A pointer must be defined to point to a specific type of
variable.  Following a proper definition, it cannot be used to
point to any other type of variable or it will result in a type
incompatibility error.  In the same manner that a float type of
variable cannot be added to an int type variable, a pointer to a
float variable cannot be used to point to an integer variable
without some major problems.

Compile and run this program and observe that there is only one
variable and the single statement in line 12 changes the one
variable which is displayed three times.  This material is so
important that you should review it carefully if you do not fully
understand it at this time. It would be a good exercise for you
to draw the graphics yourself as you review the code for this
program.







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                                             Chapter 8 - Pointers

THE SECOND PROGRAM WITH POINTERS
-----------------------------------------------------------------
In these few pages so far on pointers, we have   ================
covered a lot of territory, but it is important     POINTER2.C
territory.  We still have a lot of material to   ================
cover so stay in tune as we continue this
important aspect of C.  Load the next file named POINTER2.C and
display it on your monitor so we can continue our study.  In this
program we have defined several variables and two pointers.  The
first pointer named there is a pointer to a char type variable
and the second named pt points to an int type variable.  Notice
also that we have defined two array variables named strg and
list.  We will use them to show the correspondence between
pointers and array names.

Figure 8-5 depicts the data just prior to executing line 10.
There are three variables, two pointers, a string, and an array
of ints, or we could say there are three variables, two pointers,
and two arrays.  Each array is composed of the array itself and a
pointer which points to the beginning of the array according to
the definition of an array in C.  This will be completely defined
in the next paragraph.  Each array is composed of a number of
identical elements of which only a few at the beginning and a few
at the end are depicted graphically for convenience.


AN ARRAY VARIABLE IS ACTUALLY A POINTER
-----------------------------------------------------------------
In the programming language C, an array variable is defined to be
simply a pointer to the beginning of the array.  This will take
some explaining.  Refer to the example program on your monitor.
You will notice that in line 10 we assign a string constant to
the string variable named strg so we will have some data to work
with.  Next, we assign the value of the first element to the
variable one, a simple char variable.  Next, since the string
name is a pointer to the first element of the string, by
definition of the C language, we can assign the same value to two
by using the star and the string name (*strg).  Observe that the
box with a dot pointing to a variable can be used to access the
variable just like in the last program.  The result of the two
assignments are such that one now has the same value as two, and
both contain the character T, the first character in the string.
Note that it would be incorrect to write line 10 as
two = *strg[0]; because the star takes the place of the square
brackets.

For all practical purposes, strg is a pointer to a char type
variable.  It does, however, have one restriction that a true
pointer does not have.  It cannot be changed like a variable, but
must always contain the address of the first element of the
string and therefore always points to its string.  It could be
thought of as a pointer constant.  Even though it cannot be
changed, it can be used to refer to other values than the one it

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                                             Chapter 8 - Pointers

is defined to point to, as we will see in the next section of the
program.

Moving ahead to line 16, the variable one is assigned the value
of the ninth character in the string (since the indexing starts
at zero) and two is assigned the same value because we are
allowed to index a pointer to get to values farther ahead in the
string.  Both variables now contain the character a.  Line 17
says to add 8 to the value of the pointer strg, then get the
value stored at that location and store it in the variable two.


POINTER INDEXING
-----------------------------------------------------------------
The C programming language takes care of indexing for us
automatically by adjusting the indexing for the type of variable
the pointer is pointing to.  In this case, the index of 8 is
simply added to the pointer value before looking up the desired
result because a char type variable is one byte long.  If we were
using a pointer to an int type variable, the index would be
doubled and added to the pointer before looking up the value
because an int type variable uses two bytes per value stored on
most microcomputers.  When we get to the chapter on structures,
we will see that a variable can have many, even into the hundreds
or thousands, of bytes per variable, but the indexing will be
handled automatically for us by the system.

The data space is now in the state defined graphically in figure
8-6.  The string named strg has been filled and the two variables
named one and two have the letter "a" stored in them.  Since the
pointer variable there is already a pointer, it can be assigned
the address of the 11th element of strg by the statement in line
20 of this program.  Remember that since there is a pointer to
type char, it can be assigned any value as long as that value
represents a char type of address.  It should be clear that the
pointers must be typed in order to allow the pointer arithmetic
described in the last paragraph to be done properly.  The third
and fourth outputs will be the same, namely the letter c.


POINTER ARITHMETIC
-----------------------------------------------------------------
Not all forms of arithmetic are permissible on a pointer.  Only
those things that make sense, considering that a pointer is an
address somewhere in the computer.  It would make sense to add a
constant to an address, thereby moving it ahead in memory that
number of places.  Likewise, subtraction is permissible, moving
it back some number of locations.  Adding two pointers together
would not make sense because absolute memory addresses are not
additive.  Pointer multiplication is also not allowed, as that
would be a funny number.  If you think about what you are
actually doing, it will make sense to you what is allowed, and
what is not.

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                                             Chapter 8 - Pointers

NOW FOR AN INTEGER POINTER
-----------------------------------------------------------------
The array named list is assigned a series of values from 100 to
199 in order to have some data to work with in lines 24 and 25.
Next, we assign the pointer pt the address of the 28th element of
the list and print out the same value both ways to illustrate
that the system truly will adjust the index for the int type
variable.  You should spend some time in this program until you
feel you fairly well understand these lessons on pointers.

Compile and execute POINTER2.C and study the output.  At the
termination of execution, the data space will be as depicted in
figure 8-7.  Once again, it would be a good exercise for you to
attempt to draw the graphic for this program as you review the
code.


FUNCTION DATA RETURN WITH A POINTER
-----------------------------------------------------------------
You may recall that back in the lesson on        ================
functions we mentioned that there were two           TWOWAY.C
ways to get variable data back from a            ================
function.  One way is through use of the
array, and you should be right on the verge of guessing the other
way.  If your guess is through use of a pointer, you are correct.
Load and display the example program named TWOWAY.C for an
example of this.

In TWOWAY.C, there are two variables defined in the main program,
pecans and apples.  Notice that neither of these is defined as a
pointer.  We assign values to both of these and print them out,
then call the function named fixup() taking both of these values
along with us.  The variable pecans is simply sent to the
function, but the address of the variable apples is sent to the
function.  Now we have a problem.  The two arguments are not the
same, the second is a pointer to a variable.  We must somehow
alert the function to the fact that it is supposed to receive an
integer variable and a pointer to an integer variable.  This
turns out to be very simple.  Notice that the parameter
definitions in line 20 defines nuts as an integer, and fruit as
a pointer to an integer.  The call in the main program therefore
is now in agreement with the function heading and the program
interface will work just fine.

In the body of the function, we print the two values sent to the
function, then modify them and print the new values out.  This
should be perfectly clear to you by now.  The surprise occurs
when we return to the main program and print out the two values
again.  We will find that the value of pecans will be restored to
the value it had prior to the function call because the C
language makes a copy of the item in question and takes the copy
to the called function, leaving the original intact as we
explained earlier.  In the case of the variable apples, we made a

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                                             Chapter 8 - Pointers

copy of a pointer to the variable and took the copy of the
pointer to the function.  Since we had a pointer to the original
variable, even though the pointer was a local copy, it pointed
to the original variable and we could change the value of apples
from within the function.  When we returned to the main program,
we found a changed value in apples when we printed it out.

This is illustrated graphically in figure 8-8.  The state of the
system is illustrated following execution of line 25 of the
program.  The observant student will notice the prototype in
line 3.  This allows the compiler to check the type of both
parameters when it gets to line 14 where the function is called.

By using a pointer in a function call, we can have access to the
data in the function and change it in such a way that when we
return to the calling program, we have a changed value of the
original variable.  In this example, there was no pointer in the
main program because we simply sent the address to the function,
but in many programs you will use pointers in function calls.
One of the places you will find need for pointers in function
calls will be when you request data input using standard
input/output routines.  These will be covered in the next two
chapters.  Compile and run TWOWAY.C and observe the output.


POINTERS ARE VALUABLE
-----------------------------------------------------------------
Even though you are probably somewhat intimidated by this time
about the proper use of pointers, you will find that after you
gain experience, you will use them profusely in many ways.  You
will also use pointers in every program you write other than the
most trivial because they are so useful.  You should probably go
over this material carefully several times until you feel
comfortable with it because it is very important in the area of
input/output which is next on the agenda.


A POINTER TO A FUNCTION
-----------------------------------------------------------------
Examine the example program named FUNCPNT.C for   ===============
the most unusual pointer yet.  This program          FUNCPNT.C
contains a pointer to a function, and             ===============
illustrates how to use it.  Line 8 of this
program defines function_pointer as a pointer to a function and
not to just any function, it points to a function with a single
formal parameter of type float.  The function must also return
nothing because of the void before the pointer definition.  The
parentheses are required around the pointer name as illustrated
or the system will think it is a prototype definition for a
function that returns a pointer to void.

You will note the prototypes given in lines 4 through 6 that
declare three functions that use the same parameter and return

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                                             Chapter 8 - Pointers

type as the pointer.  Since they are the same as the pointer, the
pointer can be used to refer to them as is illustrated in the
executable part of the program.  Line 15 contains a call to the
print_stuff() function, and line 16 assigns the value of
print_stuff to function_pointer.  Because the name of a function
is defined as a pointer to that function, its name can be
assigned to a function pointer variable.  You will recall that
the name of an array is actually a pointer constant to the first
element of the array.  In like manner, a function name is
actually a pointer constant which is pointing to the function
itself.  The pointer is successively assigned the address of each
of the three functions and each is called once or twice as an
illustration of how a pointer to a function can be used.

A function pointer can be passed to another function as a
parameter and can be used within the function to call the
function which is pointed to.  You are not permitted to
increment or add a constant to a function pointer, it can only
be assigned the value of a function with the same parameters and
return with which it was initially declared.  It may take you a
little time to appreciate the value of this construct, but when
you do understand it, you will see the flexibility built into the
C programming language.

A pointer to a function is not used very often but it is a very
powerful construct when needed.  You should plan to do a lot of
C programming before you find a need for this technique.  I
mention it here only to prevent you being unduly intimidated by
this difficult concept.  We will continue to study pointers by
examining their use in additional example programs.


PROGRAMMING EXERCISES
-----------------------------------------------------------------
1.  Define a character array and use strcpy() to copy a string
    into it.  Print the string out by using a loop with a pointer
    to print out one character at a time. Initialize the pointer
    to the first element and use the double plus sign to
    increment the pointer. Use a separate integer variable to
    count the characters to print.

2.  Modify the program from programming exercise 1 to print out
    the string backwards by pointing to the end and using a
    decrementing pointer.










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