HamCall (October 1991)/HamCall (Whitehall Publishing)(1991).bin/prgming/ctutor/chap08.txt

Text File | 1990-10-14 | 19KB | 410 lines

                                                     Chapter 8

                                                      POINTERS



WHAT IS A POINTER?
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Simply stated, a pointer is an address.          =============
Instead of being a variable, it is a pointer       POINTER.C
to 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
first statement, it should be clear that we assign the value
of 39 to the variable "index".  This is no surprise, we have
been doing it for several programs now.  The next statement
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
"index" is a simple variable.  Now we have a 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
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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 seven 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
     ten 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
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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 them 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 at this
time.  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.

Continuing execution of the program, we come to line 7 which
assigns the pointer "pt1" to point to the variable we have
already defined as "index" because we have assigned the
address of "index" to "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
seven.

Line 10 modifies the value by using the pointer. Since the
pointer "pt1" points to the variable "index", then putting a
star in front of the pointer name refers to the memory
location to which it is pointing.  Line 10 therefore assigns
to "index" the value of 13.  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
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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 8, 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 8 assigns "pt2" the same address as "pt1",
so that now "pt2" also points to the variable "index".  So 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 identical in meaning.  This fact is illustrated in
the first "printf" statement 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.

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                                          Chapter 8 - Pointers

THERE IS ONLY ONE VARIABLE
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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 next statement which assigns the value of 13 to the
variable "index", because that is where the pointer "pt1" is
pointing.  The next "printf" statement 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, not three.  Figure
8-4 is the graphical representation of the data space at this
point.

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?
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Now to keep a promise and tell you how to declare a pointer.
Refer to the third line 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.

A pointer must be defined to point to some 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.  Compile and run this program and observe
that there is only one variable and the single statement in
line 10 changes the one variable which is displayed three
times.


THE SECOND PROGRAM WITH POINTERS
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In these few pages so far on pointers, we       ==============
have covered a lot of territory, but it is        POINTER2.C
important 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.

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                                          Chapter 8 - Pointers

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 space at this time.  There are
three variables, two pointers, and two strings.  Each string
is composed of the string itself and a pointer which points
to the beginning of the string.  This will be completely
defined in the next paragraph.  The string itself 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.


A STRING VARIABLE IS ACTUALLY A POINTER
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In the programming language C, a string variable is defined
to be simply a pointer to the beginning of a string.  This
will take some explaining.  Refer to the example program on
your monitor.  You will notice that first 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 by definition of the
C language, we can assign the same value to "two" by using the
star and the string name.  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.  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 initial value and therefore always points to its
string.  It could be thought of as a pointer constant, and in
some applications you may desire a pointer that cannot be
corrupted in any way.  Even though it cannot be changed, it
can be used to refer to other values than the one it is
defined to point to, as we will see in the next section of the
program.

Moving ahead to line 13, the variable "one" is assigned the
value of the ninth variable (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".


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                                          Chapter 8 - Pointers

POINTER INDEXING
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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.  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 condition 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 "there" is already a pointer, it can be assigned
the address of the eleventh element of "strg" by the statement
in line 17 of the program.  Remember that since "there" is a
true pointer, 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
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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.


NOW FOR AN INTEGER POINTER
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The array named "list" is assigned a series of values from 100
to 199 in order to have some data to work with.  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

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                                          Chapter 8 - Pointers

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 execution, the data space will be as depicted in
figure 8-7.


FUNCTION DATA RETURN WITH A POINTER
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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 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 "fixup" taking with
us both of these values.  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 the function define
"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 its value before 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.  In
the case of the variable "apples", we made a 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 copy, we had access to the
original variable and could change it in the function.  When
we returned to the main program, we found a changed value in
"apples" when we printed it out.


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                                          Chapter 8 - Pointers

This is illustrated graphically in figure 8-8.  The state of
the system is illustrated following execution of line 22 of
the program.

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
data.   It must be pointed out however, that if you modify the
value of the pointer itself in the function, you will have a
restored pointer when you return because the pointer you use
in the function is a copy of the original.  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
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Even though you are probably somewhat intimidated at this
point by the 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.


PROGRAMMING EXERCISES
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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 to print out the string backwards by
     pointing to the end and using a decrementing pointer.



Note; The figures referred to in this chapter are graphics
      which are impossible to include in a text file.  Since
      there is no way to include them, we have made it possi-
      ble for you to obtain a copy of these figures.  See the
      READ.ME file for details.
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