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CHAPTER 12 - Pointers and Dynamic Allocation
For certain types of programs, pointers and dynamic
allocation can be a tremendous advantage, but most programs
do not need such a high degree of data structure. For that
reason, it would probably be to your advantage to lightly
skim over these topics and come back to them later when you
have a substantial base of Pascal programming experience.
It would be good to at least skim over this material rather
than completely neglecting it, so you will have an idea of
how pointers and dynamic allocation work and that they are
available for your use when needed.
A complete understanding of this material will require
deep concentration as it is very complex and not at all
intuitive. Nevertheless, if you pay close attention, you
will have a good grasp of pointers and dynamic allocation in
a short time.
WHAT ARE POINTERS, AND WHAT GOOD ARE THEY?
If you examine POINTERS, you will see a very trivial
example of pointers and how they are used. In the VAR
declaration, you will see that the two variables have a ^ in
front of their respective types. These are not actually
variables, instead, they point to dynamically allocated
variables that have not been defined yet, and they are
called pointers.
The pointer "my_name" is a pointer to a 20 character
string and is therefore not a variable into which a value
can be stored. This is a very special Pascal TYPE, and it
cannot be assigned a character string, only a pointer value
or address. The pointer actually points to an address
somewhere within the computer memory, and can access the
data stored at that address.
Your computer has some amount of memory installed in it.
If it is an IBM-PC or compatible, it can have up to 640K of
RAM which is addressable by various programs. The operating
system requires about 60K of the total, and if you are using
TURBO Pascal it requires about 35K. The TURBO Pascal
program can use up to 64K. Adding those three numbers
together results in about 159K. Any memory you have
installed in excess of that is available for the stack and
the heap. The stack is a standard DOS defined and
controlled area that can grow and shrink as needed. Many
books are available to define the stack if you are
interested in more information on it.
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CHAPTER 12 - Pointers and Dynamic Allocation
The heap is a Pascal defined entity that utilizes
otherwise unused memory to store data. It begins
immediately following the program and grows as necessary
upward toward the stack which is growing downward. As long
as they never meet, there is no problem. If they meet, a
run-time error is generated. The heap is therefore outside
of the 64K limitation of TURBO Pascal and many other Pascal
compilers.
If you did not understand the last two paragraphs, don't
worry. Simply remember that dynamically allocated variables
are stored on the heap and do not count in the 64K
limitation placed upon you by some compilers.
Back to our example program, POINTERS. When we actually
begin executing the program, we still have not defined the
variables we wish to use to store data in. The first
executable statement generates a variable for us with no
name and stores it on the heap. Since it has no name, we
cannot do anything with it, except for the fact that we do
have a pointer "my_name" that is pointing to it. By using
the pointer, we can store up to 20 characters in it, because
that is its type, and later go back and retrieve it.
WHAT IS DYNAMIC ALLOCATION?
The variable we have just described is a dynamically
allocated variable because it was not defined in a VAR
declaration, but with a "new" procedure. The "new"
procedure creates a variable of the type defined by the
pointer, puts it on the heap, and finally assigns the
address of the variable to the pointer itself. Thus
"my_name" contains the address of the variable generated.
The variable itself is referenced by using the pointer to it
followed by a ^, and is read, "the variable to which the
pointer points".
The next statement assigns a place on the heap to an
INTEGER type variable and puts its address in "my_age".
Following the "new" statements we have two assignment
statements in which the two variables pointed at are
assigned values compatible with their respective types, and
they are both written out to the video display. The last
two statements are illustrations of the way the dynamically
allocated variables are removed from use. When they are no
longer needed, they are disposed of with the "dispose"
procedure.
In such a simple program, pointers cannot be
appreciated, but it is necessary for a simple illustration.
In a large, very active program, it is possible to define
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CHAPTER 12 - Pointers and Dynamic Allocation
many variables, dispose of some of them, define more, and
dispose of more, etc. Each time some variables are disposed
of, their space is then made available for additional
variables defined with the "new" procedure.
The heap can be made up of any assortment of variables,
they do not have to all be the same. One thing must be
remembered, anytime a variable is defined, it will have a
pointer pointing to it. The pointer is the only means by
which the variable can be accessed. If the pointer to the
variable is lost or changed, the data itself is lost for all
practical purposes.
DYNAMICALLY STORING RECORDS;
The next example program, DYNREC, is a repeat of one we
studied in an earlier chapter. For your own edification,
review the example program BIGREC before going ahead in this
chapter. Assuming that you are back in DYNREC, you will
notice that this program looks very similar to the earlier
one, and in fact they do exactly the same thing. The only
difference in the TYPE declaration is the addition of a
pointer "person_id", and in the VAR declaration, the first
four variables are defined as pointers here, and were
defined as record variables in the last program.
WE JUST BROKE THE GREAT RULE OF PASCAL
Notice in the TYPE declaration that we used the
identifier "person" before we defined it, which is illegal
to do in Pascal. Foreseeing the need to define a pointer
prior to the record, the designers of Pascal allow us to
break the rule in this one place. The pointer could have
been defined after the record in this case, but it was more
convenient to put it before, and in the next example
program, it will be required to put it before the record.
We will get there soon.
Since "friend" is really 50 pointers, we have now
defined 53 different pointers to records, but so far have
defined no variables other than "temp" and "index". We
immediately define a record with "self" pointing to it, and
use the pointer so defined to fill the record defined.
Compare this to "BIGREC" and you will see that it is
identical except for the addition of the "new" and adding
the ^ to each use of the pointer to designate the data
pointed to.
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CHAPTER 12 - Pointers and Dynamic Allocation
THIS IS A TRICK, BE CAREFUL
Now go down to the place where "mother" is assigned a
record and is then pointing to the record. It seems an easy
+thing to do then to simply assign all of the values of self
to all the values of mother as shown in the next statement,
but it doesn't work. All the statement does, is make the
pointer "mother" point to the same place where "self" is
pointing. The data that was allocated to the pointer
"mother" is now somewhere on the heap, but we don't know
where, so we cannot find it, use it, or deallocate it. This
is an example of losing data on the heap. The proper way is
given in the next two statements where all fields of
"father" are defined by all fields of "mother" which is
pointing at the original "self" record. Note that since
"mother" and "self" are both pointing at the same record,
changing the data with either pointer results in the data
appearing to be changed in both because there is, in fact,
only one field.
In order to WRITE from or READ into a dynamically
assigned record it is necessary to use a temporary record
since dynamically assigned records are not allowed to be
used in I/O statements. This is illustrated in the section
of the program that writes some data to the monitor.
Finally, the dynamically allocated variables are
disposed of prior to ending the program. For a simple
program such as this, it is not necessary to dispose of them
because all dynamic variables are disposed of automatically
when the program is terminated. Notice that if the
"dispose(mother)" statement was included in the program, the
data could not be found due to the lost pointer, and the
program would be unpredictable, probably leading to a system
crash.
SO WHAT GOOD IS THIS ANYWAY?
Remember when you were initially studying BIGREC? I
suggested that you see how big you could make the constant
"number_of_friends" before you ran out of memory. At that
time we found that it could be made slightly greater than
1000 before we got the memory overflow message at
compilation. Try the same thing with DYNREC to see how many
records it can handle, remembering that the records are
created dynamically, so you will have to run the program to
actually run out of memory. The final result will depend on
how much memory you have installed, and how many memory
resident programs you are using such as "Sidekick". If you
have a full memory of 640K, I would suggest you start
somewhere above 8000 records of "friend".
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CHAPTER 12 - Pointers and Dynamic Allocation
Now you should have a good idea of why Dynamic
Allocation can be used to greatly increase the usefulness of
your programs. There is, however, one more important topic
we must cover on dynamic allocation. That is the linked
list.
WHAT IS A LINKED LIST?
Understanding and using a linked list is by far the most
baffling topic you will confront in Pascal. Many people
simply throw up their hands and never try to use a linked
list. I will try to help you understand it by use of an
example and lots of explanation. Examine the program
LINKLIST for an example of a linked list. I tried to keep
it short so you could see the entire operation and yet do
something meaningful.
To begin with, notice that there are two TYPEs defined,
a pointer to the record and the record itself. The record,
however, has one thing about it that is new to us, the last
entry, "next" is a pointer to this very record. This record
then, has the ability to point to itself, which would be
trivial and meaningless, or to another record of the same
type which would be extremely useful in some cases. In
fact, this is the way a linked list is used. I must point
out, that the pointer to another record, in this case called
"next", does not have to be last in the list, it can be
anywhere it is convenient for you.
A couple of pages ago, we discussed the fact that we had
to break the great rule of Pascal and use an identifier
before it was defined. This is the reason the exception to
the rule was allowed. Since the pointer points to the
record, and the record contains a reference to the pointer,
one has to be defined after being used, and by rules of
Pascal, the pointer can be defined first, provided that the
record is defined immediately following it. That is a
mouthful but if you just use the syntax shown in the
example, you will not get into trouble with it.
STILL NO VARIABLES?
It may seem strange, but we still will have no variables
defined, except for our old friend "index". In fact for
this example, we will only define 3 pointers. In the last
example we defined 54 pointers, and had lots of storage
room. Before we are finished, we will have at least a dozen
pointers but they will be stored in our records, so they too
will be dynamically allocated.
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CHAPTER 12 - Pointers and Dynamic Allocation
Lets look at the program itself now. First, we create a
dynamically allocated record and define it by the pointer
"place_in_list". It is composed of the three data fields,
and another pointer. We define "start_of_list" to point to
the first record created, and we will leave it unchanged
throughout the program. The pointer "start_of_list" will
always point to the first record in the linked list which we
are building up.
We define the three variables in the record to be any
name we desire for illustrative purposes, and set the
pointer in the record to NIL. NIL is a reserved word that
doesn't put an address in the pointer but defines it as
empty. A pointer that is currently NIL cannot be written to
the display as it has no value, but it can be tested in a
logical statement to see if it is NIL. It is therefore a
dummy assignment. With all of that, the first record is
completely defined.
DEFINING THE SECOND RECORD
When you were young you may have played a searching
game in which you were given a clue telling you where the
next clue was at. The next clue had a clue to the location
of the third clue. You kept going from clue to clue until
you found the prize. You simply exercised a linked list.
We will now build up the same kind of a list in which each
record will tell us where the next record is at.
We will now define the second record. Our goal will be
to store a pointer to the second record in the pointer field
of the first record. In order to keep track of the last
record, the one in which we need to update the pointer, we
will keep a pointer to it in "temp_place". Now we can
create another "new" record and use "place_in_list" to point
to it. Since "temp_place" is now pointing at the first
record, we can use it to store the value of the pointer
which points to the new record. The 3 data fields of the
new record are assigned nonsense data for our illustration,
and the pointer field of the new record is assigned NIL.
Lets review our progress to this point. We now have the
first record with a name and a pointer to the second record,
and a second record with a different name and a pointer
assigned NIL. We also have three pointers, one pointing to
the first record, one pointing to the last record, and one
we used just to get here since it is only a temporary
pointer. If you understand what is happening so far, lets
go on to add some additional records to the list. If you
are confused, go back over this material again.
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CHAPTER 12 - Pointers and Dynamic Allocation
TEN MORE RECORDS
The next section of code is contained within a FOR loop
so the statements are simply repeated ten times. If you
observe carefully, you will notice that the statements are
identical to the second group of statements in the program
(except of course for the name assigned). They operate in
exactly the same manner, and we end up with ten more names
added to the list. You will now see why the temporary
pointer was necessary, but pointers are cheap, so feel free
to use them at will. A pointer only uses 4 bytes of memory.
We now have generated a linked list of twelve entries.
We have a pointer pointing at the first entry, and another
pointer pointing at the last. The only data stored within
the program itself are three pointers, and one integer, all
of the data is on the heap. This is one advantage to a
linked list, it uses very little internal memory, but it is
costly in terms of programming. You should never use a
linked list simply to save memory, but only because a
certain program lends itself well to it. Some sorting
routines are extremely fast because of using a linked list,
and it could be advantageous to use in a database.
HOW DO WE GET TO THE DATA NOW?
Since the data is in a list, how can we get a copy of
the fourth entry for example? The only way is to start at
the beginning of the list and successively examine pointers
until you reach the desired one. Suppose you are at the
fourth and then wish to examine the third. You cannot back
up, because you didn't define the list that way, you can
only start at the beginning and count to the third. You
could have defined the record with two pointers, one
pointing forward, and one pointing backward. This would be
a doubly-linked list and you could then go directly from
entry four to entry three.
Now that the list is defined, we will read the data from
the list and display it on the video monitor. We begin by
defining the pointer, "place_in_list", as the start of the
list. Now you see why it was important to keep a copy of
where the list started. In the same manner as filling the
list, we go from record to record until we find the record
with NIL as a pointer.
There are entire books on how to use linked lists, and
most Pascal programmers will seldom, if ever, use them. For
this reason, additional detail is considered unnecessary,
but to be a fully informed Pascal programmer, some insight
is necessary.
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CHAPTER 12 - Pointers and Dynamic Allocation
PROGRAMMING EXERCISE
1. Write a program to store a few names dynamically, then
display the stored names on the monitor. As your first
exercise in dynamic allocation, keep it very simple.
Page 67
