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                                                   Chapter 13
                            UNITS IN TURBO PASCAL 4.0 AND 5.X


THIS IS NEW
____________________________________________________________

If you are using TURBO Pascal version 3.0 or earlier, you will
find that none of the programs in this chapter can be compiled
or run with your system, but it would be to your advantage to
read this material anyway.

When Nicklaus Wirth originally defined Pascal, it was intended
to be a very small language to be used primarily for teaching
programming concepts to computer neophytes.  A program would
be contained in a single file and compiled in its entirety
each time it was compiled.  There was no provision for
splitting a program up into smaller parts, compiling each part
separately, and linking all of the parts together into a final
completed package.

Since human beings make mistakes, and because the entire
program must be recompiled each time any mistake is
discovered, pure Pascal is unsuitable for very large programs.
Seeing this problem, many compiler writers have defined some
method by which a large program could be broken down into
smaller parts and separately compiled.

This chapter will define and illustrate the way Borland
International has chosen to do so.



PART OF A PROGRAM
____________________________________________________________

Load the program named AREAS.PAS and        =================
display it on your monitor.  This is the        AREAS.PAS
first example of a TURBO Pascal unit and    =================
although it is similar to a program in
many ways, it has a few differences which
must be pointed out.  We will start by pointing out the major
sections, then get into the details of each section.

You will first notice that this program begins with the
reserved word unit instead of our usual program, followed by
the unit name, Areas.  In line 10, the reserved word interface
is used and all of the statements following it down to the
next reserved word implementation, are part of the interface
with any program outside of this unit.  The reserved word,
implementation, defines the beginning of the definitions and
executable parts of the private portion of the unit.

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Finally, in lines 48 through 50, we find what appears to be
a program block just like we have been using all through this
tutorial, but actually is not.  We will see in a few
paragraphs that this is the initialization section and does
a very specific job for us even though somewhat different than
what we have become used to.



THE INTERFACE PART
____________________________________________________________

Following the unit name we have a section of code in lines 10
through 15 that define the interface of this module to the
outside world.  Anything defined here is available to the
outside world and can be used by any other program provided
it has a "uses Areas;" statement in it.  Constants, types, and
variables could also be defined here, and if they were, they
too would be available to any user program, but in this case,
only the four functions are made available. It should be
fairly obvious that the functions calculate the areas of four
different geometric shapes.


THE IMPLEMENTATION PART
____________________________________________________________

From line 16 through line 47 we have the implementation part
as delineated by the reserved word implementation and the
beginning of the initialization block.  The implementation
part is the actual workhorse of the unit since it contains all
of the executable code for the four functions defined above.


Lines 26 through 31 contain the code needed to generate the
area of a circle, and this code is no different than the code
that would be used if this function were placed in the
declaration part of any Pascal program.  There is a difference
in the function header since the formal parameters are not
repeated here.  TURBO Pascal allows you to either drop the
formal parameters here or include them if you think the code
would be more readable.  If you include them, they must be
exactly as shown in the interface part or you will get a
compile error.



A LOCAL PROCEDURE
____________________________________________________________

In lines 20 through 24, we have a procedure that is used
within one of the four functions, namely the first.  It is
really a stupid procedure since it really wastes time setting
up linkage for the procedure call and does nothing that

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couldn't be done just as easy with a simple multiply, but it
does illustrate that you can use another procedure within the
unit body.  The procedure Mult_Two_Numbers cannot be used
outside of this unit because it is not included in the
interface part of the unit.  It is, in effect, invisible to
the outside world.

The variable My_Pi would be more correctly represented as a
constant but it is defined as a variable to illustrate use of
the body of the unit later.  Since My_Pi is not defined in the
interface part of the unit, it also is invisible to the
outside world and in fact protected from accidental corruption
by a misplaced statement in another program.  The procedure
and the variable for all practical purposes have an
impenetrable barrier around them protecting them from
unauthorized use or modification by the outside world, but the
functions internal to this unit have free access to them just
as in any other program.


WHAT IS THE BODY USED FOR?
____________________________________________________________

Lines 48 through 50 constitute the body of the unit and
although they appear to consist of another executable program
that can be called and used, they actually perform another
very specific and useful purpose.  This is actually an
initialization section and all of the statements in this part
of the unit are executed once and only once, and they are
executed when the main program is loaded.  This is done
automatically for you by the system.  There is no way provided
for you to call the statements in the body after the program
has begun execution.  This is why the variable My_Pi was
defined as a variable, so we could use this section to
initialize it to a useful value.

The body can actually have function and procedure calls that
are executed when the program is loaded, as well as loops or
conditional statements.

If you would like to execute some statements during
initialization and again during the execution of the program
one or more times, you can write a procedure or function to
accomplish your desires and call it at the appropriate times
in the main program.


SELECTIVE NAMING OF FUNCTIONS AND PROCEDURES
____________________________________________________________

If you will study the interface part of this unit you will
find that everything you need to use this unit is contained
within it, provided that you know enough about plane geometry
to understand the functions.  You should strive for this

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understanding in all of your interfaces so that the
implementation doesn't even require consultation.  Keep in
mind, that if you need to, you can include comments to further
define the functions in the interface part of the unit.

At this time, you should compile this unit.  You will have to
compile it to disk rather than only to memory so it will be
available for use later in this chapter.  You do this by using
the menus to change the Compile/Destination to the Disk
option.  Note that it will not generate an .EXE file but
instead a .TPU file.  This is Borland's filename extension for
a unit.


ANOTHER UNIT
____________________________________________________________

Load the file named PERIMS.PAS for another   ================
example of a unit.  This is similar to the      PERIMS.PAS
last except that it does not contain an      ================
internal procedure, and it is composed of
three procedures that calculate the
perimeters of geometric shapes, all of which are visible to
the outside world because they are included in the interface
part of the unit.  Once again, we have a private variable
named My_Pi and a block of code (actually a single statement)
to initialize the value of My_Pi when the unit is loaded.

Be sure you compile this unit to disk in the same manner as
the last and they will be ready for use.

Now that we have several functions and procedures that can be
used to calculate the areas or perimeters of several different
shapes, we need a program to illustrate their use, so if you
load and display the program named GARDEN.PAS you will have
an example of their use.


HOW DO WE USE OUR DEFINED UNITS?
____________________________________________________________

GARDEN.PAS is a very simple program that     ================
uses one of the functions and one of the        GARDEN.PAS
procedures.  The only thing you must do is   ================
add the names of the units prior to using
the external functions or procedures.
Lines 16 and 17 each use one of our newly defined routines.
As you can see, there is nothing magic about the new routines,
and once you include the unit names in a uses statement, the
new routines are in a sense, an extension to the Pascal
language.  Compile and run this program and see that it really
does what you expect it to do.



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ONE MORE EXAMPLE OF UNIT USE
____________________________________________________________

Load and display the program named          =================
SHAPES4.PAS for another example of using a     SHAPES4.PAS
predefined unit.  In line 3, this program   =================
includes our new unit named Areas so all
four of the area functions are available,
and in fact, all four are used within the body of the program.
This program should not be difficult for you to understand and
you will be left to study it on your own.  You should observe
that this program is repeated in chapter 14 in a different
form for users of TURBO Pascal 3.0.


MULTIPLE USES OF AN IDENTIFIER
____________________________________________________________

Suppose we wanted to move the variable named My_Pi to the
interface section in both of the units we defined earlier.
Then in the program named GARDEN.PAS when we included both of
the units in the uses statement, both variables named My_Pi
would be available for use so we would have a bit of a problem
defining which one we really meant to use.  TURBO Pascal has
a way to tell the system which one you wish to use by using
a qualifier in much the same way that you use a field of a
record.  The variable name Areas.My_Pi would refer to that
variable from the unit named Areas, and the name Perims.My_Pi
would refer to the variable from the unit named Perims.

You could even define a new variable of the same name in your
main program and refer to it by the qualified name
Garden.My_Pi if you chose to.  This is not recommended as it
would get very confusing to you.  The compiler would be very
happy to compile and run such a program, because it would not
get confused.

It is not illustrated in the example program, but this
technique applies to procedure and function names as well.
If you used the same procedure name in two different units,
you could specify which procedure you intend to use by using
the dot notation with the unit name and the procedure name.
Unit_Name.Procedure_Name would therefore refer to the
procedure named Procedure_Name that is a part of the unit
named Unit_Name.


WHY USE UNITS?
____________________________________________________________

There are basically three reasons to use units in your
programming.  First, some programs are so large that they
should be broken up into smaller chunks for ease of handling
and reasonable compilation size.  In fact some are so large

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that they cannot be compiled all at one time since TURBO
Pascal has an upper limit of 64K of code which can be compiled
at once.  Most other compilers have a similar limit also.

Secondly, once you complete the code to perform a certain job,
you may wish to use the same code in another program to do the
same job.  If you put the code in a unit, it is ready to
simply call and use again.  This is becoming a rather
important topic in software engineering usually referred to
as "Reusable Software".


THIS IS INFORMATION HIDING
____________________________________________________________

Finally, it is sometimes important to hide a portion of code
from the rest of the program to assure that it cannot be
unduly modified by an error somewhere else in the program.
This too is becoming an important area of software engineering
and is usually referred to as information hiding.


PROGRAMMING EXERCISE
____________________________________________________________

1.   Move My_Pi to the interface in both units and change one
     of the values slightly to see if you can read in the
     right one at the right time.  Define another variable of
     the same name in your main program and see if you can
     differentiate between all three values.  Note that, due
     to the nature of this exercise, no answer is given for
     it on the distribution disk.























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