HPAVC

home *** CD-ROM | disk | FTP | other *** search

/ HPAVC / HPAVC CD-ROM.iso / pc / PASCTXT.ZIP / CHAP13.TXT < prev next >

Wrap

Text File | 1988-01-15 | 14.5 KB | 389 lines

CHAPTER 13 - Units in TURBO Pascal 4.0 THIS IS FOR TURBO PASCAL 4.0 USERS ONLY 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 Niclaus 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 has chosen to do so. PART OF A PROGRAM Load the program named AREAS and display it on your monitor. This is the 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 unusal "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 next reserved word used is "implementation" and gives the definitions and executable parts of the private portion of the unit. 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. Page 84 CHAPTER 13 - Units in TURBO Pascal 4.0 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 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 Page 85 CHAPTER 13 - Units in TURBO Pascal 4.0 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 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. It 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 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 Page 86 CHAPTER 13 - Units in TURBO Pascal 4.0 file but instead a .TPU file. This is Borland's filename extension for a unit. ANOTHER UNIT Load the file named PERIMS for another example of a unit. This is similar to the 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 perimiters of several different shapes, we need a program to illustrate their use, so if you load and display the program named GARDEN you will have an example of their use. HOW DO WE USE OUR DEFINED UNITS? GARDEN is a very simple program that uses one of the functions and one of the 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. ONE MORE EXAMPLE OF UNIT USE Load and display the program named SHAPES4 for another example of using a 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. Page 87 CHAPTER 13 - Units in TURBO Pascal 4.0 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 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. 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 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. Page 88 CHAPTER 13 - Units in TURBO Pascal 4.0 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. Page 89