HPAVC

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

/ HPAVC / HPAVC CD-ROM.iso / pc / PASCTXT.ZIP / CHAP2.TXT < prev next >

Wrap

Text File | 1988-01-15 | 16.4 KB | 389 lines

CHAPTER 2 - Getting started in Pascal YOUR FIRST PASCAL PROGRAM Lets get right into a program that really does nothing but is an example of the most trivial Pascal program. Load Turbo Pascal, select TRIVIAL as a Work file, and select Edit. This assumes that you have been successful in learning how to use the TURBO Pascal system. If you are using TURBO Pascal 4.0, you will need to load TRIVIAL.PAS from the File menu. You should now have the most trivial Pascal program possible on your display, and we can take a look at each part to define what it does. The first line is required in the standard Pascal definition and is the program name which can be any name you like, as long as it follows the rules for an identifier given in the next paragraph. It can have no blanks, otherwise it would be considered as two words and it would confuse the compiler. The first word "program" is the first of the reserved words mentioned earlier and it is the indicator to the Pascal compiler that this is the name of the program. Notice that the line ends with a semicolon. Pascal uses the semicolon as the statement separator and although all statements do not actually end in a semicolon, most do, and use of the semicolon will clear up later in your mind. TURBO Pascal version 3.0 does not require the "program" statement, but to remain compatible with standard Pascal, it will simply ignore the entire statement. I like to include a program name both to keep me thinking in standard Pascal, and to add a little more indication of the purpose of each program. WHAT IS AN IDENTIFIER? All identifiers, including program name, procedure and function names, type definitions, and constant and variable names, will start with an alphabetical character and be composed of any combination of alphabetic and numeric characters with no embedded blanks. Upper or lower case alphabetic characters are not significant and may be mixed at will. (If you find this definition confusing at this point, don't worry about it, it will be clear later but it must be defined early). The standard definition of Pascal requires that any implementation (i.e. any compiler written by some company) must use at least 8 characters of the identifier as significant and may ignore the remaining characters if more are used. Most implementations use far Page 7 CHAPTER 2 - Getting started in Pascal more than 8. TURBO Pascal uses at least 63 characters in an identifier as being significant. Standard Pascal does not allow the use of underlines in an identifier but most implementations of Pascal allow its use after the first character. Both TURBO Pascal compilers allow the use of the underline as an allowable character in an identifier, so it will be freely used throughout this tutorial. The underline is used in the program name "Puppy_Dog" which should be on your display at this time. Returning to the example program, the next line is a blank line which is ignored by all Pascal compilers. More will be said about that at the end of this chapter. NOW FOR THE PROGRAM The next two lines comprise the actual Pascal program, which in this case does absolutely nothing. It is an illustration of the minimum Pascal program. The two words "begin" and "end" are the next two reserved words we will consider. Any logical grouping of Pascal code can be isolated by bracketing it with the two reserved words "begin" and "end". You will use this construct repeatedly as you write Pascal code so it is well to learn it thoroughly. Code to be executed by conditional jumps will be bracketed by "begin" and "end", as will code within a loop, and code contained within a subroutine (although they are called "procedures" in Pascal), and in many other ways. In the present program, the "begin" and "end" are used to bracket the main program and every Pascal program will have the main program bracketed in this manner. Because there is nothing to do in this program, there are no statements. Finally, although it could be very easily overlooked, there is one more very important part of the program, the period following "end". The period is the signal to the compiler that it has reached the end of the executable statements and is therefore finished compiling. Every Pascal program will have one, and only one period in it and that one period will be at the end of the program. I must qualify that statement in this regard, a period can be used in comments, and in text to be output. In fact there are some data formats that require using a period as part of their structure. The statement is true however, that there is only one period in the executable part of a Pascal program. Think of a Pascal program as one long sentence with one period at the end. That should pretty well describe our first program. Now it is time to compile and run it. To do so you must Page 8 CHAPTER 2 - Getting started in Pascal exit the editor using Ctrl-K-D, unless you modified the exit command. Compile the program, and run it to observe the result. Since this program doesn't do anything, it is not very interesting, so let's get one that does something. A PROGRAM THAT DOES SOMETHING Load the Pascal program WRITESM and view it on your monitor. The filename is sort of cryptic for "Write Some" and it will give a little output to the monitor. The program name is "Kitty_Cat" which says nothing about the program itself but can be any identifier we choose. We still have the begin and end to define the main program area followed by the period. However, now we have two additional statements between the begin and end. "Writeln" is a special word and it is probably not surprising that it means to write a line of data somewhere. Without a modifier, (to be explained in due time), it will write to the default device which, in the case of our IBM compatible, is the video display. The data within the parentheses is the data to be output to the display and although there are many possibilities of display information, we will restrict ourselves to the simplest for the time being. Any data between apostrophes will simply be output as text information. The special word "Writeln" is not a reserved word but is defined by the system to do a very special job for you, namely to output a line of data to the monitor. It is, in fact, a procedure supplied for you by the writers of TURBO Pascal as a programming aid for you. You can, if you so desire, use this name for some other purpose in your program but doing so will not allow you to use the standard output procedure. It will then be up to you to somehow get your data out of the program. Note carefully that some words are reserved and cannot be redefined and used for some other purpose, and some are special since they can be redefined. You will probably not want to redefine any of the special words for a long time so simply use them as tools. Notice the semicolon at the end of line 4. This is the statement separator referred to earlier and tells Pascal that this line is complete as it stands, nothing more is coming that could be considered part of this statement. The next statement, in line 5, is another statement that will be executed sequentially following the statement in line 4. This program will output the two lines of text and stop. Page 9 CHAPTER 2 - Getting started in Pascal Now it is time to go try it. Exit the editor, then compile and run the program. You should get the two lines of text output to the video display every time you run it. When you grow bored of running WRITESM lets go on to another example. ANOTHER PROGRAM WITH MORE OUTPUT Load and edit WRITEMR. This new program has three lines of output but the first two are different because another special word is introduced to us, namely Write. Write causes the text to be output in exactly the same manner as Writeln, but Write does not cause a carriage return. Writeln causes its output to take place then returns the "carriage" to the first character of the next line. The end result is that all three of the lines will be output on the same line when the program is run. Notice that there is a blank at the end of each of the first two lines so that the formatting will look nice. Exit the editor now and try the new program. Now might be a good time for you to return to editing WRITEMR and add a few more output commands to see if they do what you think they should do. When you tire of that, we will go on to the next file and learn about comments within a Pascal program. ADDING COMMENTS IN THE PROGRAM The file named PASCOMS is similar to the others except that comments have been added to illustrate their use. Pascal defines comments as anything between (* and *) or anything between { and }. Originally only the wiggly brackets were defined but since many keyboards didn't have them available, the parenthesis star combination was defined as an extension and is universal by now, so you can use either. Most of the comments are self explanatory except for the one within the code. Since comments can go from line to line, the two lines that would print "send money" are not Pascal code but are commented out. Try compiling and running this program, then edit the comments out so that "send money" is printed also. A fine point should be mentioned here. Even though some compilers allow comments to start with (* and end with }, or to start with { and end with *), it is very poor programming practice and should be discouraged. The ANSI Pascal standard allows such usage but TURBO Pascal does not allow this funny use of comment delimiters. Page 10 CHAPTER 2 - Getting started in Pascal TURBO Pascal does not allow you to nest comments using the same delimiters but it does allow you to nest one type within the other. This could be used as a debugging aid. If you generally use the (* and *) for comments, you could use the { and } in TURBO Pascal to comment out an entire section of code during debugging even if it had a few comments in it. This is a trick you should remember when you reach the point of writing programs of significant size. When you have successfully modified and run the program with comments, we will go on to explain good formatting practice and how Pascal actually searches through your source file (Pascal program) for its executable statements. It should be mentioned that the program PASCOMS does not indicate good commenting style. The program is meant to illustrate where and how comments can be used and looks very choppy and unorganized. Further examples will illustrate good use of comments to you as you progress through this tutorial. GOOD FORMATTING PRACTICE Observe GOODFORM to see an example of good formatting style. It is important to note that Pascal doesn't give a hoot where you put carriage returns or how many blanks you put in when a blank is called for as a delimiter. Pascal only uses the combination of reserved words and end-of- statement semicolons to determine the logical structure of the program. Since we have really only covered two executable statements, I have used them to build a nice looking program that can be easily understood at a glance. Compile and run this program to see that it really does what you think it should do. VERY POOR FORMATTING PRACTICE Edit UGLYFORM now to see an example of terrible formatting style. It is not really apparent at a glance but the program you are looking at is exactly the same program as the last one. Pascal doesn't care which one you ask it to run because to Pascal, they are identical. To you they are considerably different, and the second one would be a mess to try to modify or maintain sometime in the future. UGLYFORM should be a good indication to you that Pascal doesn't care about programming style or form. Pascal only cares about the structure, including reserved words and delimiters such as blanks and semicolons. Carriage returns are completely ignored as are extra blanks. You can put extra blanks nearly anywhere except within reserved words or Page 11 CHAPTER 2 - Getting started in Pascal variable names. You should pay some attention to programming style but don't get too worried about it yet. As time goes by you will develop a style of statement indentation, adding blank lines for clarity, and a method of adding clear comments to Pascal source code. Programs are available to read your source code, and put it in a "pretty" format, but that is not important now. Not only is the form of the program important, the names used for variables can be very helpful or hindering as we will see in the next chapter. Feel free to move things around and modify the format of any of the programs we have covered so far and when you are ready, we will start on variables in the next chapter. Be sure you compile and run UGLYFORM. PROGRAMMING EXERCISES 1. Write a program that displays your name on the video monitor. 2. Modify your program to display your name and address on one line, then modify it by changing the Write's to Writeln's so that the name and address are on different lines. Page 12