Chapter 2 Con't

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Pascal Tutorial - Chapter 2



Lets get right into a program that really does nothing, but is an example of the most trivial Pascal program. Load Turbo Pascal, then load TRIVIAL.PAS into the integrated environment as a work file. This assumes you have been successful in learning how to use the TURBO Pascal system. 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 contains the program name which can be any name you like, as long as it follows the rules for an identifier given in the next section. 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. The second word, Puppy_Dog, is the program name itself. Notice that the line ends with a semicolon. Pascal uses the semicolon as a statement separator and although all statements do not actually end in a semicolon, most do, and the proper use of the semicolon will clear up later in your mind. TURBO Pascal does not require the program statement, but to remain compatible with standard Pascal, it will simply ignore the entire statement. It is recommended that you include a program name both to aid your thinking in standard Pascal, and to add a little more indication of the purpose of each program. WHAT IS AN IDENTIFIER? All identifiers, including the 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 than 8 are used. Most implementations use far more than 8. All versions of TURBO Pascal use 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. All versions of TURBO Pascal compilers permit the use of the underline 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, line 2 is a blank line. Blank lines are ignored by all Pascal compilers. More will be said about the blank line at the end of this chapter. NOW FOR THE PROGRAM Lines 3 and 4 comprise the actual Pascal program, which in this case does absolutely nothing. This 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 between the begin and end reserved words. Finally, although it could be very easily overlooked, there is one more very important part of the program, the period following the reserved word 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. Think of a Pascal program as one long sentence with one period at the end. Ignore lines 9 through 13 for a few minutes and we will describe them fully later. That should pretty well describe our first program. Now it is time for you to compile and run it. Since this program doesn't do anything, it is not very interesting, so let's look at one that does something. A PROGRAM THAT DOES SOMETHING Example Program ------> WRITESM.PAS Load the Pascal program WRITESM.PAS and view it on your monitor. The filename is sort of cryptic for "Write Some" and it will display a little output on 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, which will be fully 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 kinds of data we may wish to display, we will restrict ourselves to the simplest for the time being. Any information 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. Until you gain considerable programming experience, simply use them as tools. Notice the semicolon at the end of line 4. This is the statement separator referred to earlier and tells the Pascal compiler 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. Now it is time to go try it. Compile and run this program. You should see the two lines of text output to the video display every time you run this program. When you grow bored of running WRITESM.PAS let's go on to another example. ANOTHER PROGRAM WITH MORE OUTPUT Example program ------> WRITEMR.PAS Examine the example program named WRITEMR.PAS. 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 is a procedure which causes the text to be output in exactly the same manner as Writeln, but Write does not cause a carriage return to be output. 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 of text will be output on the same line of the monitor 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. Compile and execute the new program. Now might be a good time for you to return to editing WRITEMR.PAS 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 Example program ------> PASCOMS.PAS The file named PASCOMS.PAS 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, lines 11 and 12 that would normally print "send money", are not Pascal code but are commented out by the comment delimiters in lines 10 and 13. Try compiling and running this program, then edit the comment delimiters 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 permit this funny use of comment delimiters. 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 named PASCOMS.PAS 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. THE RESULT OF EXECUTION SECTION You should now be able to discern the purpose for lines 20 through 26 of this program. Each of the example programs in this tutorial lists the result of execution in a comments section at the end of the program. This makes it possible to study this tutorial anywhere if you print out a hard copy of the example programs. With this text, and a hard copy of the example programs containing the result of execution, you do not need access to a computer to study. Of course you would need access to a computer to write, compile, and execute the programming exercises, which you are heartily encouraged to do. GOOD FORMATTING PRACTICE Example program ------> GOODFORM.PAS Examine GOODFORM.PAS 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 Example program ------> UGLYFORM.PAS Examine UGLYFORM.PAS 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.PAS 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 variable names. You should pay some attention to programming style but don't get too worried about it yet. It would be good for you to simply use the style illustrated throughout this tutorial until you gain experience with Pascal. 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 execute UGLYFORM.PAS.

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

Chapter 3 Con't

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