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- 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 TRIVIAL.PAS
- 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
- 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 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.
-
- Page 2-1
-
- Chapter 2 - Getting Started in Pascal
-
- 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 which is
- 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. Hit <alt> r to compile and run the
- program, then <alt> F5 to view the result of execution. Since this
- program doesn't do anything, it is not very interesting, so let's
- look at one that does something.
-
-
-
-
- Page 2-2
-
- Chapter 2 - Getting Started in Pascal
-
-
- A PROGRAM THAT DOES SOMETHING
- _________________________________________________________________
-
- Load the Pascal program WRITESM.PAS and view it ===============
- on your monitor. The filename is sort of WRITESM.PAS
- 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 the program in the
- same manner as you did for the first example 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.
-
-
-
-
-
-
- Page 2-3
-
- Chapter 2 - Getting Started in Pascal
-
- ANOTHER PROGRAM WITH MORE OUTPUT
- _________________________________________________________________
-
- Examine the example program named WRITEMR.PAS. ===============
- This new program has three lines of output but WRITEMR.PAS
- 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
- _________________________________________________________________
-
- The file named PASCOMS.PAS is similar to the ===============
- others except that comments have been added to PASCOMS.PAS
- 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. 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.
-
- 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.
-
-
- Page 2-4
-
- Chapter 2 - Getting Started in Pascal
-
- 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 similar comments section at the
- end of the program. This makes it possible to study this tutorial
- anywhere once you print out the example programs as described in
- the READ.ME file on the distribution disk. 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
- _________________________________________________________________
-
- Examine GOODFORM.PAS to see an example of good ================
- formatting style. It is important to note that GOODFORM.PAS
- 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
- _________________________________________________________________
-
- Examine UGLYFORM.PAS now to see an example of ================
- terrible formatting style. It is not really UGLYFORM.PAS
- 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
-
- Page 2-5
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- Chapter 2 - Getting Started in Pascal
-
- 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.
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