The Programmer Disk/The Programmer Disk (Microforum).iso/xpro/tutor/pro30/chap02.txt

Text File | 1991-02-04 | 16KB | 354 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      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.

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                            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.




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                            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.






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                            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.


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                            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

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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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