ProfitPress Mega CDROM2 …eeware (MSDOS)(1992)(Eng)/ProfitPress-MegaCDROM2.B6I/PROG/PASCAL/PASTUT24.ZIP/PTUTRTXT.ZIP/CHAP02.TXT

Text File | 1989-12-01 | 16.2 KB | 414 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         TRIVIAL.PAS
most trivial Pascal program. Load Turbo     =================
Pascal, select TRIVIAL.PAS 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 through 5.5, 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
a 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 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

                                                     Page 2-1

                                    Getting Started in Pascal

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.
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.  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, the next line 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.  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 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.  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.
Ignore lines 9 through 13 for a few minutes and we will
describe them fully later.

                                                     Page 2-2

                                    Getting Started in Pascal


That should pretty well describe our first program.  Now it
is time to compile and run it after you exit the editor.  The
means of doing this is given for each compiler as an aid to
get you started.

  TURBO Pascal 3.0 - <ctrl>k, r

  TURBO Pascal 4.0 - <alt>r

  TURBO Pascal 5.x - <alt>r  (then <alt>F5 to view the
results)

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.PAS and     =================
view it on your monitor.  The filename is      WRITESM.PAS
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 so
simply use them as tools.


                                                     Page 2-3

                                    Getting Started in Pascal

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.  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.



ANOTHER PROGRAM WITH MORE OUTPUT
____________________________________________________________

Examine the example program named           =================
WRITEMR.PAS.  This new program has three       WRITEMR.PAS
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
____________________________________________________________

The file named PASCOMS.PAS is similar to    =================
the others except that comments have been      PASCOMS.PAS
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

                                                     Page 2-4

                                    Getting Started in Pascal

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.

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 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.





                                                     Page 2-5

                                    Getting Started in Pascal

GOOD FORMATTING PRACTICE
____________________________________________________________

Examine GOODFORM.PAS to see an example of    ================
good formatting style.  It is important to     GOODFORM.PAS
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
____________________________________________________________

Examine UGLYFORM.PAS now to see an example   ================
of terrible formatting style.  It is not       UGLYFORM.PAS
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.


                                                     Page 2-6

                                    Getting Started in Pascal


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 2-7