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1988-01-15
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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 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
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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
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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.
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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.
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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
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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.
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