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CHAP2.TXT
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1988-11-27
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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.
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 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
more than 8. TURBO Pascal uses up to 127 characters in an
identifier as being significant. Since nearly all Pascal
compilers use the underline as an allowable character in an
identifier, 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.
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CHAPTER 2 - Getting started in Pascal
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 by the
BEGIN and END statement. Since 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 exit the
editor, using Ctrl-K-D, unless you modified the exit
command. Then compile the program, and finally run it and
observe the result.
Since that program didn't do much, it was not very
interesting, so let's get one that does something.
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CHAPTER 2 - Getting started in Pascal
A PROGRAM THAT DOES SOMETHING
Load the Pascal program WRITESM and edit it. The name
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
is any identifier we choose to make it. 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 another reserved 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.
Notice the semicolon at the end of each line. 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. This
program will output the two lines of text and stop. Now it
is time to go try it. Exit the editor, then compile and run
the program.
You should now get the two lines 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
reserved 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.
It is time to confess to a little lie. WRITELN and WRITE
are not actually reserved words, they are actually
predefined functions which we have not discussed yet. For
the time being, it is easiest to think of them as reserved
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CHAPTER 2 - Getting started in Pascal
words. When we get to the proper point, we will redefine
them properly.
Now might be a good time for you to go back 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
your 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 probably 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.
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.
GOOD FORMATTING PRACTICE
Edit GOODFORM now 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
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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
variable names. You should pay 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 clear commenting of 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
can 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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