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Chapter 2
GETTING STARTED IN C
YOUR FIRST C PROGRAM
-----------------------------------------------------------------
The best way to get started with C is to actu- ===============
ally study a program, so load the file named TRIVIAL.C
TRIVIAL.C and display it on the monitor. You ===============
are looking at the simplest possible C program.
There is no way to simplify this program or to leave anything out.
Unfortunately, the program doesn't do anything.
The word main is very important, and must appear once, and only
once in every C program. This is the point where execution is
begun when the program is run. We will see later that this does
not have to be the first statement in the program but it must
exist as the entry point. Following the main program name is a
pair of parentheses which are an indication to the compiler that
this is a function. We will cover exactly what a function is in
due time. For now, I suggest that you simply include the pair of
parentheses.
The two curly brackets, properly called braces, are used to
define the limits of the program itself. The actual program
statements go between the two braces and in this case, there are
no statements because the program does absolutely nothing. You
can compile and run this program, but since it has no executable
statements, it does nothing. Keep in mind, however, that it is a
valid C program. When you compile this program, you may get a
warning depending on how your compiler is set up. You can ignore
the warning and we will discuss it later in this tutorial.
A PROGRAM THAT DOES SOMETHING
-----------------------------------------------------------------
For a much more interesting program, load the ===============
program named WRTSOME.C and display it on your WRTSOME.C
monitor. It is the same as the previous ===============
program except that it has one executable
statement between the braces. The executable statement is a call
to a function supplied as a part of your C library. Once again,
we will not worry about what a function is, but only how to use
this one named printf(). In order to output text to the monitor,
the desired text is put within the function parentheses and
bounded by quotation marks. The end result is that whatever is
included between the quotation marks will be displayed on the
monitor when the program is run.
Notice the semi-colon at the end of the line. C uses a semi-
colon as a statement terminator, so the semi-colon is required as
a signal to the compiler that this line is complete. This
Page 2-1
Chapter 2 - Getting Started in C
program is also executable, so you can compile and run it to see
if it does what you think it should. It should cause the text
between the quotation marks to appear on the monitor.
You may get two warnings when you compile this program and each
of the remaining programs in this chapter. All warnings in this
chapter can be ignored.
ANOTHER PROGRAM WITH MORE OUTPUT
-----------------------------------------------------------------
Load the program WRTMORE.C and display it on ===============
your monitor for an example of more output WRTMORE.C
and another small but important concept. You ===============
will see that there are four program statements
in this program, each one being a call to the function printf().
The top line will be executed first, then the next, and so on,
until the fourth line is complete. The statements are executed
in order from top to bottom.
Notice the funny character near the end of the first line, namely
the backslash. The backslash is used in the printf() statement
to indicate that a special control character is following. In
this case, the "n" indicates that a newline is requested. This
is an indication to return the cursor to the left side of the
monitor and move down one line. It is commonly referred to as a
carriage return/line feed. Any place within text that you
desire, you can put a newline character and start a new line.
You could even put it in the middle of a word and split the word
between two lines. The C compiler considers the combination of
the backslash and letter n as one character.
A complete description of this program is now possible. The
first printf() outputs a line of text and returns the carriage.
(Of course, there is no carriage, but the cursor is moved to the
next line on the monitor. The terminology carries over from the
days of teletypes.) The second printf() outputs a line but does
not return the carriage so that the third line is appended to the
end of the second, then followed by two carriage returns,
resulting in a blank line. Finally the fourth printf() outputs a
line followed by a carriage return and the program is complete.
After compiling and executing WRTMORE.C, the following text
should be displayed on your monitor;
This is a line of text to output.
And this is another line of text.
This is a third line.
Compile and run this program to see if it gives you this output.
It would be a good idea at this time for you to experiment by
adding additional lines of printout to see if you understand how
Page 2-2
Chapter 2 - Getting Started in C
the statements really work. Add a few carriage returns in the
middle of a line to prove to yourself that it works as stated,
then compile and execute the modified program. The more you
modify and compile the example programs, the more you will learn
as you work your way through this tutorial.
LET'S PRINT SOME NUMBERS
-----------------------------------------------------------------
Load the file named ONEINT.C and display it on ================
the monitor for our first example of how to ONEINT.C
work with data in a C program. The entry point ================
main should be clear to you by now as well as
the beginning brace. The first new thing we encounter is line 3
containing int index; which is used to define an integer variable
named index. The word int is a keyword in C, and can not be used
for anything else. It defines a variable that can store a value
in the range from -32768 to 32767 in most C compilers for
microcomputers. The variable name, index, can be any name that
follows the rules for an identifier and is not one of the
keywords for C. The final character on the line, the semi-colon,
is the statement terminator as discussed earlier.
Note that, even though we have defined a variable, we have not
yet assigned a value to it, so it contains an undefined value.
We will see in a later chapter that additional integers could
also be defined on the same line, but we will not complicate the
present situation.
Observing the main body of the program, you will notice that
there are three statements that assign a value to the variable
index, but only one at a time. The statement in line 5 assigns
the value of 13 to index, and its value is printed out by line 6.
(We will see how shortly. Just trust me for the time being.)
Later, the value of 27 is assigned to index, and finally 10 is
assigned to it, each value being printed out. It should be
intuitively clear that index is indeed a variable and can store
many different values but only one value at a time of course.
Please note that many times the words "printed out" are used to
mean "displayed on the monitor". You will find that in many
cases experienced programmers take this liberty, probably due to
the printf() function being used for monitor display.
HOW DO WE PRINT NUMBERS?
-----------------------------------------------------------------
To keep our promise, let's return to the printf() statements for
a definition of how they work. Notice that they are all identical
and that they all begin just like the printf() statements we have
seen before. The first difference occurs when we come to the %
character. This is a special character that signals the output
routine to stop copying characters to the output and do something
Page 2-3
Chapter 2 - Getting Started in C
different, namely output the value of a variable. The % sign is
used to signal the output of many different types of variables,
but we will restrict ourselves to only one for this example. The
character following the % sign is a d, which signals the output
routine to get a decimal value and output it. Where the decimal
value comes from will be covered shortly. After the d, we find
the familiar \n, which is a signal to return the video "carriage",
and the closing quotation mark.
All of the characters between the quotation marks define the
pattern of data to be output by this statement. Following the
output pattern, there is a comma followed by the variable name
index. This is where the printf() statement gets the decimal
value which it will output because of the %d we saw earlier.
We could add more %d output field descriptors anywhere within the
brackets and more variables following the description to cause
more data to be printed with one statement. Keep in mind
however, that the number of field descriptors and the number of
variable definitions must be the same or the runtime system will
generate something we are not expecting.
Much more will be covered at a later time on all aspects of input
and output formatting. A reasonably good grasp of these
fundamentals are necessary in order to understand the following
lessons. It is not necessary to understand everything about
output formatting at this time, only a fair understanding of the
basics.
Compile and run ONEINT.C and observe the output. Two programming
exercises in this chapter are based on this program.
HOW DO WE ADD COMMENTS IN C?
-----------------------------------------------------------------
Load the file named COMMENTS.C and observe it ================
on your monitor for an example of how comments COMMENTS.C
can be added to a C program. Comments are ================
added to make a program more readable to you
but represent nonsense to the compiler, so we must tell the
compiler to ignore the comments completely by bracketing them with
special characters. The slash star combination is used in C for
comment delimiters, and are illustrated in the program at hand.
Please note that the program does not illustrate good commenting
practice, but is intended to illustrate where comments can go in
a program. It is a very sloppy looking program.
The first slash star combination introduces the first comment and
the star slash at the end of the first line terminates this
comment. Note that this comment is prior to the beginning of the
program illustrating that a comment can precede the program
itself. Good programming practice would include a comment prior
to the program with a short introductory description of the
Page 2-4
Chapter 2 - Getting Started in C
program. The comment in line 3 is after the main program entry
point and prior to the opening brace for the program code itself.
The third comment starts after the first executable statement in
line 5 and continues for four lines. This is perfectly legal
because a comment can continue for as many lines as desired until
it is terminated. Note carefully that if anything were included
in the blank spaces to the left of the three continuation lines
of the comment, it would be part of the comment and would not be
compiled, but totally ignored by the compiler. The last comment,
in line 11, is located following the completion of the program,
illustrating that comments can go nearly anywhere in a C program.
Experiment with this program by adding comments in other places
to see what will happen. Comment out one of the printf()
statements by putting comment delimiters both before and after it
and see that it does not get executed causing a line of printout.
Comments are very important in any programming language because
you will soon forget what you did and why you did it. It will be
much easier to modify or fix a well commented program a year from
now than one with few or no comments. You will very quickly
develop your own personal style of commenting.
Some C compilers will allow you to "nest" comments which can be
very handy if you need to "comment out" a section of code during
debugging. Since nested comments are not a part of the ANSI-C
standard, none will be used in this tutorial. Check the
documentation for your compiler to see if they are permitted
with your implementation of C. Even though they may be allowed,
it is a good idea to refrain from their use, since they are
rarely used by experienced C programmers.
A VERY USEFUL EXTENSION
-----------------------------------------------------------------
Load the program named CPPCOMS.C for an example ===============
program with a very useful extension to the C CPPCOMS.C
programming language in recent years. Comments ===============
in C++, a language that was based on C, begin
with a double slash and continue to the end of the line. Many
compiler writers are adding this comment style to their
compilers in addition to the slash-star delimiter defined already.
Using the older style slash-star comment, it is fairly easy to
begin a comment, and forget to terminate it, inadvertently
commenting out a block of code. In some circumstances, this can
be very difficult to track down, so the designer of C++ added the
single line comment. If your compiler supports this style of
comment, you may choose to use it, but you must keep in mind that
if you ever want to use a different compiler that doesn't support
this extension, you may have a lot of code to modify. Of course,
that assumes you add lots of comments to your code.
Page 2-5
Chapter 2 - Getting Started in C
GOOD FORMATTING STYLE
-----------------------------------------------------------------
Load the file GOODFORM.C and observe it on your ==============
monitor. It is an example of a well formatted GOODFORM.C
program. Even though it is very short and ==============
therefore does very little, it is very easy to see
at a glance what it does. With the experience you have already
gained in this tutorial, you should be able to very quickly grasp
the meaning of the program in it's entirety. Your C compiler
ignores all extra spaces and all carriage returns giving you
considerable freedom in formatting your program. Indenting and
adding spaces is entirely up to you and is a matter of personal
taste. Compile and run the program to see if it does what you
expect it to do.
Now load and display the program UGLYFORM.C ================
and observe it. How long will it take you to UGLYFORM.C
figure out what this program will do? It ================
doesn't matter to the compiler which format
style you use, but it will matter to you when you try to debug
your program. Compile this program and run it. You may be
surprised to find that it is the same program as the last one,
except for the formatting. Don't get too worried about format-
ting style yet. You will have plenty of time to develop a style
of your own as you learn the C language. Be observant of styles
as you see C programs in magazines and books.
This should pretty well cover the basic concepts of programming
in C, but as there are many other things to learn, we will forge
ahead to additional program structure. It will definitely be to
your advantage to do the programming exercises at the end of each
chapter. They are designed to augment your studies and teach you
to use your compiler.
PROGRAMMING EXERCISES
-----------------------------------------------------------------
1. Write a program to display your name on the monitor.
2. Modify the program to display your address and phone number
on separate lines by adding two additional printf()
statements.
3. Remove line 5 from ONEINT.C by commenting it out, then
compile and execute the resulting program to see the value
of an uninitialized variable. This can be any value within
the allowable range. If it happens to have the value of
zero, that is only a coincidence, but then zero is probably
the most probably value to be in an uninitialized variable
because there are lots of zero values floating around in a
computers memory.
Page 2-6
Chapter 2 - Getting Started in C
4. Add the following two lines just prior to the closing brace
of ONEINT.C to see what it does. Study it long enough to
completely understand the result.
printf("Index is %d\n it still is %d\n it is %d",
index, index, index);
Page 2-7
