C/C++ Interactive Guide/c-cplusplus-interactive-guide.iso/c_ref/csource4/252_01/chap02.txt

Text File | 1990-02-15 | 13.8 KB | 349 lines

                                                    Chapter 2
                                         GETTING STARTED IN C


YOUR FIRST C PROGRAM
____________________________________________________________

The best way to get started with C is to        =============
actually study a program, so load the file        TRIVIAL.C
named 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.


A PROGRAM THAT DOES SOMETHING
____________________________________________________________

For a much more interesting program, load the   =============
program named WRTSOME.C and display it on         WRTSOME.C
your 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

                                                     Page 2-1

                             Chapter 2 - Getting Started in C

semi-colon as a statement terminator, so the semi-colon is
required as a signal to the compiler that this line is
complete.  This program is also executable, so you can compile
and run it to see if it does what you think it should.


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.  The second printf() outputs a line but does not
return the carriage so that the third line is appended to 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 the statements really work.

                                                     Page 2-2

                             Chapter 2 - Getting Started in C

LET'S PRINT SOME NUMBERS
____________________________________________________________

Load the file named ONEINT.C and display it    ==============
on the monitor for our first example of how       ONEINT.C
to 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 have a value 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.

Note that, even though we have defined a variable, we have not
yet assigned a value to it.  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 4
assigns the value of 13 to index, and its value is printed out
by line 5.  (We will see how shortly.)  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 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

                                                     Page 2-3

                             Chapter 2 - Getting Started in C

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, and after the
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 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 get confused and probably quit with a runtime error.

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.


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.C
comments can be added to a C program.          ==============
Comments are added to make a program more
readable to you but the compiler must ignore
the comments.  The slash star combination is used in C for
comment delimiters.  They 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 program.  The next comment 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

                                                     Page 2-4

                             Chapter 2 - Getting Started in C

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.  The last comment 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 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.


GOOD FORMATTING STYLE
____________________________________________________________

Load the file GOODFORM.C and observe it on     ==============
your monitor.  It is an example of a well        GOODFORM.C
formatted 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 formatting style yet.  You will have plenty of

                                                     Page 2-5

                             Chapter 2 - Getting Started in C

time to develop a style of your own as you learn the language.
Be observant of styles as you see C programs in magazines,
books, and other publications.

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.


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.


































                                                     Page 2-6