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

Text File | 1990-02-15 | 14KB | 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 i