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Chapter 2 - Getting started in C
YOUR FIRST C PROGRAM
The best way to get started with C is to actually look
at a program, so load the file named TRIVIAL.C into your
editor 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 your monitor. It is the
same as the previous program except that it has one
executable statement between the braces.
The executable statement is another function. Once
again, we will not worry about what a function is, but only
how to use this one. In order to output text to the
monitor, it 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 program is also executable, so you can
compile and run it to see if it does what you think it
should.
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Chapter 2 - Getting started in C
ANOTHER PROGRAM WITH MORE OUTPUT
Load the program WRTMORE.C and display it on your
monitor for an example of more output and another small but
important concept. You will see that there are four program
statements in this program, each one being a "printf"
function statement. 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 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 the third line is appended to that 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.
Compile and run this program to see if it does what you
expect it to do. 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.
LETS PRINT SOME NUMBERS
Load the file named ONEINT.C and display it on the
monitor for our first example of how 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 the line containing "int index;", which is used
to define an integer variable named "index". The "int" is a
reserved word in C, and can therefore not be used for
anything else. It defines a variable that can have a value
from -32768 to 32767 on most microcomputer implementations
of C. Consult your users manual for the exact definition
for your compiler. The variable name, "index", can be any
name that follows the rules for an identifier and is not one
of the reserved words for C. Consult your manual for an
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Chapter 2 - Getting started in C
exact definition of an identifier for your compiler. The
final character on the line, the semi-colon, is the
statement terminator used in C.
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 first one
assigns the value of 13 to "index", and its value is printed
out. (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. 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 a variable. The % sign is used to signal the
start 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,
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 it is important that
the number of field descriptors and the number of variable
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Chapter 2 - Getting started in C
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 this topic is 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 COMMENTS.C and observe it on your monitor
for an example of how 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 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. 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 printed out.
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Chapter 2 - Getting started in C
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 compilers allow you to "nest" comments which can
be very handy if you need to "comment out" a section of code
during debugging. Check your compiler documentation for the
availability of this feature with you particular compiler.
Compile and run COMMENTS.C at this time.
GOOD FORMATTING STYLE
Load the file GOODFORM.C and observe it on your
monitor. It is an example of a well 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 concerning how you format 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 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
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.
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