The First C Program
The types of variables,
constants & keywords the next logical step is to combine them to form
instructions. However, instead of this, we would write our first C program now.
Once we have done that we would see in detail the instructions
that it made use of.
Before we begin with our first C program do remember the following rules that are applicable to
all C programs:
(a) Each
instruction in a C program is written as a separate statement. Therefore a
complete C program would comprise of a series of statements.
(b) The
statements in a program must appear in the same order in which we wish them to
be executed;
unless of course the logic of the problem demands a deliberate
‘jump’ or transfer of control to a statement, which is out of sequence.
(c) Blank spaces
may be inserted between two words to improve the readability of the statement.
However, no blank spaces are allowed within a variable, constant or
keyword.
(d) All
statements are entered in small case letters.
(e) C has no
specific rules for the position at which a statement is to be written. That’s
why it is often called a free-form language.
(f) Every C
statement must end with a ;. Thus ; acts as a statement terminator.
(g) Let us now
write down our first C program. It would simply calculate simple interest for a
set of values representing principle, number of years and rate of
interest.
/* Calculation of simple interest */ /* Author gekay Date: 25/05/2004 */ main( ) { int
p, n ; float r, si ;
p = 1000 ; n = 3 ;
r = 8.5 ;
/* formula for simple
interest */ si = p * n * r / 100 ;
printf (
"%f" , si ) ;
}
Now a few useful tips about the program...
− Comment about the program should be enclosed within
/* */. For example, the first two statements
in our program are comments.
− Though comments are not necessary, it is a good practice
to begin a program with a comment indicating the purpose of the program, its
author and the date on which the program was written.
− Any number of comments can be written at any place in the
program. For example, a comment can be written before the statement, after the
statement or within the statement as shown below:
/* formula */ si = p * n * r / 100 ;
si = p * n * r / 100 ;
/* formula */
si = p * n * r / /* formula */ 100 ;
− Sometimes it is not so obvious as to what a particular
statement in a program accomplishes. At such times it is worthwhile mentioning
the purpose of the statement (or a set of statements) using a comment. For
example:
/* formula for simple
interest */
si = p * n * r / 100 ;
− Often programmers seem to ignore writing of comments. But
when a team is building big software well commented code is almost essential
for other team members to understand it.
− Although a lot of comments are probably not necessary in
this program, it is usually the case that programmers tend to use too few
comments rather than too many. An adequate number of comments can save hours of
misery and suffering when you later try to figure out what the program
does.
− The normal language rules do not apply to text written
within /* .. */. Thus we can type this text in small case, capital or a
combination. This is because the comments are solely given for the
understanding of the programmer or the fellow programmers and are completely
ignored by the compiler.
− Comments cannot be nested. For example,
/* Cal of SI /* Author sam date 01/01/2002 */ */
is invalid.
− A comment can be split over more than one line, as
in,
/* This is
a jazzy
comment */
Such a comment is often called a multi-line comment.
− main( ) is a
collective name given to a set of statements. This name has to be main( ), it
cannot be anything else. All statements that belong to main( ) are enclosed
within a pair of braces { } as shown
below.
main( )
{
statement 1 ;
statement 2 ;
statement 3 ; }
− Technically speaking main( ) is a function. Every function
has a pair of parentheses ( ) associated
with it. We would discuss functions and
their working in great detail in Chapter 5.
− Any variable used in the program must be declared before
using it. For example,
int p, n ;
float r, si ;
− Any C statement always ends with a ;
For example,
float r, si ;
r = 8.5 ;
− In the statement,
si = p * n * r / 100 ;
* and / are the arithmetic operators. The arithmetic
operators available in C are +, -, * and /. C is very rich in operators. There
are about 45 operators available in C. Surprisingly there is no operator for
exponentiation... a slip, which can be forgiven considering the fact that C has
been developed by an individual, not by a committee.
− Once the value of si is calculated it needs to be
displayed on the screen. Unlike other languages, C does not contain any instruction
to display output on the screen. All output to screen is achieved using
readymade library functions. One such function is printf( ). We have used it
display on the screen the value contained in si.
The general form of printf( ) function is,
printf ( "<format string>", <list of
variables> ) ;
<format string> can contain,
%f for printing real
values %d for printing integer values
%c for printing character values
In addition to format specifiers like %f, %d and %c the
format string may also contain any other characters. These characters are
printed as they are when the printf( ) is executed.
Following are some examples of usage of printf( )
function:
printf ( "%f", si ) ; printf ( "%d %d %f
%f", p, n, r, si ) ; printf ( "Simple interest = Rs. %f", si ) ;
printf ( "Prin = %d \nRate = %f", p, r ) ;
The output of the last statement would look like
this...
Prin = 1000 Rate = 8.5
What is ‘\n’ doing in this statement? It is called newline
and it takes the cursor to the next line.
Therefore, you get the output split
over two lines. ‘\n’ is one of the several Escape Sequences available in C.
These are discussed in detail in Chapter 11. Right now, all that we can say is
‘\n’ comes in handy when we want to format the output properly on separate
lines.
printf( ) can not only print values of variables, it can
also print the result of an expression. An expression is nothing but a valid
combination of constants, variables and operators. Thus, 3, 3 + 2, c and a + b
* c – d all are valid expressions. The
results of these expressions can be printed as shown below:
printf ( "%d %d %d %d", 3, 3 + 2, c, a + b * c – d
) ;
Note that 3 and c also represent valid expressions.
Comments
Post a Comment