STRUCTURE OF C LANGUAGE PROGRAM

1 ) Comment line
2) Preprocessor directive
3 ) Global variable declaration
4) main function( )
{
Local variables;
Statements;
}
User defined function
}
}
Comment line
It indicates the purpose of the program. It is represented as
/*……………………………..*/

Comment line is used for increasing the readability of the program. It is useful in
explaining the program and generally used for documentation. It is enclosed within the
decimeters. Comment line can be single or multiple line but should not be nested. It can
be anywhere in the program except inside string constant & character constant.

Preprocessor Directive:

#include<stdio.h> tells the compiler to include information about the standard

input/output library. It is also used in symbolic constant such as #define PI 3.14(value).
The stdio.h (standard input output header file) contains definition
&declaration of system defined function such as printf( ), scanf( ), pow( ) etc.
Generally printf() function used to display and scanf() function used to read value

Global Declaration:
This is the section where variable are declared globally so that it can be access by all
the functions used in the program. And it is generally declared outside the function :

main()
It is the user defined function and every function has one main() function from where
actually program is started and it is encloses within the pair of curly braces.
The main( ) function can be anywhere in the program but in general practice it is placed
in the first position.
Syntax :
main()
{
……..
……..
……..
}
The main( ) function return value when it declared by data type as

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int main( )
{
return 0
}

The main function does not return any value when void (means null/empty) as
void main(void ) or void main()
{
printf (“C language”);
}

Output: C language

The program execution start with opening braces and end with closing brace.
And in between the two braces declaration part as well as executable part is mentioned.
And at the end of each line, the semi-colon is given which indicates statement
termination.

/*First c program with return statement*/

#include <stdio.h>
int main (void)
{
printf ("welcome to c Programming language.\n");
return 0;
}

Output: welcome to c programming language.

Steps for Compiling and executing the Programs

A compiler is a software program that analyzes a program developed in a particular
computer language and then translates it into a form that is suitable for execution on a
particular computer system. Figure below shows the steps that are involved in entering,
compiling, and executing a computer program developed in the C programming
language and the typical Unix commands that would be entered from the command line.

Step 1: The program that is to be compiled is first typed into a file on the computer
system. There are various conventions that are used for naming files, typically be any
name provided the last two characters are “.c” or file with extension .c. So, the file
name prog1.c might be a valid filename for a C program.

A text editor is usually used to enter the C program into a file. For example, vi is a
popular text editor used on Unix systems. The program that is entered into the file is
known as the source program because it represents the original form of the program
expressed in the C language.

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Step 2: After the source program has been entered into a file, then proceed to have it
compiled. The compilation process is initiated by typing a special command on the
system. When this command is entered, the name of the file that contains the source
program must also be specified. For example, under Unix, the command to initiate
program compilation is called cc. If we are using the popular GNU C compiler, the
command we use is gcc.
Typing the line
gcc prog1.c or cc prog1.c

In the first step of the compilation process, the compiler examines each program
statement contained in the source program and checks it to ensure that it conforms to
the syntax and semantics of the language. If any mistakes are discovered by the
compiler during this phase, they are reported to the user and the compilation process
ends right there. The errors then have to be corrected in the source program (with the
use of an editor), and the compilation process must be restarted. Typical errors reported
during this phase of compilation might be due to an expression that has unbalanced
parentheses (syntactic error), or due to the use of a variable that is not “defined”
(semantic error).

Step 3: When all the syntactic and semantic errors have been removed from the
program, the compiler then proceeds to take each statement of the program and
translate it into a “lower” form that is equivalent to assembly language program needed
to perform the identical task.

Step 4: After the program has been translated the next step in the compilation process
is to translate the assembly language statements into actual machine instructions. The
assembler takes each assembly language statement and converts it into a binary format
known as object code, which is then written into another file on the system. This file
has the same name as the source file under Unix, with the last letter an “o” (for object)
instead of a “c”.
Step 5: After the program has been translated into object code, it is ready to be linked.
This process is once again performed automatically whenever the cc or gcc command
is issued under Unix. The purpose of the linking phase is to get the program into a final
form for execution on the computer.
If the program uses other programs that were previously processed by the compiler,
then during this phase the programs are linked together.
Programs that are used from the system’s program library are also searched and linked
together with the object program during this phase.

The process of compiling and linking a program is often called building.

The final linked file, which is in an executable object code format, is stored in another
file on the system, ready to be run or executed. Under Unix, this file is called a.out by
default. Under Windows, the executable file usually has the same name as the source
file, with the c extension replaced by an exe extension.

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Step 6: To subsequently execute the program, the command a.out has the effect of
loading the program called a.out into the computer’s memory and initiating its
execution.

When the program is executed, each of the statements of the program is sequentially
executed in turn. If the program requests any data from the user, known as input, the
program temporarily suspends its execution so that the input can be entered. Or, the
program might simply wait for an event, such as a mouse being clicked, to occur.
Results that are displayed by the program, known as output, appear in a window,
sometimes called the console. If the program does not produce the desired results, it is
necessary to go back and reanalyze the program’s logic. This is known as the
debugging phase, during which an attempt is made to remove all the known problems
or bugs from the program. To do this, it will most likely be necessary to make changes
to original source program.

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/* Simple program to add two numbers…………………….*/

include <stdio.h>
int main (void)
{
int v1, v2, sum; //v1,v2,sum are variables and int is data type declared
v1 = 150;
v2 = 25;
sum = v1 + v2;
printf ("The sum of %i and %i is= %i\n", v1, v2, sum);
return 0;
}
Output:
The sum of 150 and 25 is=175

CHARACTER SET
A character denotes any alphabet, digit or special symbol used to represent information.
Valid alphabets, numbers and special symbols allowed in C are

The alphabets, numbers and special symbols when properly combined form constants,
variables and keywords.

Identifiers
Identifiers are user defined word used to name of entities like variables, arrays,
functions, structures etc. Rules for naming identifiers are:
1) name should only consists of alphabets (both upper and lower case), digits and
underscore (_) sign.
2) first characters should be alphabet or underscore
3) name should not be a keyword

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4) since C is a case sensitive, the upper case and lower case considered differently, for
example code, Code, CODE etc. are different identifiers.
5) identifiers are generally given in some meaningful name such as value, net-salary,
age, data etc. An identifier name may be long, some implementation recognizes only
first eight characters, most recognize 31 characters. ANSI standard compiler recognize
31 characters. Some invalid identifiers are 5cb, int, res#, avg no etc.

Keyword

There are certain words reserved for doing specific task, these words are known as
reserved word or keywords. These words are predefined and always written in lower
case or small letter. These keywords can’t be used as a variable name as it assigned
with fixed meaning. Some examples are int, short, signed,
unsigned, default, volatile, float, long, double, break, continue, typedef, static, do,
for, union, return, while, do, extern, register, enum, case, goto, struct, char, auto,
const etc.

Data types
Data types refer to an extensive system used for declaring variables or functions of
different types before its use. The type of a variable determines how much space it
occupies in storage and how the bit pattern stored is interpreted. The value of a variable
can be changed any time.

C has the following 4 types of data types

 basic built-in data types: int, float, double, char
 Enumeration data type: enum
 Derived data type: pointer, array, structure, union
 Void data type: void

A variable declared to be of type int can be used to contain integral values only—that is,
values that do not contain decimal places. A variable declared to be of type float can be
used for storing floating- point numbers (values containing decimal places). The double
type is the same as type float, only with roughly twice the precision. The char data type
can be used to store a single character, such as the letter a, the digit character 6, or a
semicolon similarly A variable declared char can only store character type value.

There are two types of type qualifier in c

Size qualifier: short, long
Sign qualifier: signed, unsigned

When the qualifier unsigned is used the number is always positive, and when signed is
used number may be positive or negative. If the sign qualifier is not mentioned, then by
default sign qualifier is assumed. The range of values for signed data types is less than
that of unsigned data type. Because in signed type, the left most bit is used to represent

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sign, while in unsigned type this bit is also used to represent the value. The size and
range of the different data types on a 16 bit machine is given below:

Basic data type Data type with type Size Range

qualifier (byte)

char char or signed char 1 -128 to 127

Unsigned char
1 0 to 255

int int or signed int 2 -32768 to 32767

unsigned int 2 0 to 65535
short int or signed shortint 1 -128 to 127
unsigned short int 1 0 to 255
long int or signed long int 4 -2147483648 to 2147483647
unsigned long int 4 0 to 4294967295
float Float 4 -3.4E-38 to 3.4E+38
double double 8 1.7E-308 to 1.7E+308
Long double 10 3.4E-4932 to 1.1E+4932

CONSTANTS
Constant is a any value that cannot be changed during program execution. In C, any
number, single character, or character string is known as a constant. A constant is an
entity that doesn’t change whereas a variable is an entity that may change.

For example, the number 50 represents a constant integer value. The character string
"Programming in C is fun.\n" is an example of a constant character string. C constants
can be divided into two major categories:
 Primary Constants
 Secondary Constants

These constants are further categorized as

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Numeric constant
Character constant
String constant

Numeric constant: Numeric constant consists of digits. It required minimum size of 2

bytes and max 4 bytes. It may be positive or negative but by default sign is always
positive. No comma or space is allowed within the numeric constant and it must have at
least 1 digit. The allowable range for integer constants is -32768 to
32767. Truly speaking the range of an Integer constant depends upon the compiler.
For a 16-bit compiler like Turbo C or Turbo C++ the range is –32768 to 32767.
For a 32-bit compiler the range would be even greater. Mean by a 16-bit or a 32- bit
compiler, what range of an Integer constant has to do with the type of compiler.
It is categorized a integer constant and real constant. An integer constants are whole
number which have no decimal point. Types of integer constants are:
Decimal constant: 0-------9(base 10)
Octal constant: 0-------7(base 8)
Hexa decimal constant: 0----9, A------F(base 16)

In decimal constant first digit should not be zero unlike octal constant first digit must be
zero(as 076, 0127) and in hexadecimal constant first two digit should be 0x/ 0X (such as
0x24, 0x87A). By default type of integer constant is integer but if the value of integer
constant is exceeds range then value represented by integer type is taken to be
unsigned integer or long integer. It can also be explicitly mention integer and unsigned
integer type by suffix l/L and u/U.
Real constant is also called floating point constant. To construct real constant we must
follow the rule of ,
-real constant must have at least one digit.
-It must have a decimal point.

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-It could be either positive or negative.
-Default sign is positive.
-No commas or blanks are allowed within a real constant. Ex.: +325.34
426.0
-32.76

To express small/large real constant exponent (scientific) form is used where number is
written in mantissa and exponent form separated by e/E. Exponent can be positive or
negative integer but mantissa can be real/integer type, for example
3.6*105=3.6e+5. By default type of floating point constant is double, it can also be
explicitly defined it by suffix of f/F.

Character constant
Character constant represented as a single character enclosed within a single quote.
These can be single digit, single special symbol or white spaces such as
‘9’,’c’,’$’, ‘ ’ etc. Every character constant has a unique integer like value in machine’s
character code as if machine using ASCII (American standard code for information
interchange). Some numeric value associated with each upper and lower case
alphabets and decimal integers are as:

A------------ Z ASCII value (65-90)

a-------------z ASCII value (97-122)
0-------------9 ASCII value (48-59)
; ASCII value (59)

String constant
Set of characters are called string and when sequence of characters are enclosed
within a double quote (it may be combination of all kind of symbols) is a string constant.
String constant has zero, one or more than one character and at the end of the string
null character (\0) is automatically placed by compiler. Some examples are “,sarathina” ,
“908”, “3”,” ”, “A” etc. In C although same characters are enclosed within single and
double quotes it represents different meaning such as “A” and ‘A’ are different because
first one is string attached with null character at the end but second one is character
constant with its corresponding ASCII value is 65.

Symbolic constant
Symbolic constant is a name that substitute for a sequence of characters and,
characters may be numeric, character or string constant. These constant are generally
defined at the beginning of the program as
#define name value , here name generally written in upper case for example

#define MAX 10
#define CH ‘b’
#define NAME “sony”

Variables

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Variable is a data name which is used to store some data value or symbolic names
for storing program computations and results. The value of the variable can be change
during the execution. The rule for naming the variables is same as the naming identifier.

Before used in the program it must be declared. Declaration of variables specify its
name, data types and range of the value that variables can store depends upon its data
types.

Syntax:
int a;
char c;
float f;
Variable initialization

When we assign any initial value to variable during the declaration, is called initialization
of variables. When variable is declared but contain undefined value then it is called
garbage value. The variable is initialized with the assignment operator such as
Data type variable name=constant;
Example: int a=20;
Or int a;
a=20;
statements

Expressions
An expression is a combination of variables, constants, operators and function call.
It can be arithmetic, logical and relational for example:-
int z= x+y // arithmatic expression
a>b //relational
a==b // logical
func(a, b) // function call

Expressions consisting entirely of constant values are called constant expressions.

So, the expression
121 + 17 - 110
is a constant expression because each of the terms of the expression is a constant
value. But if i were declared to be an integer variable, the expression
180 + 2 – j
would not represent a constant expression.

CONTROL STATEMENT
Generally C program statement is executed in a order in which they appear in the
program. But sometimes we use decision making condition for execution only a part of
program, that is called control statement. Control statement defined how the control is
transferred from one part to the other part of the program. There are several control
statement like if...else, switch, while, do....while, for loop, break, continue, goto etc.

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Loops in C
Loop:-it is a block of statement that performs set of instructions. In loops
Repeating particular portion of the program either a specified number of time or until a
particular no of condition is being satisfied.

There are three types of loops in c

1.While loop
2.do while loop
3.for loop
While loop

Syntax:-
while(condition)
{
Statement 1;
Statement 2;
}
Or while(test condition)
Statement;
The test condition may be any expression .when we want to do something a fixed no of
times but not known about the number of iteration, in a program then while loop is used.
Here first condition is checked if, it is true body of the loop is executed else, If condition
is false control will be come out of loop.
Example:-

/* wap to print 5 times welcome to C” */

#include<stdio.h>
void main()
{
int p=1;
While(p<=5)
{
printf(“Welcome to C\n”);
P=p+1;
}
}

Output: Welcome to C
Welcome to C
Welcome to C
Welcome to C
Welcome to C

So as long as condition remains true statements within the body of while loop will get
executed repeatedly.

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do while loop
This (do while loop) statement is also used for looping. The body of this loop may
contain single statement or block of statement. The syntax for writing this statement is:

Syntax:-
Do
{
Statement;
}
while(condition);

Example:-
#include<stdio.h>
void main()
{
int X=4;
do
{
Printf(“%d”,X);
X=X+1;

}whie(X<=10);
Printf(“ ”);
}
Output: 4 5 6 7 8 9 10

Here firstly statement inside body is executed then condition is checked. If the condition
is true again body of loop is executed and this process continue until the condition
becomes false. Unlike while loop semicolon is placed at the end of while.

There is minor difference between while and do while loop, while loop test the condition
before executing any of the statement of loop. Whereas do while loop test condition
after having executed the statement at least one within the loop.
If initial condition is false while loop would not executed it’s statement on other hand do
while loop executed it’s statement at least once even If condition fails for first time. It
means do while loop always executes at least once. Notes:
Do while loop used rarely when we want to execute a loop at least once.

for loop
In a program, for loop is generally used when number of iteration are known in advance.
The body of the loop can be single statement or multiple statements. Its syntax for
writing is:

Syntax:-
for(exp1;exp2;exp3)

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{
Statement;
}

Or
for(initialized counter; test counter; update counter)
{
Statement;
}

Here exp1 is an initialization expression, exp2 is test expression or condition and exp3
is an update expression. Expression 1 is executed only once when loop started and
used to initialize the loop variables. Condition expression generally uses relational and
logical operators. And updation part executed only when after body of the loop is
executed.

Example:-
void main()
{
int i;
for(i=1;i<10;i++)
{
Printf(“ %d ”, i);
}
}

Output:-1 2 3 4 5 6 7 8 9

Nesting of loop
When a loop written inside the body of another loop then, it is known as nesting of loop.
Any type of loop can be nested in any type such as while, do while, for. For example
nesting of for loop can be represented as :
void main()
{
int i,j;
for(i=0;i<2;i++)
for(j=0;j<5; j++)
printf(“%d %d”, i, j);
}

Output: i=0
j=0 1 2 3 4
i=1
j=0 1 2 3 4

Break statement(break)

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Sometimes it becomes necessary to come out of the loop even before loop condition
becomes false then break statement is used. Break statement is used inside loop and
switch statements. It cause immediate exit from that loop in which it appears and it is
generally written with condition. It is written with the keyword as break. When break
statement is encountered loop is terminated and control is transferred to the statement,
immediately after loop or situation where we want to jump out of the loop instantly
without waiting to get back to conditional state.
When break is encountered inside any loop, control automatically passes to the first
statement after the loop. This break statement is usually associated with if statement.

Example :
void main()
{
int j=0;
for(;j<6;j++)
if(j==4)
break;
}

Output:
0123
Continue statement (key word continue)

Continue statement is used for continuing next iteration of loop after skipping some
statement of loop. When it encountered control automatically passes through the
beginning of the loop. It is usually associated with the if statement. It is useful when we
want to continue the program without executing any part of the program.
The difference between break and continue is, when the break encountered loop is
terminated and it transfer to the next statement and when continue is encounter control
come back to the beginning position.
In while and do while loop after continue statement control transfer to the test condition
and then loop continue where as in, for loop after continue control transferred to the
updating expression and condition is tested.

Example:-
void main()
{
int n;
for(n=2; n<=9; n++)
{
if(n==4)
continue;
printf(“%d”, n);
}
}
Printf(“out of loop”);

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}

Output: 2 3 5 6 7 8 9 out of loop

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