INTRODUCTION TO C PROGRAMMING

LEARNING

ABHISHEK DWIVEDI
IT DEPARTMENT
AVIT, CHENNAI.
ABHISHEK DWIVEDI 3 July 2010

WHICH TOPIC U WANNA STUDY ???
INTRODUCTION
CONSTANTS & VARIABLES
OPERATORS & EXPRESSIONS
STRUCTURE OF A C PROGRAM
CONTROL STRUCTURES
ARRAYS AND STRINGS
FUNCTIONS
STORAGE CLASSES
STRUCTURES & UNIONS
POINTERS
DYNAMIC MEMORY ALLOCATION
FILE MANAGEMENT IN C
COMMAND LINE ARGUMENTS

Introducing C
 C is a programming language developed at AT & T
Bell Laboratories of USA in 1972, designed and
written by “Dennis Ritchie”.

 C is highly portable i.e., software written for one
computer can be run on another computer.

 An important feature of ‘C’ is its ability to extend
itself. A C program is basically a collection of
functions.


Historical Development of C

ALGOL  Algorithmic Language
CPL  Combined Programming Language
BCPL  Basic Combined Programming
Language

C Tokens

 A token is an atomic unit (smallest indivisible
units) in a program.

 The most basic elements in a C program
recognized by the compiler are a single character or
a group of characters called C tokens.

 The compiler cannot breakdown the token any
further. For example, the words main, ‘{‘ (brace), ‘(‘
(parenthesis) are all tokens of C program.


Types of tokens.
1. Keywords
Examples: float, int, double, while,
for.
2. Identifiers
Examples: main, amount
3. Constants
Examples: 12.4, 7894
4. Strings
Examples: “CSM”, “Thursday”
5. Special Symbols
Examples: [,], {, }, (, )
6. Operators
Examples: +, *, /


The C character set

The C character set includes the upper case letters A to
Z, the lower case a to z, the decimal digits 0 to 9 and
certain special characters.

Letters a, b, c, ……………z

Digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9

Special characters ~,.;:?‘“!()[]{}/<>=+
-$#@&*%^


Character/ Meaning Character/ Meaning
Symbol Symbol
~ tilde , Comma
. Period ; Semicolon
? Ques. mark : Colon
“ Double Quote ‘ Single Quote
( Left parenthesis ) Right
Parenthesis
[ Left Bracket ] Right Bracket
{ Left Brace } Right Brace
/ Slash Back Slash
< Less than > Greater than
= Equal to ! Exclamatory
Mark
- Minus + Plus
# Hash $ Dolor Sign
& Ampersand * Asterisk (or
star)
% Percent ^ Carat
_ Underscore | Vertical Bar

Identifiers
Identifiers are distinct names given to program
elements such as constants, variables, etc.
An Identifier is a sequence of letters, digits, and the
special character ‘_’ (underscore).
1. It must start with either a letter or
underscore. ‘_’
2. No commas or blanks are allowed within a
variable name.
3. The upper case and lower case letters are
treated as distinct, i.e., identifiers are
case-sensitive.
4. An identifier can be of any length.
5. No special symbol can be used in a variable
name.


Keywords
 Keywords are predefined tokens in C.
These are also called reserved words.
 Key words have special meaning to the
C compiler. These key words can be
used only for their intended action; they
cannot be used for any other purpose.
 C has 32 keywords.


The standard keywords are
auto break case char const
continue default do double else
enum extern float for goto
if int long register return
short signed sizeof static struct
switch typedef union unsigned void
volatile while


Data types
A data type defines a set of values and
the operations that can be performed on
them.
Every datatype item (constant, variable
etc.) in a C program has a datatype
associated with it.
C also has a special datatype called void,
which, indicates that any data type, i.e., no
data type, does not describe the data
items.


Size (No. of
Data types Description Range
Bytes)
Char Single character 1 0 to 255

Int An integer 2 -32768 to +32767

Float Floating point number 4 -2,147,483,648 to
+2,147,483,647
Double Floating point number 8 Approximately 15 digits
of Precision
Void No data type 0

signed char Character 1 -128 to 127

unsigned char Unsigned character 1 0 to 255

short signed int Short signed integer 2 -32768 to +32767

short unsigned int Short unsigned integer 3 0 to 65535

Long singed int Long signed integer 4 -2,147,483,648 to
+2,147,483,647


Constants and Variables
A constant is a literal, which remain
unchanged during the execution of a
program.
A constant is a fixed value that cannot be
altered during the execution of a program.
C constants can be classified into two
categories.
 Primary Constants
 Secondary Constants


Rules for constructing Integer constants
An integer constant must have at least one digit.
It should not contain either a decimal point or exponent.
If a constant is positive, it may or may not be preceded by a
plus sign. If it is a negative, it must be preceded by a
minus sign.
Commas, blanks and non-digit characters are not allowed in
integer constants.
The value of integer constant cannot exceed specified limits.
The valid range is
–32768 to +32767.


Real constants

Real values are often called floating-point constants. There
are two ways to represent a real constant decimal form and
exponential form.

In exponential form of representation, the real constant is
represented in two parts. The part appearing before ‘e’ is
called mantissa, whereas the part following ‘e’ is called
exponent.


Rules for constructing Real constants
The mantissa part and the exponential part should be
separated by a letter e.
The mantissa part may have a positive or negative sign.
Default sign of mantissa part is positive.
The exponent must have at least one digit, which must be
a positive or negative integer. Default sign is
positive.
Range of real constants expressed in exponential form is
- 3.4e38 to 3.4e38.


Rules for constructing Characters constants

A character constant is a single alphabet, a single digit or a
single special symbol enclosed within single inverted
commas. Both the inverted commas point to the left.
For example, ‘A’ is valid character constant whereas A
is not.
The maximum length of a character constant can be 1
character.

Note: Every character has its ASCII (American Standard
Code for Information Interchange) value. That means every
character is interchange with integer constant. For
example, ‘A’ value is 65 and ‘a’ value is 97.


String constants
A string constant is a sequence of characters enclosed in
double quotes. The characters may be letters,
numbers, blank space or special characters.
Note that “” is null string or empty string. And the
single string constant “A” is not equivalent to the
single character constant ‘A’.
Each string constant must end with a special character ‘0’.
This character is called null character and used to
terminate the string. The compiler automatically
places a null ‘0’ character at the end of every
string constant.


Escape sequence
Some non-printing characters and some other characters such as double
quote (“), single quote (‘), question mark (?) and backslash (), require an
escape sequence. A list of commonly used backslash character constants is
given below.
Escape Escape
Meaning ASCII value Meaning ASCII value
Sequence Sequence
a Bell 7 r Carriage 13
return

b Back Space 8 ” Double Quote 34

t Tab 9 ’ Single Quote 39

n New line 10 ? Question 63
Mark

v Vertical tab 11 Back Slash 92
f Form feed 12 0 Null 0

Variables
A variable can be considered as a name given to the location
in memory.
The term variable is used to denote any value that is referred
to a name instead of explicit value.
A variable is able to hold different values during execution of
a program, where as a constant is restricted to just one
value.
For example, in the equation 2x + 3y = 10; since x and y can
change, they are variables, whereas 2,3 and 10 cannot
change, hence they are constants. The total equation
is known as expression.


Rules for constructing variable names
The name of a variable is composed of one to several characters,
the first of which must be a letter .
No special characters other than letters, digits, and underscore can
be used in variable name. Some compilers permit underscore as the
first character.
Commas or Blanks are not allowed with in a variable name.
Upper case and Lower case letters are significant. That is the
variable “income” is not same as “INCOME”.
The variable name should not be a C key word. Also it should not
have the same name as a function that is written either by user or
already exist in the C library.


C Instructions
There are basically four types of instructions in C:
Type Declaration Instruction
Input/Output Instruction
Arithmetic Instruction
Control Instruction


The purpose of each instructions
 Type Declaration to declare the type of
instruction variables used in a C program

 Input/Output instruction To perform the function of
supplying input data to a
program and obtaining the
output results from it.

 Arithmetic instruction to perform arithmetic
operations between
constants and variables.

 Control instruction to control the sequence of
execution of various
statements in a C program.


Operators & Expressions
An operator is a symbol that tells the computer to
perform certain mathematical or logical
manipulations.
Operators are used in program to manipulate data
and variables. The data items that operators
act upon are called operands.
Some operators require two operands, while
others act upon only one operand. The
operators are classified into unary, binary and
ternary depending on whether they operate on
one, two or three operands respectively.

Types of operators
C has four classes of operators
1. Arithmetic Operators
2. Relational Operators
3. Logical Operators
4. Bit-wise Operators
In addition, C has some special operators, which
are unique to C, they are
1. Increment & Decrement Operators
2. Conditional Operators
3. Assignment Operators, etc.


Arithmetic Operators
There are five arithmetic operators in C.
The following table lists the arithmetic operators allowed in C:

Operator Meaning

+ Addition

_ Subtraction; also for unary minus

* Multiplication

/ Division

% Modulo division (remainder after integer
division)

Relational Operators
Relational Operators are symbols that are used to test the relationship
between two variables or between a variable and a constant. We often
compare two quantities, and depending on their relation takes certain
decisions. These comparisons can be done with the help of relational
operators.
C has six relational operators as shown below.

Operator Meaning
> Greater than
>= Greater than or Equal to
< Less than
<= Less than or Equal to
== Equal to
!= Not equal to

Logical Operators
Logical Operators are symbols that are used to combine or negate
expressions containing relational operators.
C has three logical operators as defined below.

Operator Meaning

&& Logical AND

|| Logical OR

! Logical NOT


Bitwise operators
The lowest logical element in the memory is bit. C allows the
programmer to interact directly with the hardware of a particular
system through bitwise operators and expression.
These operators work only with int and char datatypes and cannot
be used with float and double type.
The following table shows the bitwise operators that are available in
C.
Operator Meaning
- One’s Complement
| Bitwise OR
& Bitwise AND
^ Bitwise Exclusive OR (XOR)
>> Right Shift
<< Left Shift

Increment & Decrement operators
C has two very useful operators for adding and subtracting a variable.
These are the increment and decrement operators, ++ and -- . These two
operators are unary operators.
The increment operator ++ adds 1 to its operand, and the decrement
operator -- subtracts 1 from its operand. Therefore, the following are
equivalent operations.
++i is equivalent to i = i + 1;
--i is equivalent to i = i – 1;
These operators are very useful in loops.


Assignment operators
In addition to usual assignment operator =, C has a set of shorthand operators, that
simplifies the coding of a certain type of assignment statement.
It is of the form
var op = exp
where var is a variable, op is a C binary arithmetic operator and exp is an
expression.

Statement Equivalent Statement
a+=b a=a+b
a-=b a=a-b
a * =b a=a*b
a*=b+c a = a * ( b+ c)
a%=b a=a%b
a*=a a=a*a


Conditional Operator
C provides a peculiar operator ? : which is useful in reducing the code. It is
ternary operator requiring three operands.
The general format is
exp1 ? exp2 : exp3;
where exp1, exp2 and exp3 are expressions.
In the above conditional expression, exp1 is evaluated first. If the value of
exp1 is non zero (true), then the value returned will be exp2. if the value
of exp1 is zero (false), then the value returned will be exp3.


Hierarchy (precedence) of operators
The priority or precedence in which the operations of an arithmetic
statement are performed is called the hierarchy of operators.
The operators of at the higher level of precedence are evaluated first. The
operators of the same precedence are evaluated either from left to right
or from right to left, depending on the level. This is known as the
Associativity property of an operator.
PRECEDENCE OF OPERATORS (Arithmetic operators only)

Operator Description Associativity Rank
* Multiplication Left to right 3
/ Division “ 3
% Modulo “ 3
+ Addition “ 4
- Subtraction “ 4


Structure of a ‘C’ program
C programs consist of one or more functions. Each function performs a
specific task. A function is a group or sequence of C statements that are
executed together.
The following is a simple C program that prints a message on the screen.

/* A simple program for printing a message */
# include <stdio.h>
# include <conio.h>
void main( )
{
clrscr( );
printf(“Welcome to C”);
getch( );
}


Description
The first line
/* A simple program for printing a message */
is a comment line. Comments in the c program are optional and may
appear anywhere in a C program. Comments are enclosed between /*
and */.
The second line
# include <stdio.h>
tells the compiler to read the file stdio.h and include its contents in this
file. stdio.h, one of header files, contain the information about input and
output functions. stdio.h means Standard Input Output Header file. This
file contains the information about printf() function.


Description (contd…)
The third line
# include <conio.h>
tells the compiler to read the file conio.h and include its contents in this file.
conio.h means Consoled Input Output Header file. This file contains the
information about clrscr() and getch() functions.
The fourth line
void main( )
is the stat of the main program. The word main is followed by a pair of ordinary
parenthesis ( ), which indicates that main is also a function.
The fifth line
{
the left brace represents the beginning of the program.
The sixth line
clrscr( );
tells the compiler to clear the screen and kept the cursor at left side corner.

Description (contd…)
The seventh line
printf( “Welcome to C”);
this function causes its arguments to be printed on the screen on the
computer.
The eight line
getch( );
is reads the single character directly from the keyboard without printing
on the screen.
The ninth line
}
the right brace represents the ending of the program.


Rules to write a C program
1. All C statements must end with semicolon.
2. C is case-sensitive. That is, upper case and lower case characters
are different. Generally the statements are typed in lower case.
3. A C statement can be written in one line or it can split into multiple
lines.
4. Braces must always match upon pairs, i.e., every opening brace {
must have a matching closing brace }.
5. Every C program starts with void main( ) function.
6. Comments cannot be nested. For example,
/* Welcome to ‘C’ ,/* programming*/ */
A comment can be split into more than one line.


Execution of C Program
Steps to be followed in writing and running a C program.
Creation of Source Program
Create a C program file in various C compilers are available under MS-DOS,
Turbo C Editor etc.
Compilation of the Program
Turbo C compiler is user friendly and provides integrated program
development environment. Thus, selecting key combination can do
compilation. That means press Alt + F9 for compilation.
Program Execution
In Turbo C environment, the RUN option will do the compilation and
execution of a program. Press Ctrl + F9 for execution the program.


printf( ) Function: Writing Output Data
The printf( ) function is used to write information to standard output
(normally monitor screen). The structure of this function is
printf(format string, list of arguments);
The format string contains the following:
1. Characters that are simply printed on the screen.
2. Specifications that begin with a % sign and define the output format
for display of each item.
3. Escape sequence characters that begin with a sign such as n, t, b
etc.
Character Argument Resulting Output

c Character A single character
d Integer Signed decimal integer
s String Prints character strings
f Floating Single floating point number
point

scanf( ) Function: getting user input
The real power of a technical C program is its ability to interact with the
program user. This means that the program gets input values for variables
from users.
The scanf( ) function is a built-in C function that allows a program to get
user input from the keyboard. The structure of this function is
scanf(format string &list of arguments);
Examples
scanf(“%d”, &a );
scanf(“%d %c %f ”,&a, &b, &c );


CONTROL STRUCTURES
The control flow statements of a language determine the order in which
the statements are executed.
We also need to be able to specify that a statement, or a group of
statements, is to be carried out conditionally, only if some condition is
true.
Also we need to be able to carry out a statement or a group of statements
repeatedly based on certain conditions.
These kinds of situations are described in C using Conditional Control and
Loop Control structures.


Conditional & loop structures
A conditional structure can be implemented in C using
The if statement
The if-else statement
The nested if-else statement
The switch statement.
whereas loop control structures can be implemented in C using
while loop
do-while loop
for statement


The if statement
The if statement is used to control the flow of execution of statements.
The general form of if statement is
if (condition)
statement;
Suppose if it is required to include more than one statement, then a
compound statement is used, in place of single statement. The form of
compound statement is
if (condition)
{
statement1;
statement2;
}
If the condition is true, then the statement/statements will be executed.
If the condition is false, then the statement/statements will not be
executed.

The if statement : Program
Program
/* Inputting year is Leap or not */
#include<stdio.h>
#include<conio.h>
void main() Output 1
{ Enter year:1990
int year; Not leap year
clrscr(); Output 2
printf(“Enter year:”); Enter year:1996
Leap year
scanf(“%d”,&year);
if(year%4==0)
printf(“Leap year”);
if(year%4!=0)
printf(“Not leap year”);
getch();
}

The if-else Statement
The general form of if-else statement is…
if (condition)
statement1;
else
statement2;
If the condition is true, then statement1 is executed. Otherwise if the condition is
false, then the statement2 is executed. Here statements statement1 and statement2
are either simple statements or compound statements.
That is…
if (condtion)
{
statements /* if block */
}
else
{
statements /* else */
}


The if-else Statement : Program
Program
/* Single digit or not */
#include<stdio.h>
#include<conio.h>
void main()
{ Output 1
Enter a number:5
int n;
Single digit
clrscr();
Output 2
printf(“Enter a number:”); Enter a number:12
scanf(“%d”,&n); Not single digit
if(n<=9)
printf(“Single digit”);
else
printf(“Not single digit”);
getch();
}


Nested if-else Statements
When a series of conditions are involved, we can use more than one if-else
statement in nested form.
This form is also known as if-else if-else statements. The general form of
if-else if-else statement is
if (condition)
statements;
else if (condition)
statements;
else
statements;
Note that a program contains number of else if statements and must be
ended with else statement.


Nested if-else Statements : Program
Program
/* To check whether +ve, -ve or zero */
#include<stdio.h>
#include<conio.h>
void main()
{ Output 1
int n; Enter a number: -2
clrscr(); -ve
printf(“Enter a number:”); Output 2
scanf(“%d”,&n); Enter a number:0
If(n>0) zero
printf(“+ve”); Output 3
else if(n<0) Enter a number:5
printf(“-ve”); +ve
else
printf(“zero”);
getch();
}


The Switch Statement
The Switch statement is an extension of the if-else if-else statement. The
switch makes one selection when there are several choices to be made. The
direction of the branch taken by the switch statement is based on the value of
any int (or int compatible) variable or expression.
The general form of Switch statement is shown below.
switch (variable)
{
case constant1:statement 1;
“ “ “
“ “ “
“ “ “
case constant n:statement n;
default :statement;
}


The Switch Statement : Program
Program
/* Letter -> color name */
#include<stdio.h>
#include<conio.h>
void main()
Output 1
{
char x;
Enter a char:w
clrscr(); w->white
printf(“Enter a char:”); Output 2
scanf(“%c”,&x); Enter a char:b
switch(x) b->black
{ Output 3
case ‘w’:printf(“w->white”); Enter a char:c
break;
No color
case ‘b’:printf(“b->black”);
break;
default:printf(“No color”);
}
getch();
}


The exit( ) Function
The exit( ) is a function in the standard library of C. This function causes
immediate termination of the program and execution control return to
the operating system.
In general, the termination to exit( ) function is 0 to indicate that
termination is normal. Other arguments may be used to indicate some
sort of an error.


LOOPS
A portion of program that is executed repeatedly is called a loop.
The C programming language contains three different program
statements for program looping. They are
 For loop
 While loop
 Do-While loop


The For Loop
The for loop is used to repeat the execution statement for some fixed
number of times.
The general form of for loop is
for(initialization;condition;increment/decrement)
statement;
where the statement is single or compound statement.
initialization is the initialization expression, usually an assignment to the
loop-control variable. This is performed once before the loop actually
begins execution.
condition is the test expression, which evaluated before each iteration of
the loop, which determines when the loop will exist.
increment is the modifier expression, which changes the value of loop
control variable. This expression is executed at the end of each loop.


The For Loop : Program
Program
/* Print 1 to 10 numbers */
#include<stdio.h>
Output
#include<conio.h> 1
void main() 2
3
{
4
int i; 5
clrscr(); 6
7
for(i=1;i<=10;i++)
8
printf(“n%d”,i); 9
getch(); 10
}


The While Loop
The while loop is best suited to repeat a statement or a set of statements
as long as some condition is satisfied.
The general form of while loop is
initial expression;
while(conditional-expression)
{
statement;
increment/decrement;
}
where the statement (body of the loop) may be a single statement
or a compound statements. The expression (test condition) must results
zero or non-zero.


The While Loop : Program
Program
/* Print a message 3 times */
#include<stdio.h>
#include<conio.h>
void main() Output
{ Jiffy Solutions
int i=1; Jiffy Solutions
clrscr(); Jiffy Solutions
while(i<=3)
{
printf(“nJiffy Solutions”);
i++;
}
getch();
}


The do-while loop
The structure of do-while loop is similar to while loop. The difference is
that in case of do-while loop the expression is evaluated after the body of
loop is executed. In case of while loop the expression is evaluated before
executing body of loop.
The general form of do-while statement is
do
{
statement;
}while(expression);
where statement is a single statement or compound statement. In
contrast to while loop statement (body of loop), do-while loop is executed
one or more times.


ARRAYS
An Array is a collection of same data type. The elements of an array are
referred by a common name and are differentiate from one another by their
position with in an array. The elements of an array can be of any data type but
all elements in an array must be of the same type.
The general form of declaring a array is
type array_name[size];
where type is a valid datatype, array_name is the name of the array and size
is the number of elements that array_name contains.
Example:
int A[100];

int  data type of elements that an array
A  name of array
100  size of an array


ARRAYS (Contd…)
The individual elements of an array can be referenced by means of its subscript (or
index)
Suppose A is an array of 20 elements, we can reference each element as
A[0]  1st element
A[1]  2nd element
A[2]  3rd element
:
:
A[19]  20th element
Note: Subscript enclosed within parenthesis.
In C subscript starts from 0. That is, if we declare an array of size n, then we can refer the
elements from 0 to (n-1)th element.
Arrays are of 3 types. They are
Single Dimensional Array
Double Dimensional Array
Multi Dimensional Array

Single Dimensional Array
The general form of Single Dimensional array is:
datatype variable[size];
Example:
int A[20];
Initialization of arrays during declaration
Similar to other datatypes, the array also can be initialized at the time of
declaration.
int num[5] ={3,2,1,5,4};
char name[15] = {‘c’,’o’,’m’,’p’,’u’,’t’,’e’,’r’,’s’};
float rate[] = {20.5,15.75,12.34};


Single Dimensional Array : Program
Program illustrating Single Dimensional Array
#include<stdio.h>
#include<conio.h>
void main()
{
int a[5],i;
clrscr();
printf(“Enter 5 elements into an array:”);
for(i=0;i<=4;i++)
scanf(“%d”,&a[i]);
printf(“The 5 elements are:”);
for(i=0;i<=4;i++)
printf(“n%d”,a[i]);
getch();
}


Two-Dimensional Array
The general form of Two-Dimensional Arrays is
type array_name[row_size][column_size];
Example:
int a[2][2];
Initializing Two-Dimensional Arrays
Like the one-dimensional arrays, following their declaration with a list of
initial values enclosed in braces may initialize two-dimensional arrays.
For example,
int a[2][2] ={1,2,5,4};
initializes the elements of the first row to zero and the second row to one.
The initialization is done row by row. The above statement can be
equivalently written as
int a[2][2]={{1,2},{5,4}};


Two-Dimensional Array : Programs
Program Illustrating Two Dimensional arrray printf(“The 4 elements are:n”);
#include<stdio.h> for(i=0;i<=1;i++)
#include<conio.h> {
void main()
for(j=0;j<=1;j++)
{
{
printf(“%d ”,a[i][j]);
int a[2][2],i,j; }
clrscr(); printf(“n”);
printf(“Enter 4 elements into array:”); }
for(i=0;i<=1;i++) getch();
}
{
for(j=0;j<=1;j++)
{
scanf(“%d”,&a[i][j]);
}
}


Handling of Character Strings
A string is an array of characters. There is no string built-in data type in
C. But we can declare string as an array of characters. To recognize a
character array, it should end with a null character (‘0’).
For example, the string SCIENCE would be stored as
‘S’ ‘C’ ‘I’ ‘E’ ‘N’ ‘C’ ‘E’ ‘0’
The length of a string is the number of characters it contains
excluding null character. Hence, the number of locations needed to store
a string is one more than length of string.
In this example, the length of the string is 7.


Declaring and initializing string variables
The general form of declaration of string variable is
char string-name[size];
where, string-name is the name of a string and size is the maximum
number of characters the string-name can contain.
Example:
char name[30];
String variables can be initialized at the time of declaration.
Example:
char name[30] = “Millennium”;

A string can also be initialized at the time of declaration in the following ways.
char name[30] = “Millennium”;
or
char name[30] ={‘M’,’i’,’l’,’l’,’n’,’n’,’i’,’u’,’m’,’0’};


Reading and writing strings
The scanf(), printf() function is used with %s with format specification to
read and print a string.
Example:
char str[30];
scanf(“%s”,str);
printf(“%s”,str);
In the case of reading strings, the ampersand (&) is not required before
the string variable name. As mentioned earlier, one of the limitations of
the scanf() function is that it is not capable of holding multiword strings,
even though it can read them.


Reading and writing strings : Program
Program
#include<stdio.h>
#include<conio.h>
void main()
{
char line[80];
clrscr();
printf(“Enter a linen”);
scanf(“%s”,line);
printf(“The entered line is:%s”,line);
getch();
}


String handling Functions: string.h
Every C compiler provides a large set of string handling library functions,
which are contained in the header file string.h
The following table shows some of the functions available in string.h
header file.
Function Meaning
strcat() String concatenate. Append one string to another. First
character of string2 overwrites null character of string1.
strlen() Returns the length of the string not counting the null
character.
strlwr() Converts a string to lower case.
strupr() Converts a string to upper case.
strcpy() Copies a string into another.
strcmp() Compares two strings
strrev() Reverses a string

strcat() function
The strcat() function concatenates the source string at the end of the
target string. For example “Computing” and “Techniques” on
concatenation would result in string “ComputingTechniques”.
The general form is
strcat(string1, string2);
strong2 appends to string1 and the first character to string2 overwrites
null character of first string1. This function returns the first argument
i.e., string1. The string2 remains unchanged.


strcat() function : Program
Program
#include<stdio.h>
#include<conio.h>
#include<string.h>
void main() Output
Enter String1:Jiffy
{
Enter String2:Solutions
char s1[30],s2[15]; The entire string is:
clrscr(); Jiffy Solutions
printf(“Enter String1:”);
gets(s1);
printf(“Enter String2:”);
gets(s2);
printf(“The entire string is:%s”,strcat(s1,s2));
getch();
}


strcmp() function
strcmp() function compares two strings to find out whether they
are same or different.
The two strings are compared character by character until there
is a mismatch or end of one of the strings is reached, whichever
occurs first.
The general form is:
strcmp(string1, string2);
If the two strings are same, strcmp() returns a value 0.
If they are not same, it returns the numeric difference between
the ASCII values of the first non-matching characters.
That is, it returns less than 0 if string1 is less than string2, and
greater than 0 if string1 is greater than string2.


strcmp() function : Program
Program
#include<stdio.h>
#include<conio.h>
#include<string.h>
void main()
{
char s1[25],s2[25]; Output
int c;
Enter String1:abc
clrscr();
Enter String2:ABC
printf(“Enter string1:”);
String1 > String2
gets(s1);
printf(“Enter string2:”);
gets(s2);
c=strcmp(s1,s2);
if(c>0)
printf(“String1 > String2”);
else if(c<0)
printf(“String2 > String1”);
else
printf(“Both are equal”);
getch();
}

strcpy() function
strcpy(String1, String2);
The strcpy() function is used to copy the character string from String2 to
String1.
This function returns the result string String1 the String2 remains
unchanged. String2 may be a character array or a string constant.


strcpy() function : Program
Program
#include<stdio.h>
#include<conio.h>
#include<string.h>
Output
void main()
Enter String1:Millennium
{ The String2 is:Millennium
char s1[15],s2[15];
clrscr();
printf(“Enter String1:”)
gets(s1);
printf(“The String2 is:%s”,strcpy(s2,s1));
getch();
}

strlen() function
This function strlen() counts the number of characters
present in a string. The counting ends at the first null
character.
The general form is
strlen (String1);
The strlen() function returns the length of the argument
String1 excluding the null character. The argument may be
a string constant.


strlen() function : Program
Program
#include<stdio.h>
#include<conio.h>
#include<string.h>
void main() Output
{ Enter a string:
char str[30]; Millennium Software
clrscr(); Solutions
printf(“Enter a string:”); The length of a string
gets(str); is:29
printf(“The length of a string
is:%d”,strlen(str));
getch();
}


strupr(), strlwr(), strrev() functions
The strupr() function is used to convert the string
into upper case.
The strlwr() function is used to convert the string
into lower case.
The strrev() function prints the entire string in
reverse order.


strupr(), strlwr(), strrev() functions : Program
Program
#include<stdio.h>
#include<conio.h>
#include<string.h>
Output
void main() Enter a string:millennium
{ The upper case string
char str[15]; is:MILLENNIUM
clrscr(); The lower case string
printf(“Enter a string:”); is:millennium
gets(str); The reverse string
is:muinnellim
printf(“The upper case string is:%s”,strupr(str));
printf(“nThe lower case string is:%s”,strlwr(str));
printf(“nThe reverse string is:%s”,strrev(str));
getch();
}


Functions
Functions are building blocks of C. Function performs the same set of instructions
on different sets of data or at different portions of a program.
C functions can be classified into two categories, namely
library functions and user-defined functions.
main is an example of user-defined functions. printf and scanf belong to the
category of library functions.
The main distinction between these two categories is that library functions are not
required to be written by us whereas a user-defined function has to be developed
by the user at the time of writing a program.
The general form of C function is
Return-type Function-name (parameter list)
parameter declaration;
{
Body of function;
}

Advantages of user-defined functions:
It facilitates top-down modular programming. In this
programming style, the high level logic of the overall problem is
solved first while the details of each lower-level function are
addressed later.
The length of a source program can be reduced by using functions at
appropriate places. This factor is particularly critical with
microcomputers where memory space is limited.
Many other programs may use a function. This means that a C
programmer can build on what others have already done, instead
of starting over, from scratch.
As mentioned earlier, it is easy to locate and isolate a faulty function for
further investigations.


Category of functions
A function, depending on whether arguments are
present or not and whether a value is returned or
not, may belong to one of the following categories:
Category 1: Functions with no arguments and no
return values.
Category 2: Functions with arguments and no
return values.
Category 3: Functions with arguments and return
values.


Functions with no arguments and no return values
Program
#include<stdio.h>
#include<conio.h>
Function Declaration
void printline(); void main()
{
clrscr();
printline(); /*Function Declaration*/
printf(“This illustrates the use of C functionsn”);
printline();
getch();
}
void printline() /*Return Type & Function Name*/
{
int i;
for(i=1;i<=40;i++)
printf(“-”);
printf(“n”);
}


Arguments but no Return values
Program
#include<stdio.h>
#include<conio.h>
void swap(int,int);
void main()
{
int a,b;
clrscr();
printf(“Enter 2 numbers:”);
scanf(“%d%d”,&a,&b);
swap(a,b); /* Function Call*/
getch();
}
void swap(int x, int y) /*Called Function*/
{
int z;
z = x;
x = y;
y = z;
printf(“nAfter swapping:%d,%d”,x,y);
}

Arguments with Return Values
Program
#include<stdio.h>
#include<conio.h>
int big(int,int);
void main()
{
int a,b,max;
clrscr();
scanf(“%d%d”,&a,&b);
max = big(a,b);
printf(“nThe biggest number is:%d”,max);
getch();
}
int big(int x, int y)
{
if(x>y)
return x;
else
return y;
}

Recursion
Recursion is a technique to be used to call itself. long int fact(int n)
In C, it is possible for the functions to call {
themselves. long int f;
A function is called recursive if a statement with in if(n==1)
the body of a function calls the same function itself. return 1;
Program else
#include<stdio.h> f = n*fact(n-1);
return f;
#include<conio.h> }
long int fact(int);
void main()
{
int n;
long int res;
clrscr();
printf(“Enter a positive number:”);
scanf(“%d”,&n);
res = fact(n);
printf(“The factorial is:%ld”,res);
getch();
}


Important points about functions
All C programs must contain atleast one function. [The main() function
serves this rule]
A function can return only one value. Thus we should not specify two
values to return.
The return type in function declaration is optional. If no return type is
specified it is assumed to be an integer which is default.
When a function is not returning any value, void type can be used as
return type.
Parameter list is optional.
‘C’ provides a statement return
return expression
Return statement is used in function definition to communicate the
return value to the calling function.


Important points (Contd…)
Return statement indicates exit from the function and return to the point
from where the function was invoked.
There may be any number of return statements in function definition,
but only one return statement will activate in a function call.
The variable declarations within the function (between braces { }) are
local to the function and are not available outside the function.
If there is no return statement, the program will return to the calling
point after it reaches the end of the function body (}).
A function call can be used wherever a variable of same type is used
(except the left side of an assignment statement).
There should be one to one correspondence between the actual and
formal parameters in type, order and number.
C allows recursion. That is a function can call itself.
A C function cannot be defined in another function.

Functions with Arrays
Like the values of simple variables, it is also possible to pass the values of
an array to a function.
To pass an array to a called function, it is sufficient to list the name of the
array, without any subscripts, and the size of the array as arguments. For
example, the call
largest(a,n);
will pass all the elements contained in the array a of size n. The called
function expecting this call must be appropriately defined. The largest
function header might look like:
int largest(array,size);
int array[];
int size;


Variables
There are three basic places in a C program
where variables will be declared:
 inside the function,
in the definition of function parameters,
or outside of all functions.
These variables are called local variables, formal
parameters, and global variables respectively.


Local Variables
The body of any function comprises of two parts:
declaration of variables and a set of executable
statements.
Variables declared inside a function are called local
variables. This name derives from the fact that a
variable inside a function can be used only inside
that function.
An attempt on our part or access the local variable
of one function in another, will draw an error from
the compiler: Identifier undefined.


Global Variables Block Variables
C program consists of three sections Yet another place to declare
namely: the preprocessor directives, variables is inside any block:
the global variable section and finally these variables are called block
the functions. variables and these can be used
The variables that are declared in the only inside that block.
global variable section are called global
variables.
While a local variable can be used only
inside a function in which it is declared,
a global variable can be used anywhere
in the program.


Storage classes
The storage class of a variable dictates how, when
and where storage will be allocated for the
variable. The different storage classes available
are:
1. Auto
2. Register
3. Extern
4. Static


Auto
Automatic variables are declared inside a function in which they are to be
utilized. They are created when the function is called and destroyed
automatically when the function is exited, hence the name automatic.
Automatic variables are therefore private (or local) to the function in which
they are declared. Because of this property, automatic variables are also
refereed to as local or internal variables.
A variable declared inside a function without storage class specification is,
by default, an automatic variable.
One important feature of automatic variables is that their value cannot be
changed accidentally by what happens in some other function in the
program. This assures that we may declare and use the same variable name
in different functions in the same program without causing any confusion to
the compiler.


Auto : Program
#include<stdio.h> void function1()
#include<conio.h> {
int m = 10;
void function1();
printf(“%dn”,m);
void function2(); }
void main() void function2()
{
{
int m = 100;
int m = 1000; function1();
clrscr(); printf(“%dn”,m);
}
function2();
printf(“%dn”,m);
getch();
}


Register
It is possible for use to attribute the register storage class to certain
variables.
We can tell the compiler that a variable should be kept in one of the
machine’s registers, instead of keeping in the memory (where normal
variables are stored).
Since a register access is much faster than a memory access, keeping the
frequently accessed variables in the register will lead to faster execution of
programs. This is done as follows:
register int count;
The important point while using register variables is, registers of CPU do
not have addresses. Thus, we should not refer the address of a register
variable.
For example,
The following statement will result an error :
register int i;
scanf(“%d”,&i);

Register : Program
#include<stdio.h>
#include<conio.h>
void main()
{
register int count;
int sum;
clrscr();
for(count=0;count<10;count++)
sum = sum + count;
printf(“The sum is:%d”,sum);
getch();
}


Extern
This is the default storage class for all global variables.
Extern storage class variables get initialized (to zero in the
case of integers) automatically, retain their value
throughout the execution of the program and can be
shared by different modules of the same program.
However, assuming “int intvar;”is present in a.c., to be also
to have proper binding with the same variable, b.c
(another file) should have “extern int intvar”.


Extern : Program
a.c file b.c file
#include<stdio.h> float f = 84.237;
extern int intvar;
#include<conio.h>
funct(char c, int intvar)
int intvar; {
extern float f; char c1,c2;
:
void main()
:
{ }
char ch;
funct(ch,intvar);
printf(“%f”,f);
getch();
}


Static
The static storage class has a number of implications depending upon its
usage.
The default storage class for all local variables is auto. This can be
changed to static by prefixing the declaration with the keyword static as
in
static int intvar.
A local variable with static storage class is still a local variable as far as its
scope is concerned, it is still available only inside the function in which it is
declared. But certain other properties of the variable change; It gets
initialized to zero automatically, is initialized only once, (during program
startup) and it retains its value throughout the execution of the program.
When applied to a global variable, the global variable becomes
inaccessible outside the file in which it is declared.
This storage class can be applied to functions too. Similar to case 2, the
functions become inaccessible outside the file.


Static : Program
Program:
#include<stdio.h>
#include<conio.h>
void main() Output
{ The value of n is 1
function1( ); The value of n is 2
function1( ); The value of n is 3
function1( );
getch();
}
void function1()
{
static int n;
n++;
printf(“nThe value of n is:%d”,n);
}


Storage Type Default Where Scope Life Storage
class initial declared
value

Auto Local garbage within the function within the function until function is no Memory
or value where it is declared longer active
none

Register Local garbage within the function within the function until function is no CPU
value where it is declared longer active registers

Static Local Zero within the function within the function until program ends, Memory
where it is declared value of the variable
persists between
different function
calls

Extern Global Zero A heat of all All files including other While any of these Memory
functions within a files where declared files are active.
file extern That is, as long as
the program’s
execution doesn’t
come to an end


Structures
A structure is a convenient tool for Example:
handling a group of logically related struct book
data items. These fields are called
structure elements or members. {
char name[20];
Declaring A Structure
char author[15];
The general form of a structure int pages;
declaration statement is given below:
float price;
struct <structure name> }
{
structure element1;
structure element2;
structure element3;
……….
……….
}


Important points while declaring a structure type:

The closing brace in the structure type declaration must be
followed by a semicolon.
It is important to understand that a structure type
declaration does not tell the compiler to reserve any
space in memory. All a structure declaration does
is, it defines the ‘form’ of the structure.
Usually structure type declaration appears at the top of the
source code file, before any variables or functions are
defined.


ACCESSING STRUCTURES ELEMENTS
We can assign values to the members of a structure in a number of ways.
As mentioned earlier, the members themselves are not variables. They
should be linked to the structure variables in order to make them
meaningful members. The link between a member and a variable is
established using the member operator ‘.’ Which is also known as ‘dot
operator’ or ‘period operator’.
The general form is
Structure-Variable. Structure-Member;
For example,
book1.price;
We can also use scanf to give the values through the keyboard.
scanf(“%s”,book1.name);
scanf(“%d”,&book1.pages);
are valid input statements.

ARRAYS OF STRUCTURES
we may declare an array of structures, each element of the array
representing a structure variable. For example,
struct class student[100];
defines an array called student, that consists of 100 elements. Each
element is defined to be of the type struct class. Consider the following
declaration:
struct marks
{
int sub1;
int sub2;
int sub3;
}s[5];


ARRAYS WITHIN STRUCTURES
C permits the use of arrays as structure members.
We have already used arrays of characters inside a
structure. Similarly, we can use single-or multi-
dimensional arrays of type int or float.
For example, the following structure declaration is
valid:
struct marks
{
int number;
float sub[3];
}s[2];


STRUCTURES WITHIN STRUCTURES
Structures within a structure means The salary structure contains a
nesting of structures. Nesting of
structures is permitted in C. Let us member named allowance which itself
consider the following structure is a structure with three members.
definition:
struct salary The members contained in the inner
{ structure namely dearness,
char name[20]; house_rent, and city can be referred to
char dept[10]; as
struct
employee.allowance.dearness
{ employee.allowance.house_rent
int dearness; employee.allowance.city
int house_rent;
int city;
}allowance;
}employee;


UNIONS
Unions are a concept borrowed from structures and therefore follow the
same syntax as structures. However, there is major distinction between
them in terms of storage. In structures, each member has its own storage
location, whereas all the members of a union use the same location. This
implies that, although a union may contain many members of different
types, it can handle only one member at a time. Like structures, a union
can be declared using the keyword union as follows:

union item
{
int m;
float x;
char c;
}code;

SIZE OF STRUCTURES
We normally use structures, unions, and arrays to create variables of large
sizes. The actual size of these variables in terms of bytes may change
from machine to machine. We may use the unary operator sizeof to tell
us the size of a structure (or any variable).
The expression
sizeof(struct x)
will evaluate the number of bytes required to hold all the members of the
structure x. If y is a simple structure variable of type struct x, then the
expression
sizeof(y)
would also give the same answer.


Pointers
Pointers are another important feature of C language. They are a
powerful tool and handy to use once they are mastered. There are a
number of reasons for using pointers.
A pointer enables us to access a variable that is defined outside the
function.
Pointers are more efficient in handling the data tables.
Pointers reduce the length and complexity of a program.
They increase the execution speed.
The use of a pointer array to character strings results in saving of data
storage space in memory.


ACCESSING THE ADDRESS OF A VARIABLE

The actual location of a variable in the memory is system
dependent and therefore, the address of a variable is not known
to us immediately. We can determine the address of the variable
with the help of the operator & in C. We have already seen the use
of this address operator in the scanf function. The operator &
immediately preceding a variable returns the address of the
variable associated with it.
For example,
p = &quantity;
would assign the address to the variable p. The & operator can be
remembered as ‘address of’.


ACCESSING ADDRESSES OF VARIABLES : PROGRAM

#include<stdio.h>
#include<conio.h>
void main()
{
char a;
int x;
float p, q;
clrscr();
a = ‘A’;
x = 125;
p = 10.25, q = 18.76;
printf(“%c is stored at address %u”, a, &a);
printf(“n%d is stored at address %u”, x, &x);
printf(“n%f is stored at address %u”, p, &p);
printf(“n%f is stored at address %u”, q, &q);
getch();
}

DECLARING POINTERS
In C, every variable must be declared for its For example,
type. Since pointer variables contain int *p;
addresses that belong to a separate data declares the variable p as a pointer
type, they must be declared as pointers variable that points to an integer data
before we use them. type.
The declaration of a pointer variable takes Remember that the type int refers to
the following form: the data type of the variable being
datatype *pt_name; pointed to by p and not the type of the
value of the pointer.
This tells the compiler three things about
the variable pt_name. Similarly, the statement
float *x;
The asterisk (*) tells the variable declares x as a pointer to a floating
pt_name is a pointer variable.
point variable.
pt_name needs a memory location.
pt_name points to a variable of type
datatype.


INITIALIZING POINTERS
Once a pointer variable has been A pointer variable can be
declared, it can be made to point to a initialized in its declaration itself.
variable using an assignment For example,
statement such as int x, *p=&x;
p = &quantity; is perfectly valid. It declares x as
an integer variable and p as a
which causes p to point to quantity.
pointer variable and then
That is, p now contains the address of initializes p to the address of x.
quantity. Note carefully that this is an
This is known as pointer initialization. initialization of p, not *p. And
Before a pointer is initialized, it should also remember that the target
not be used. variable x is declared first. The
statement
int *p=&x, x;
is not valid.


ACCESSING A VARIABLE THROUGH ITS POINTER

Once a pointer has been assigned the address of a
variable, the question remains as to how to access
the value of the variable using the pointer. This is
done by using another unary operator * (asterisk),
usually known as the indirection operator.
Consider the following statements:
int quantity, *p, n;
quantity = 179;
p = &quantity;
n = *p;


ACCESSING A VARIABLE THROUGH ITS POINTER
(CONTD…)

The first line declares quantity and n as integer variables and p as
a pointer variable pointing to an integer.
The second line assigns the value 179 to quantity.
The third line assigns the address of quantity to the pointer
variable p.
The fourth line contains the indirection operator *. When the
operator * is placed before a pointer variable in an expression,
the pointer returns the value of the variable of which the pointer
value is the address.
In this case, *p returns the value of the variable quantity,
because p is the address of quantity. The * can be remembered
as ‘value of address’. Thus the value of n would be 179.


POINTERS AND ARRAYS
When an array is declared, the If we declare p as an integer pointer,
compiler allocates a base address and then we can make the pointer p to
sufficient amount of storage to contain point to the array x by the following
all the elements of the array in assignment:
contiguous memory locations. p = x;
The base address is the location of the This is equivalent to
first element (index 0) of the array. The p = &x[0];
compiler also defines the array name When handling arrays, instead of
as a constant pointer to the first using array indexing, we can use
element. pointers to access array elements.

Note that *(p+3) gives the value of
x[3]. The pointer accessing method is
much faster than array indexing.


DYNAMIC MEMORY ALLOCATION

C language requires the number of elements in an
array to be specified at compile time. But we may
not be able to do so always. Our initial judgment of
size, if it is wrong, may cause failure of the program
or wastage of memory space.
Many languages permit a programmer to
specify an array’s size at run time. The process of
allocating memory at run time is known as dynamic
memory allocation.
In C language there are four library routines
known as “memory management functions” that
can be used for allocating and freeing memory
during program execution.


Memory Allocation Functions
Function Task

malloc Allocates requested size of bytes and returns a pointer to the first byte
of the allocated space.

calloc Allocates space for an array of elements, initializes them to zero and then
returns a pointer to the memory.

Free Frees previously allocated space.

realloc Modifies the size of previously allocated space.


Allocating a Block of Memory
A block of memory may be allocated Use of malloc Function
using the function malloc. The malloc #include<stdio.h>
function reserves a block of memory #include<conio.h>
of specified size and returns a pointer void main()
of type void. This means that we can {
assign it to any type of pointer. int *p, n, i;
It takes the following form: clrscr();
printf(“Enter n value:”);
ptr = (datatype *)malloc(byte-size);
scanf(“%d”,&n);
ptr is a pointer of type datatype. The p = (int) malloc(sizeof(int)*n);
malloc returns a pointer (of datatype) printf(“nEnter %d numbers:”,n);
to an area of memory with size byte- for(i=0;i<n;i++)
size. scanf(“%d”,*(p+i));
Example: printf(“nThe numbers are:”);
for(i=0;i<n;i++)
x = (int *) malloc (sizeof(int) * n);
printf(“n%d”,*(p+i));
getch();
}

Allocating Multiple Blocks of Memory
calloc is another memory allocation function that is normally used for
requesting memory space at run time for storing derived data types such as
arrays and structures.
While malloc allocates a single block of storage space, calloc allocates
multiple blocks of storage, each of the same size, and then sets all bytes to
zero.
The general form of calloc is:
ptr = (datatype *) calloc (n,elem-size);
The above statement allocates contiguous space for n blocks, each of size
elem-size bytes. All bytes are initialized to zero and a pointer to the first byte
of the allocated region is returned. If there is not enough space, a NULL
pointer is returned.


Releasing the Used Space
Compile-time storage of a variable is allocated and
released by the system in accordance with its
storage class.
With the dynamic run-time allocation, it is our
responsibility to release the space when it is not
required.
The release of storage space becomes important
when the storage is limited. We may release that
block of memory for future use, using the free
function:
free(ptr);
ptr is a pointer to a memory block which has
already been created by malloc or calloc.


Altering the Size of a Block
It is likely that we discover later, the previously allocated memory is not
sufficient and we need additional space for more elements.
It is also possible that the memory allocated is much larger than
necessary and we want to reduce it.
In both the cases, we can change the memory size already allocated
with the help of the function realloc. This process is called the
reallocation of memory. For example, if the original allocation is done by
the statement
ptr = malloc(size);
then reallocation of space may be done by the statement
ptr = realloc(ptr, newsize);


POINTERS AND CHARACTER STRINGS

Program
We know that a string is an array of #include<stdio.h>
characters, terminated with a null #include<conio.h>
character. void main()
{
Like in one-dimensional arrays, we can char *str;
use a pointer to access the individual int i=0;
characters in a string. clrscr();
printf(“Enter a string:”);
gets(str);
while(*str!=’0’)
{
i++;
str++;
}
printf(“nThe length is:%d”,i);
getch();
}

POINTERS AND FUNCTIONS
Program : Pointers as function Parameters
#include<stdio.h>
In functions we can pass the duplicate #include<conio.h>
values for the actual parameters, but void main()
we can pass the address of a variable as {
an argument to a function in the int a,b;
normal fashion. clrscr();
When we pass addresses to a function, scanf(“%d %d”,&a,&b);
the parameters receiving the addresses printf(“nBefore exchange: a=%d, b=%d”,a, b);
should be pointers. The process of swap(&a,&b);
calling a function using pointers to pass printf(“nAfter exchange: a=%d, b=%d”,a, b);
the addresses of variable is known as getch();
}
call by address. void swap(int *x, int *y)
{
int z;
z = *x;
*x = *y;
*y = z;
}

POINTERS AND STRUCTURES
We know that the name of an array stands for Program: Pointers to Structure variables
the address of its zeroth element. The same #include<stdio.h>
thing is true of the names of arrays of structure #include<conio.h>
variables. struct invent
{
Suppose product is an array variable of struct char name[20] ;
type. The name product represents the int number;
address of its zeroth element. }product[3], *ptr;
Consider the following declaration: void main()
{
struct inventory clrscr();
{ printf(“INPUTnn”);
printf(“Enter name and number(3 records):”);
char name[30]; for(ptr = product;ptr<product+3;ptr++)
int number; scanf(“%s %d”,ptr->name,&ptr->number);
printf(“nnOUTPUT”);
} product[3], *ptr;
for(ptr = product;ptr<product+3;ptr++)
printf(“n%st%d”,ptr->name,ptr->number);
getch();
}


File Management in C
A file is a place on the disk where a group of related data is
stored. Like most other languages, C supports a number of
functions that have the ability to perform basic file
operations, which include:
 Naming a file,
 Opening a file,
 Reading data from a file,
 Writing data to a file, and
 Closing a file.


High level I/O functions
Function Name Operation

fopen() Creates a new file for use
Opens an existing file for use.
fclose() Closes a file which has been opened for use.
Getc() Reads a character from a file.
putc() Writes a character to a file.
fprintf() Writes a set of data values to a file.
fscanf() Reads a set of data values from a file.
getw() Reads an integer from a file.
putw() Writes an integer to file.
fseek() Sets the position to a desired point in the file
Ftell() Gives the current position in the file
rewind() Sets the position to the beginning of the file.

DEFINING AND OPENING A FILE
If we want to store data in a file in the secondary
memory, we must specify certain things about the file,
to the operating system. They include:
Filename.
Data Structure.
Purpose.
Following is the general format for declaring
and opening a file:
FILE *fp;
fp = fopen(“filename”, “mode”);


DEFINING AND OPENING A FILE (CONTD…)

The first statement declares the variable fp as a “pointer to the data type
FILE”. As stated earlier, FILE is a structure that is defined in the I/O library.
The second statement opens the file named filename and assigns as
identifier to the FILE the pointer fp. This pointer which contains all the
information about the file is subsequently used as a communication link
between the system and the program.
The second statement also specifies the purpose of opening this file. The
mode does this job. Mode can be one of the following:
r open the file for reading only.
w open the file for writing only.
a open the file for appending (or adding) data to it.
Note that both the filename and mode are specified as strings. They
should be enclosed in double quotation marks.


DEFINING AND OPENING A FILE (CONTD…)

When trying to open a file, one of the Consider the following statements:
following things may happen: FILE *p1, *p2;
When the mode is ‘writing’ a file with p1 = fopen(“data”,”r”);
the specified name is created if the file p2 = fopen(“results”,”w”);
does not exist. The contents are
deleted, if the file already exists. Many recent compilers include
additional modes of operation. They
When the purpose is ‘appending’, the
include:
file is opened with the current contents
r+ The existing file is opened to the
safe. A file with the specified name is
beginning for both reading and writing.
created if the file does not exist.
w+ Same as w except both for reading
If the purpose is ‘reading’, and if it and writing
exists, then the file is opened with the a+ Same as a except both for reading
current contents safe; otherwise an and writing.
error occurs. We can open and use a number of files
at a time. This number however
depends on the system we use.


CLOSING A FILE

A file must be closed as soon as all
operations on it have been completed.
fclose(file_pointer);


INPUT/OUTPUT OPERATIONS ON FILES

Once a file is opened, reading out of or writing to it is accomplished using the standard
I/O routines that are listed.
The getc and putc Functions
The simplest file I/O functions are getc and putc. These are analogous to getchar and
putchar functions and handle one character at a time. Assume that a file is opened
with mode w and file pointer fp1. Then, the statement
putc(c, fp1);
writes the character contained in the character variable c to the file associated with
FILE pointer fp1. Similarly, getc is used to read a character from a file that has been
opened in read mode. For example, the statement
c = getc(fp2);
would read a character from the file whose file pointer is fp2.
The file pointer moves by one character position for every operation of getc
or putc. The getc will return an end-of-file marker EOF, when end of the file has been
reached. Therefore, the reading should be terminated when EOF is encountered.


Program : Writing to and Reading from a File
PROGRAM
#include<stdio.h>
#include<conio.h>
void main()
{
FILE *f1;
char c;
clrscr();
printf(“Data Inputnn”);
f1 = fopen(“INPUT”, “w”);
while((c=getchar()!=EOF)
putc(c,f1);
fclose(f1);
printf(“nData Outputnn”);
f1 = fopen(“INPUT”,”r”);
while((c=getc(f1))!=EOF)
printf(“%c”,c);
fclose(f1);
getch();
}

The fprintf & fscanf Functions
The functions fprintf and fscanf perform I/O operations that are identical
to the familiar printf and scanf functions, except of course that they work
on files.
The first argument of these functions is a file pointer which specifies the
file to be used.
The general form of fprintf is
fprintf(fp, “control string”, list);
where fp is a file pointer associated with a file that has been opened for
writing. The control string contains output specifications for the items in
the list. The list may include variables, constants and strings. Example:
fprintf(f1, “%s %d %f”,name,age,7.5);
Here, name is an array variable of type char and age is int variable.


The fscanf Function
The general format of fscanf is
fscanf(fp, “control string”, list);
This statement would cause the reading of the items in the list from the
file specified by fp, according to the specifications contained in the
control string.
Example:
fscanf(f2, “%s %d”, item, &quantity);
Like scanf, fscanf also returns the number of items that are successfully
read. When the end of file is reached, it returns the value of EOF.


COMMAND LINE ARGUMENTS
It is a parameter supplied to a program when the program
is invoked. This parameter may represent a filename the
program should process.
For example, if we want to execute a program to copy the
contents of a file named X_FILE to another one named
Y_FILE, then we may use a command line like
C:TC>PROGRAM X_FILE Y_FILE
PROGRAM is the filename where the executable code of
the program is stored. This eliminates the need for the
program to request the user to enter the filenames during
execution.


How do these parameters get into the program?
We know that every C program should have The size of this array will be equal to the
one main function and that it marks the value of argc. For instance, for the
beginning of the program. command line given above, argc is three and
But what we have not mentioned so far is that argv is array of three pointers to strings as
it can also take arguments like other shown below:
functions. In fact main can take two argv[0] PROGRAM
arguments called argc and argv and the argv[1] X_FILE
information contained in the command line is argv[2]Y_FILE
passed on to the program through these In order to access the command line
arguments, when main is called up by the arguments, we must declare the main
system. function and its parameters as follows:
main(argc,argv);
The variable argc is an argument counter that int argc;
counts the number of arguments on the char *argv[];
command line. The argv is an argument vector The first parameter in the command line is
and represents an array of character pointers always the program name and therefore
that point to the command line arguments argv[0] always represents the program
name.


Program : Command line Arguments
#include<stdio.h> for(i=2;i<argc;i++)
#include<conio.h> fprintf(fp,"%s",argv[i]);
void main(argc,argv) fclose(fp);
printf("Contents of %s filenn",argv[1]);
int argc; fp=fopen(argv[1],"r");
char *argv[]; for(i=2;i<argc;i++)
{ {
fscanf(fp,"%s",word);
FILE *fp; printf("%s",word);
int i; }
fclose(fp);
char word[15];
printf("nn");
clrscr(); for(i=0;i<argc;i++)
fp = fopen(argv[1],"w"); printf("%*sn",i*5,argv[i]);
getch();
printf("nNo.of arguments in Command
}
line=%dnn",argc);


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INTRODUCTION TO C PROGRAMMING

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