C Language Unit-3

/* Ranking of 60 students in a class */
int main() { Array & its Advantage
/*declaring 60 varialbes */
int score0, score1,score2,……,score59; score0 scores[0]
/* Reading scores for sixty times */ score1 scores[1]
printf(“Enter the score : “); score2 scores[2]
scanf(“%d”, &score0);
…. …. …. …. score3 scores[3]
printf(“Enter the score : “); . .
scanf(“%d”, &score59); . .
/* comparing & swapping for 1770 times
* to arrange in descending order */
score59 scores[59]
swap( score0, score1);
swap( score1, score2); Sixty variables are replaced by one Array
swap( score2, score3); #include<stdio.h>
…. …. …. …. int main() { Sixty input
swap( score0,score1); int scores[60] , i , j, temp; statements are
swap( score1,score2); for(i = 0; i < 60 ;i++) { called by one loop
swap( score0,score1); printf("Enter the score : "); statement
/*printing 60 scores after sorting */ scanf("%d", &scores[i]);
printf(“%4d”, score0); }
printf(“%4d”, score1); for(i=0;i<(60-1);i++) 1770 comparing
… … … … for( j=0; j <(60 -(i+1)); j++) statements are
} if(scores[ j ] < scores[ j +1]) { included in one
void swap ( int a, int b) { temp = scores[ j ]; loop statement
int temp; scores[ j ] = scores[ j +1];
if( a < b) { scores[ j + 1] = temp;
temp = a ; a = b ; b = temp; }
} for( i = 0; i < 60; i ++) printf("%4d", scores[i]);
} }

scores[0] scores[1] scores[2] scores[3] scores[4]

...
4022 4023 4024 4025 4026 4027 4028 4029 4030 4031 ( memory
addresses)
start here scores Array
Mean can be calculated only after reading all scores. Each deviation is difference of individual score and
mean. To calculate deviations of all scores, scores must be stored in an ARRAY.
Accessing
Declaration of Array an element

Initialization of Array

Input to an element
Processing on Array

Scalar variable for single data item & Vector variable for multiple data items

Scalar Variables :
 A variable represents a data item and it can be used to store a single atomic value at a time.
These are also called scalar variables.
 Integer takes 2 bytes memory as a single unit to store its value. i.e.,the value of a scalar variable
cannot be subdivided into a more simpler data items.
 The address of first byte is the address of a variable .
Vector Variables (arrays):
 In contrast, an array is multivariable (an aggregate data type), which is also referred to a data
structure. It represents a collection of related data items of same type.
An individual data item of an array is called as ‘element’. Every element is accessed by index or
subscript enclosed in square brackets followed after the array name.
 All its elements are stored in consecutive memory locations, referred under a common array name.

Ex : int marks[10] ; /* declaration of array */
 ‘0’ is first number in computer environment. The first element of array marks is marks[0] and last
element is marks[9]. (the address of first element is the address of the array)
 An array is a derived data type. It has additional operations for retrieve and update the individual
values.
 The lowest address corresponds to the first element and the highest address to the last element.
 Arrays can have from one to several dimensions. The most common array is the string, which is
simply an array of characters terminated by a null.

Declaration of One Dimensional Arrays Elements of
Syntax : Array [3] by [4]
arrayType arrayName [ numberOfElements ];
Example :
int scores [60];
[0][0] [0][1] [0][2] [0][3]
float salaries [20];
Initialization of Array while Declaration : [1][0] [1][1] [1][2] [1][3]
int numbers [ ] = { 9, 4, 2, 7, 3 };
[2][0] [2][1] [2][2] [2][3]
char name[ ] ={‘R’,’a’,‘v’,‘i’,‘ ‘,‘T’,‘e’,‘j’,’a’,’0’ };
char greeting[ ] = “Good Morning”;
Declaration of Multi Dimensional Arrays /*passing an array to function */
Syntax : #define SIZE 10
arrayType arrayName [ Rows ][ Columns ]; int main() {
arrayType arrayName [ Planes][ Rows ][ Columns ]; float list[SIZE] ,avg;
Example : ……………
/* Each student for seven subjects */
avg = average(SIZE , list );
int marks[60][7];
/* Matrix with 3 planes and 5 rows and 4 columns */
……………
float matrix[3][5][4]; }
Initialization of Array while Declaration : float average( int n , float x[]) {
int matrix [ ][ ] = { { 4, 2, 7, 3 } , float sum=0,i;
{ 6, 1, 9, 5 } , for( i = 0; i < n ; i++)
{ 8, 5, 0, 1 } }; sum = sum + x[i];
return ( sum / n ) ;
}

Strings - One Dimensional Character Arrays
A String is sequence of characters. In ‘C’ strings are implemented by an array of characters
terminated with a null character ‘0’(back slash followed by zero ).

char name[] = “Ravi Kiran”; R a v i K i r a n 0
name
‘name’ is an array of characters has size of eleven characters including a null
character ‘0’(ascii code is zero).
char name[25] ;
scanf(“%s”, name); /*reading a string until a white space is encountered ( & operator is not required )*/
printf(“%s”, name); /*printing a string in input window */
gets(name) ; /* reading a string including white spaces until ‘n’ is encountered. */
puts(name); /* printing a string and moves cursor to new line */

String Manipulation Functions in <string.h>
strlen(s1) - returns the length of string excluding the last ‘null’ character.
strcpy(s1,s2) - copies characters in s2 into s1.
strcat(s1,s2)- concatenates s2 to s1.
strcmp(s1,s2) -compares s1 with s2 lexicographically and returns ‘0’ if two strings are
same , returns -1 if s1 is before s2 and returns +1 if s1 is after s2.
strcmpi(s1,s2) -compares s1 with s2 like strcmp() but case of characters is ignored.
strchr(s1,ch) -returns pointer to first occurrence of the character ‘ch’ in s1.
strstr(s1,s2) -returns pointer to first occurrence s2 in s1.
strrev(s1) -returns pointer to the reversed string.

Memory Address : Bit is a smallest unit of memory to store either ‘0’ or ‘1’ in memory.
Byte is unit of memory of 8 bits. Memory is a sequence of a large number of memory
locations , each of which has an address known as byte. Every byte in memory has a
sequential address number to recognized by processor.

Memory Sections of C-Runtime
RAM is temporary storage place to run programs. C-Language runtime also utilizes an
allotted memory block in RAM to run its programs.
Text Section : Memory-area that contains the machine instructions(code).It is read
only and is shared by multiple instances of a running program.
Data Section : Memory image of a running program contains storage for initialized
global variables, which is separate for each running instance of a program.
BSS (Below Stack Segment) : Memory area contains storage for uninitialized global
variables. It is also separate for each running instance of a program.
Stack : Area of memory image of a running program contains storage for automatic
variables of a function. It also stores memory address of the instruction which is the
function call, to return the value of called function.
Heap : This memory region is reserved for dynamically allocating memory for
variables at run time. Dynamic Memory Allocation calculate the required memory size
while program is being executed.
Shared Libraries: This region contains the executable image of shared libraries
being used by a program.

Two or more Permanent Manipulations using one Function
Passing Parameters By Value Passing Parameters By Reference

/* program to swap two numbers */ /* program to swap two numbers */
#include<stdio.h> #include<stdio.h>
void swap(int x, int y) void swap(int *x, int *y)
{ {
int temp; int temp;
temp = x; x = y; y = temp; temp = *x; *x = *y; *y = temp;
printf(“nIn swap() : %d %d “,x,y); printf(“nIn swap() : %d %d “,*x,*y);
} }
int main() int main()
{ {
int a = 25,b = 37; int a = 25,b = 37;
printf(“Before swap() : %d %d”,a,b); printf(“Before swap() : %d %d”,a,b);
swap (a,b); swap (&a , &b);
printf(“nAfter swap() : %d %d“,a,b); printf(“nAfter swap() : %d %d“,a,b);
} }

Output : Output :
Before swap() 25 37 Before swap() 25 37
In swap () 37 25 In swap () 37 25
After swap() 25 37 After swap() 37 25

Pointer variable – A variable holds the address of another variable

Allots some memory location Value in ‘option’
4042 (for example)
char option = ‘Y’; with a name option and ‘Y’
stores value ‘Y’ in it
option
Memory Address of variable ‘option’ 4042

Creates a pointer variable
with a name ‘ptr’
char *ptr = NULL; Which can hold a
Memory address ptr

Memory address of 4042 ‘Y’
ptr = &option; Variable ‘option’
Is copied to the option
ptr
Pointer ‘ptr’
4042
The value ‘N’ is
*ptr = ‘N’; stored in the variable 4042 ‘N’
which has the option
memory address ptr
4042 4042

Program with Using Pointers
int main() { pointer variables are declared
int n1, n2 ;
n1 n2
int *p = NULL, *q = NULL;
n1 = 6 ; p q NULL
Prints 6 6 NULL
p = & n1;
printf (“%d %d”, n1,*p );
Prints address of n1
printf (“%ld %ld”,&n1, p ); n1 6 n2 3

p q
q = & n2; Prints 6 3
*q = 3 ;
printf (“ %d %d “, *p , *q ) ;
n1 6 n2 3
p = q; pointer ‘q’ assigned with pointer ‘q’
printf (“ %d %d “, *p , *q ) ; p q

Prints 3 3
*p = 7 ; n1 6 n2 7
printf (“ %d %d “, *p , *q ) ;
} p q
Prints 7 7

When two pointers are referencing with one variable, both pointers contains address of the
same variable, and the value changed through with one pointer will reflect to both of them.

Pointer and Arrays

Even though pointers and arrays work alike and strongly related,
they are not synonymous. When an array is assigned with pointer,
the address of first element of the array is copied into the pointer.

#include<stdio.h>
int main()
Pointer is an address variable, having no
{
initialized value by default. The address
int a[3] = { 12, 5 ,7}, b[3]; stored in the pointer can be changed
int *p ,*q; time to time in the program.
Prints 12 12
p = a;
printf("%d %dn", *p, *a);
Prints 12 12
q = p; Array name is an address constant,
printf("%d %d",*p,*q); initialized with the address of the first
element (base address )in the array. The
address stored in array name cannot be
b = a; /* error */ changed in the program.
}

Pointer Arithmetic and Arrays
#include <stdio.h>
int main() {
int arr [5] = { 12, 31, 56, 19, 42 };
int *p;
p = arr + 1; Prints 31
Prints 12 31 56
printf("%d n", *p);
printf("%d %d %dn", *(p-1), *(p), *(p + 1));
--p;
printf("%d", *p); Prints 12

arr[0] or *( arr + 0 ) 12 p - 1
arr[1] or *( arr + 1 ) 31 p
arr[2] or *( arr + 2 ) 56 p +1
arr[3] or *( arr + 3 ) 19 p +2
arr[4] or *( arr + 4 ) 42 p +3

Subscript operator [ ] used to access an element of array
implements address arithmetic, like pointer.

Array of Pointers
The advantage of pointer array is that the length of each row in the array may
be different. The important application of pointer array is to store character strings
of different length. Example :
char *day[ ] = { “Sunday”, “Monday”, ”Tuesday”, “Wednesday”, “Thursday”,
“Friday”, “Saturday” };

Pointer to Pointer ( Double indirection )
Example :
int a = 25; a pa ppa
int *pa = &a;
int **ppa ; 25 4024 4056
*ppa = &pa;
printf(“%d”, *pa);  prints 25 4056 4078
4024
printf(“%d”, **ppa);  prints 25

Two Dimensional Array -- Pointers
a[0][0] a[0][1] a[0][2] a[1][0] a[1][1] a[1][2] a[2][0] a[2][1] a[2][2] a[3][0] a[3][1] a[3][2]

base_address Array name contains base address

Address of a[ i ] [ j ] = *( * ( base_address + i ) + j ) = * ( * ( a + i ) + j )

void Pointer Function Pointers
‘void’ type pointer is a generic Function pointers are pointers, which
pointer, which can be assigned to any point to the address of a function.
data type without cast during Declaration :
compilation or runtime. ‘void’ pointer <return type> (* function_pointer)
cannot be dereferenced unless it is (type1 arg1, type2 arg2, ……. );
cast.
int add ( int a, int b ) { return (a + b) ; }
int main( ) { int sub ( int a, int b ) { return (a – b) ; }
void* p;
int x = 7; int (*fp ) (int, int ) ; /* function pointer */
float y = 23.5;
p = &x; int main( ) {
printf(“x contains : %dn”, *( ( int *)p) ); fp = add;
p = &y; printf(“Sum = %dn”, fp( 4, 5 ) ) ;
printf(“y contains : %fn”, *( ( float *)p) ); fp = sub;
} printf(“Difference = %dn”, fp( 6 , 2 ) ) ;
}

Output :
x contains 7 Output :
y contains 23.500000 Sum = 9
Difference = 4

Dynamic Memory Allocation (DMA) of pointers

Static memory allocation means allocating memory by compiler. When using address operator,
the address of a variable is assigned to a pointer. Ex : int a = 20 ; int *p = &a ;

Dynamic memory allocation means allocating memory using functions like malloc() and calloc().
The values returned by these functions are assigned to pointer variables only after execution of
these functions. Memory is assigned at run time.

int main() Allocates memory in bytes and returns the address of first
{ byte to the pointer variable
int *p, *q ;
p = (int *) malloc ( sizeof( int ) ); Releases previously allocated memory space.
if( p == NULL )
{ calloc ( ) is used for allocating memory space
printf(“Out of memoryn”); during the program execution for derived data types
such as arrays, structures etc.,
exit(-1);
Example :
} struct book {
printf(“Address in p : %d“, p ); int no ; char name[20] ; float price ;
};
free ( p ); struct book b1 ;
p = NULL; b1 *ptr ;
} ptr = (book *) calloc ( 10, sizeof ( book ) );

ptr = (book * ) realloc ( ptr , 35 * sizeof ( book ) );
Modifies the size of previously allocated memory to
new size.

Standard Character Functions
Command Line Arguments
Classification of Characters
File Name : cmdline.c
control printable
iscntrl ( ) isprint ( ) int main( int argc , char* argv [ ])
{
int i ;
space graphical
isspace ( ) isgraph () printf(“Number of arguments : %d“, argc );
printf(“nName of Program : %s“, argv [0] );
alpha-numeric punctuation
for ( i = 1; i < argc ; i++ )
isalnum ( ) ispunct ( ) printf(“nUser value %d : %s “,
i , argv [ i ] );
alphabetic digit }
isalpha( ) isdigit ()

output
upper lower
isupper ( ) islower () Compile the program :
c:>tcc cmdline.c
c:>cmdline welcome to c-programming
Other character functions in <ctype.h> c:>Number of arguments : 4
Name of Program : c:cmdline.exe
toupper( ) – converts to uppercase. User value 1 : welcome
tolower ( ) – converts to lowercase. User value 2 : to
toascii ( ) – converts greater than 127 to User value 3 : c-programming
with in the range 0 – 127

Standard C-Library Functions
<stdlib.h>
int atoi(s) Converts string s to an integer
long atol(s) Converts string s to a long integer.
float atof(s) Converts string s to a double-precision quantity.
void* calloc(u1,u2) Allocate memory to an array u1, each of length u2 bytes.
void exit(u) Closes all files and buffers, and terminate the program.
void free (p) Free block of memory.
void* malloc (u) Allocate u bytes of memory.
int rand(void) Return a random positive integer.
void* realloc(p,u) Allocate u bytes of new memory to the pointer variable p.
void srand(u) Initialize the random number generator.
void systerm(s) Pass command string to the operating system.

<time.h>
clock_t clock() Returns clock ticks since program starts.
char *asctime(stuct tm) Converts date and time into ascii.
int stime(time_t *tp) Sets time.
time_t time(time_t *timer) Gets time of day.
double difftime(t1,t2) Returns difference time between two times t1 and t2.

C Language Unit-3

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C Language Unit-3