1. EENG212 ALGORITHMS & DATA STRUCTURES
POINTERS
EASTERN MEDITERRANEAN UNIVERSITY

2. POINTERS Outline Introduction
Pointer Variable Declarations and Initialization
Pointer Operators
Calling Functions by Reference
Using the const Qualifier with Pointers
Bubble Sort Using Call by Reference
Pointer Expressions and Pointer Arithmetic
The Relationship between Pointers and Arrays
Arrays of Pointers

3. Pointers Powerful, but difficult to master Simulate call-by-reference
Close relationship with arrays and strings

4. Pointer Variable Declarations and Initialization
Pointer variables
Contain memory addresses as their values
Normal variables contain a specific value (direct reference)
Pointers contain address of a variable that has a specific value (indirect reference)
Indirection – referencing a pointer value
count 7
count 7
countPtr

5. Pointer Variable Declarations and Initialization
Pointer declarations
* used with pointer variables
int *myPtr;
Declares a pointer to an int (pointer of type int *)
Multiple pointers require using a * before each variable declaration
int *myPtr1, *myPtr2;
Can declare pointers to any data type
Initialize pointers to 0, NULL, or an address
0 or NULL – points to nothing (NULL preferred)

6. Pointer Operators & (address operator) Returns address of operand
int y = 5;
int *yPtr;
yPtr = &y; // yPtr gets address of y
yPtr “points to” y
yPtr y 5
yptr
500000
600000
Address of y is value of yptr

7. Pointer Operators * (indirection/dereferencing operator)
Returns a synonym/alias of what its operand points to
*yptr returns y (because yptr points to y)
* can be used for assignment
Returns alias to an object
*yptr = 7; // changes y to 7
Dereferenced pointer (operand of *) must be an lvalue (no constants)
* and & are inverses
They cancel each other out

8. The address of a is the value of aPtr.
1 /* Fig. 7.4: fig07_04.c
2 Using the & and * operators */
3 #include <stdio.h> 4
5 int main()
6 {
7 int a; /* a is an integer */
8 int *aPtr; /* aPtr is a pointer to an integer */ 9
10 a = 7;
11 aPtr = &a; /* aPtr set to address of a */ 12
13 printf( "The address of a is %p"
"\nThe value of aPtr is %p", &a, aPtr ); 15
16 printf( "\n\nThe value of a is %d"
"\nThe value of *aPtr is %d", a, *aPtr ); 18
19 printf( "\n\nShowing that * and & are inverses of "
"each other.\n&*aPtr = %p"
"\n*&aPtr = %p\n", &*aPtr, *&aPtr ); 22
23 return 0;
24 }
The address of a is the value of aPtr.
The * operator returns an alias to what its operand points to. aPtr points to a, so *aPtr returns a.
Notice how * and & are inverses
The address of a is 0012FF88
The value of aPtr is 0012FF88
The value of a is 7
The value of *aPtr is 7
Proving that * and & are complements of each other.
&*aPtr = 0012FF88
*&aPtr = 0012FF88

9. Calling Functions by Reference
Call by reference with pointer arguments
Pass address of argument using & operator
Allows you to change actual location in memory
Arrays are not passed with & because the array name is already a pointer
* operator
Used as alias/nickname for variable inside of function
void double( int *number ) {
*number = 2 * ( *number ); }
*number used as nickname for the variable passed

10. Inside cubeByReference, *nPtr is used (*nPtr is number).
1 /* Fig. 7.7: fig07_07.c
2 Cube a variable using call-by-reference
3 with a pointer argument */ 4
5 #include <stdio.h> 6
7 void cubeByReference( int * ); /* prototype */ 8
9 int main()
10 {
11 int number = 5; 12
13 printf( "The original value of number is %d", number );
14 cubeByReference( &number );
15 printf( "\nThe new value of number is %d\n", number ); 16
17 return 0;
18 } 19
20 void cubeByReference( int *nPtr )
21 {
22 *nPtr = *nPtr * *nPtr * *nPtr; /* cube number in main */
23 }
Notice that the function prototype takes a pointer to an integer (int *).
Notice how the address of number is given - cubeByReference expects a pointer (an address of a variable).
Inside cubeByReference, *nPtr is used (*nPtr is number).
The original value of number is 5
The new value of number is 125

11. Bubble Sort Using Call-by-reference
Implement bubblesort using pointers
Swap two elements
swap function must receive address (using &) of array elements
Array elements have call-by-value default
Using pointers and the * operator, swap can switch array elements
Psuedocode
Initialize array
print data in original order
Call function bubblesort
print sorted array
Define bubblesort

12. Bubble Sort Using Call-by-reference
sizeof
Returns size of operand in bytes
For arrays: size of 1 element * number of elements
if sizeof( int ) equals 4 bytes, then
int myArray[ 10 ];
printf( "%d", sizeof( myArray ) );
will print 40
sizeof can be used with
Variable names
Type name
Constant values

13. 1 /* Fig. 7.15: fig07_15.c
2 This program puts values into an array, sorts the values into
3 ascending order, and prints the resulting array. */
4 #include <stdio.h>
5 #define SIZE 10
6 void bubbleSort( int *, const int ); 7
8 int main()
9 { 10
11 int a[ SIZE ] = { 2, 6, 4, 8, 10, 12, 89, 68, 45, 37 };
12 int i; 13
14 printf( "Data items in original order\n" ); 15
16 for ( i = 0; i < SIZE; i++ )
printf( "%4d", a[ i ] ); 18
19 bubbleSort( a, SIZE ); /* sort the array */
20 printf( "\nData items in ascending order\n" ); 21
22 for ( i = 0; i < SIZE; i++ )
printf( "%4d", a[ i ] ); 24
25 printf( "\n" ); 26
27 return 0;
28 } 29
30 void bubbleSort( int *array, const int size )
31 {
32 void swap( int *, int * );
Bubblesort gets passed the address of array elements (pointers). The name of an array is a pointer.

14. 33 int pass, j;
34 for ( pass = 0; pass < size - 1; pass++ ) 35
for ( j = 0; j < size - 1; j++ ) 37
if ( array[ j ] > array[ j + 1 ] )
swap( &array[ j ], &array[ j + 1 ] );
40 } 41
42 void swap( int *element1Ptr, int *element2Ptr )
43 {
44 int hold = *element1Ptr;
45 *element1Ptr = *element2Ptr;
46 *element2Ptr = hold;
47 }
Data items in original order
Data items in ascending order

15. Pointer Expressions and Pointer Arithmetic
Arithmetic operations can be performed on pointers
Increment/decrement pointer (++ or --)
Add an integer to a pointer( + or += , - or -=)
Pointers may be subtracted from each other
Operations meaningless unless performed on an array

16. Pointer Expressions and Pointer Arithmetic
5 element int array on machine with 4 byte ints
vPtr points to first element v[ 0 ]
at location 3000 (vPtr = 3000)
vPtr += 2; sets vPtr to 3008
vPtr points to v[ 2 ] (incremented by 2), but the machine has 4 byte ints, so it points to address 3008
pointer variable vPtr
v[0]
v[1]
v[2]
v[4]
v[3]
3000
3004
3008
3012
3016
location

17. The Relationship Between Pointers and Arrays
Arrays and pointers closely related
Array name like a constant pointer
Pointers can do array subscripting operations
Declare an array b[ 5 ] and a pointer bPtr
To set them equal to one another use:
bPtr = b;
The array name (b) is actually the address of first element of the array b[ 5 ]
bPtr = &b[ 0 ]
Explicitly assigns bPtr to address of first element of b

18. The Relationship Between Pointers and Arrays
Element b[ 3 ]
Can be accessed by *( bPtr + 3 )
Where n is the offset. Called pointer/offset notation
Can be accessed by bptr[ 3 ]
Called pointer/subscript notation
bPtr[ 3 ] same as b[ 3 ]
Can be accessed by performing pointer arithmetic on the array itself
*( b + 3 )

19. /* Fig. 7.20: fig07_20.cpp
Using subscripting and pointer notations with arrays */
#include <stdio.h>
int main() {
int b[] = { 10, 20, 30, 40 }; /* initialize array b */
int *bPtr = b; /* set bPtr to point to array b */
int i; /* counter */
int offset; /* counter */
/* output array b using array subscript notation */
printf( "Array b printed with:\nArray subscript notation\n" );
/* loop through array b */
for ( i = 0; i < 4; i++ ) {
printf( "b[ %d ] = %d\n", i, b[ i ] );
} /* end for */

20. /* output array b using array name and pointer/offset notation */
printf( "\nPointer/offset notation where\n"
"the pointer is the array name\n" );
/* loop through array b */
for ( offset = 0; offset < 4; offset++ ) {
printf( "*( b + %d ) = %d\n", offset, *( b + offset ) );
} /* end for */
/* output array b using bPtr and array subscript notation */
printf( "\nPointer subscript notation\n" );
for ( i = 0; i < 4; i++ ) {
printf( "bPtr[ %d ] = %d\n", i, bPtr[ i ] );
/* output array b using bPtr and pointer/offset notation */
printf( "\nPointer/offset notation\n" );
printf( "*( bPtr + %d ) = %d\n", offset, *( bPtr + offset ) );
return 0; /* indicates successful termination */
} /* end main */

21. Array b printed with:
Array subscript notation
b[ 0 ] = 10
b[ 1 ] = 20
b[ 2 ] = 30
b[ 3 ] = 40
Pointer/offset notation where
the pointer is the array name
*( b + 0 ) = 10
*( b + 1 ) = 20
*( b + 2 ) = 30
*( b + 3 ) = 40
Pointer subscript notation
bPtr[ 0 ] = 10
bPtr[ 1 ] = 20
bPtr[ 2 ] = 30
bPtr[ 3 ] = 40
Pointer/offset notation
*( bPtr + 0 ) = 10
*( bPtr + 1 ) = 20
*( bPtr + 2 ) = 30
*( bPtr + 3 ) = 40
Press any key to continue

22. Binary Search
Binary Search: Given a sorted array Binary Search algorithm can be used to perform fast searching of a searchkey on the sorted array.
The following program uses pointer notation to implement the binary search algorithm for the search key entered by the user in the following array:
(3, 5, 9, 11, 15, 17, 22, 25, 37, 68)

23. #include <stdio.h>
#define SIZE 10
int BinarySearch(int *, int);
int main() {
int a[SIZE]= {3, 5, 9, 11, 15, 17, 22, 25, 37, 68}, key, pos;
printf(“Enter the Search Key\n”);
scanf(“%d”,&key);
pos = BinarySearch(a, key);
if(pos == -1)
printf(“The search key is not in the array\n”);
else
printf(“The search key %d is at location %d\n”, key, pos);
return 0; }

24. int BinarySearch (int *aptr, int skey){
int low=0,high=SIZE-1,middle;
while(low <= high){
middle= (low+high)/2;
if(skey == *(aptr+middle))
return middle;
else if(skey <*(aptr+middle))
high = middle-1;
else
low = middle+1; }
return -1;
aptr[middle]