EENG212 ALGORITHMS & DATA STRUCTURES POINTERS EASTERN MEDITERRANEAN UNIVERSITY

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

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

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

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)

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

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

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" 14 "\nThe value of aPtr is %p", &a, aPtr ); 15 16 printf( "\n\nThe value of a is %d" 17 "\nThe value of *aPtr is %d", a, *aPtr ); 18 19 printf( "\n\nShowing that * and & are inverses of " 20 "each other.\n&*aPtr = %p" 21 "\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

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

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

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

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

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++ ) 17 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++ ) 23 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.

33 int pass, j; 34 for ( pass = 0; pass < size - 1; pass++ ) 35 36 for ( j = 0; j < size - 1; j++ ) 37 38 if ( array[ j ] > array[ j + 1 ] ) 39 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 2 6 4 8 10 12 89 68 45 37 Data items in ascending order 2 4 6 8 10 12 37 45

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

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

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

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 )

/* 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 */

/* 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 */

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

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)

#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; }

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]