1. C++ Programming Lecture 18 Pointers – Part IIBy
Ghada Al-Mashaqbeh
The Hashemite University
Computer Engineering Department

2. The Hashemite University
Outline
Introduction.
sizeof Operator and Arrays.
sizeof Operator and Pointers.
Bubble Sort Using Call-by-reference.
Pointer Expressions and Pointer Arithmetic.
The Relationship Between Pointers and Arrays.
Examples.
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3. sizeof Operator and Arrays
Returns size of operand in bytes
Returns the result of type size_t which is unsigned integer.
For arrays, sizeof returns
( the size of 1 element ) * ( number of elements )
if sizeof( int ) = 4, then
int myArray[10];
cout << sizeof(myArray);
will print 40
To get the size of an array (number of elements) using sizeof operator do the following:
Array size = sizeof(myArray)/ sizeof(int);
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4. sizeof Operator and Pointers
Applying sizeof operator for a pointer always returns a result of 4 regardless of the data type to which the pointer is pointing.
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5. Bubble Sort Using Call-by-reference
We will implement bubblesort function using pointers
swap function must receive the address (using &) of the array elements
array elements have call-by-value default
Using pointers and the * operator, swap is able to switch the values of the actual array elements
Psuedocode
Initialize array
print data in original order
Call function bubblesort
print sorted array
Define bubblesort
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1 // Fig. 5.15: fig05_15.cpp
2 // This program puts values into an array, sorts the values into
3 // ascending order, and prints the resulting array.
4 #include <iostream> 5
6 using std::cout;
7 using std::endl; 8
9 #include <iomanip> 10
11 using std::setw; 12
13 void bubbleSort( int *, const int ); 14
15 int main()
16 {
17 const int arraySize = 10;
18 int a[ arraySize ] = { 2, 6, 4, 8, 10, 12, 89, 68, 45, 37 };
19 int i; 20
21 cout << "Data items in original order\n"; 22
23 for ( i = 0; i < arraySize; i++ )
cout << setw( 4 ) << a[ i ]; 25
26 bubbleSort( a, arraySize ); // sort the array
27 cout << "\nData items in ascending order\n"; 28
29 for ( i = 0; i < arraySize; i++ )
cout << setw( 4 ) << a[ i ]; 31
32 cout << endl;
33 return 0;
34 }
Bubblesort gets passed the address of array elements (pointers). The name of an array is a pointer.
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36 void bubbleSort( int *array, const int size )
37 {
38 void swap( int * const, int * const ); 39
40 for ( int pass = 0; pass < size - 1; pass++ ) 41
for ( int j = 0; j < size - 1; j++ ) 43
if ( array[ j ] > array[ j + 1 ] )
swap( &array[ j ], &array[ j + 1 ] );
46 } 47
48 void swap( int * const element1Ptr, int * const element2Ptr )
49 {
50 int hold = *element1Ptr;
51 *element1Ptr = *element2Ptr;
52 *element2Ptr = hold;
53 }
swap takes pointers (addresses of array elements) and dereferences them to modify the original array elements.
Data items in original order
Data items in ascending order
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8. Pointer Expressions and Pointer Arithmetic I
Increment/decrement pointer (++ or --)
Add/subtract an integer to/from a pointer( + or += , - or -=)
Pointers may be subtracted from each other
Pointer arithmetic is meaningless unless performed on an array
5 element int array on a machine using 4 byte ints
vPtr points to first element v[ 0 ], which is at location 3000
vPtr = 3000
vPtr += 2; sets vPtr to 3008
vPtr points to v[ 2 ]
pointer variable vPtr
v[0]
v[1]
v[2]
v[4]
v[3]
3000
3004
3008
3012
3016
location
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9. Pointer Expressions and Pointer Arithmetic II
Subtracting pointers
Returns the number of elements between two addresses
vPtr2 = &v[ 2 ]; vPtr = &v[ 0 ]; vPtr2 - vPtr = 2
Pointer comparison
Test which pointer points to the higher numbered array element
Test if a pointer points to 0 (NULL)
if ( vPtr == NULL )
statement
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10. Pointer Expressions and Pointer Arithmetic III
Pointers assignment
If not the same type, a cast operator must be used
Exception: pointer to void (type void *)
Generic pointer, represents any type
No casting needed to convert a pointer to void pointer
void pointers cannot be dereferenced
Example:
#include<iostream.h>
int main() {
int x, y;
void*ptr = &x;
int *pptr = &y;
ptr = pptr;
pptr = (int*)ptr; //casting }
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11. The Relationship Between Pointers and Arrays I
Arrays and pointers are closely related
Array name is a constant pointer that contains the address of the first element in the array.
Pointers can do array subscripting operations
Having declared an array b[ 5 ] and a pointer bPtr
bPtr is equal to b
bptr == b
bptr is equal to the address of the first element of b
bptr == &b[ 0 ]
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12. The Relationship Between Pointers and Arrays II
Accessing array elements with pointers
Element b[ n ] can be accessed by *( bPtr + n )
Called pointer/offset notation
Array itself can use pointer arithmetic.
b[ 3 ] same as *(b + 3)
Pointers can be subscripted (pointer/subscript notation)
bPtr[ 3 ] same as b[ 3 ]
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Example I
// Converting lowercase letters to uppercase letters
// using a non-constant pointer to non-constant data.
#include <iostream.h>
#include <cctype> // prototypes for islower and toupper
void convertToUppercase( char * );
int main() {
char phrase[] = "characters and $32.98";
cout << "The phrase before conversion is: " << phrase;
convertToUppercase( phrase );
cout << "\nThe phrase after conversion is: “ << phrase << endl;
return 0;
} // end main
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Example I … cont.
// convert string to uppercase letters
void convertToUppercase( char *sPtr ) {
while ( *sPtr != '\0' ) { // current character is not '\0'
if ( islower( *sPtr ) ) // if character is lowercase,
*sPtr = toupper( *sPtr ); // convert to uppercase
++sPtr; // move sPtr to next character in string
} // end while
} // end function convertToUppercase
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Example II
// Copying a string using array notation and pointer notation.
#include <iostream.h>
void copy1( char *, const char * ); // prototype
void copy2( char *, const char * ); // prototype
int main() {
char string1[ 10 ];
char *string2 = "Hello";
char string3[ 10 ];
char string4[] = "Good Bye";
copy1( string1, string2 );
cout << "string1 = " << string1 << endl;
copy2( string3, string4 );
cout << "string3 = " << string3 << endl;
return 0; // indicates successful termination
} // end main
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Example II … cont.
// copy s2 to s1 using array notation
void copy1( char *s1, const char *s2 ) {
for ( int i = 0; ( s1[ i ] = s2[ i ] ) != '\0'; i++ )
; // do nothing in body
} // end function copy1
// copy s2 to s1 using pointer notation
void copy2( char *s1, const char *s2 )
for ( ; ( *s1 = *s2 ) != '\0'; s1++, s2++ )
} // end function copy2
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Additional Notes
This lecture covers the following material from the textbook:
Chapter 5: Sections 5.6 – 5.8
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