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C++ Programming Lecture 18 Pointers – Part II

C++ Programming Lecture 18 Pointers – Part II. By Ghada Al-Mashaqbeh The Hashemite University Computer Engineering Department. Outline. Introduction. sizeof Operator and Arrays. sizeof Operator and Pointers. Bubble Sort Using Call-by-reference.

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C++ Programming Lecture 18 Pointers – Part II

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  1. C++ ProgrammingLecture 18Pointers – Part II By Ghada Al-Mashaqbeh The Hashemite University Computer Engineering Department

  2. 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. The Hashemite University

  3. sizeof Operator and Arrays • sizeof • 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); The Hashemite University

  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. The Hashemite University

  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 The Hashemite University

  6. 1 // Fig. 5.15: fig05_15.cpp 2 // This program puts values into an array, sorts the values into Bubblesort gets passed the address of array elements (pointers). The name of an array is a pointer. 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++ ) 24 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++ ) 30 cout << setw( 4 ) << a[ i ]; 31 32 cout << endl; 33 return 0; 34 } The Hashemite University

  7. 36 void bubbleSort( int *array, const int size ) 37 { 38 void swap( int * const, int * const ); 39 swap takes pointers (addresses of array elements) and dereferences them to modify the original array elements. 40 for ( int pass = 0; pass < size - 1; pass++ ) 41 42 for ( int j = 0; j < size - 1; j++ ) 43 44 if ( array[ j ] > array[ j + 1 ] ) 45 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 } 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 68 89 The Hashemite University

  8. location 3000 3004 3008 3012 3016 pointer variable vPtr v[0] v[1] v[2] v[4] v[3] Pointer Expressions and Pointer Arithmetic I • Pointer arithmetic • 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 ] The Hashemite University

  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 The Hashemite University

  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 } The Hashemite University

  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 ] The Hashemite University

  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 ] The Hashemite University

  13. 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 The Hashemite University

  14. 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 The Hashemite University

  15. 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 The Hashemite University

  16. 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++ ) ; // do nothing in body } // end function copy2 The Hashemite University

  17. Additional Notes • This lecture covers the following material from the textbook: • Chapter 5: Sections 5.6 – 5.8 The Hashemite University

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