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Pointers and Strings

Pointers and Strings. Outline Introduction Pointer Variable Declarations and Initialization Pointer Operators Calling Functions by Reference Using const with Pointers Selection Sort Using Pass-by-Reference. Introduction. Pointers Powerful, but difficult to master

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Pointers and Strings

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  1. Pointers and Strings OutlineIntroductionPointer Variable Declarations and InitializationPointer OperatorsCalling Functions by ReferenceUsing const with PointersSelection Sort Using Pass-by-Reference

  2. Introduction • Pointers • Powerful, but difficult to master • Enable pass-by-reference and allow the creation of dynamic data structure (e.g., linked lists, queues, etc.) • Close relationship with arrays and strings

  3. count countPtr count 7 7 Pointer Variable Declarations and Initialization • Pointer variables • Contain memory addresses as values • Normally, variable contains specific value (direct reference) • Pointers contain address of a variable that itself has specific value (indirect reference) • Indirection • Referencing value through pointer • Pointer declarations • *indicates variable is pointer int *myPtr; declares pointer to int, pointer of type int * • Multiple pointers require multiple asterisks int *myPtr1, *myPtr2;

  4. Pointer Variable Declarations and Initialization • Can declare pointers to any data type • Pointer initialization • Initialized to0, NULL, or address • 0orNULLpoints to nothing (null pointer) • NULLdefined in <iostream>

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

  6. Pointer Operators • *(indirection or dereferencing operator) • Returns synonym for the object its pointer operand points to. • *yPtrreturnsy(becauseyPtrpoints toy) • Example: cout << *yPtr << endl;  returns the value of y  cout << y << endl; • dereferenced pointer is lvalue *yPtr= 9;// assigns 9 to y cin >> *yPtr; // receive a value from input *and&are inverses of each other

  7. 1 // Fig. 8.4: fig08_04.cpp 2 // Using the & and * operators. 3 #include <iostream> 4 5 using std::cout; 6 using std::endl; 7 8 int main() 9 { 10 int a; // a is an integer 11 int *aPtr; // aPtr is a pointer to an integer 12 13 a = 7; 14 aPtr = &a; // aPtr assigned address of a 15 16 cout << "The address of a is " << &a 17 << "\nThe value of aPtr is " << aPtr; 18 19 cout << "\n\nThe value of a is " << a 20 << "\nThe value of *aPtr is " << *aPtr; 21 22 cout << "\n\nShowing that * and & are inverses of " 23 << "each other.\n&*aPtr = " << &*aPtr 24 << "\n*&aPtr = " << *&aPtr << endl; 25 * and & are inverses of each other

  8. 26 return0; // indicates successful termination 27 28 } // end main * and & are inverses; same result when both applied to aPtr The address of a is 0012FED4 The value of aPtr is 0012FED4 The value of a is 7 The value of *aPtr is 7 Showing that * and & are inverses of each other. &*aPtr = 0012FED4 *&aPtr = 0012FED4

  9. Calling Functions by Reference • 3 ways to pass arguments to function • Pass-by-value • Pass-by-reference with reference arguments • Pass-by-reference with pointer arguments • returncan return one value from function • Arguments passed to function using reference arguments • Modify original values of arguments • Eliminate the overhead of passing large objects! • More than one value “returned”

  10. Calling Functions by Reference • Pass-by-reference with pointer arguments • Simulate pass-by-reference • Use pointers and (*) indirection operator • Pass address of argument using &operator • Arrays not passed with & because array name already pointer (pointer to the first element in the array, &arrayName[0]) • * operator is used in the function as alias/nickname for variable • A function receiving an address as an argument must define a pointer parameter to receive the address

  11. 1 // Fig. 8.6: fig08_06.cpp 2 // Cube a variable using pass-by-value. 3 #include <iostream> 4 5 using std::cout; 6 using std::endl; 7 8 int cubeByValue( int ); // prototype 9 10 int main() 11 { 12 int number = 5; 13 14 cout << "The original value of number is " << number; 15 16 // pass number by value to cubeByValue 17 number = cubeByValue( number ); 18 19 cout << "\nThe new value of number is " << number << endl; 20 21 return0; // indicates successful termination 22 23 } // end main 24 Pass number by value; result returned by cubeByValue

  12. 25 // calculate and return cube of integer argument 26 int cubeByValue( int n ) 27 { 28 return n * n * n; // cube local variable n and return result 29 30 } // end function cubeByValue Cubes and returns local variable n cubeByValue receives parameter passed-by-value The original value of number is 5 The new value of number is 125

  13. 1 // Fig. 8.7: fig08_07.cpp 2 // Cube a variable using pass-by-reference 3 // with a pointer argument. 4 #include <iostream> 5 6 using std::cout; 7 using std::endl; 8 9 void cubeByReference( int * ); // prototype 10 11 int main() 12 { 13 int number = 5; 14 15 cout << "The original value of number is " << number; 16 17 // pass address of number to cubeByReference 18 cubeByReference( &number ); 19 20 cout << "\nThe new value of number is " << number << endl; 21 22 return0; // indicates successful termination 23 24 } // end main 25 Prototype indicates parameter is pointer to int Apply address operator & to pass address of number to cubeByReference cubeByReference modified variable number

  14. 26 // calculate cube of *nPtr; modifies variable number in main 27 void cubeByReference( int *nPtr ) 28 { 29 *nPtr = *nPtr * *nPtr * *nPtr; // cube *nPtr 30 31 } // end function cubeByReference cubeByReference receives address of int variable, i.e., pointer to an int Modify and access int variable using indirection operator * The original value of number is 5 The new value of number is 125

  15. Using const with Pointers • constqualifier • Value of variable should not be modified • constused when function does not need to change a variable • Principle of least privilege • Award a function enough access to accomplish its task, but no more • Four ways to pass pointer to function • Non-constant pointer to non-constant data • Highest amount of access is granted • Non-constant pointer to constant data • Constant pointer to non-constant data • Constant pointer to constant data • Least amount of access is granted

  16. 1 // Fig. 8.10: fig08_10.cpp 2 // Converting lowercase letters to uppercase letters 3 // using a non-constant pointer to non-constant data. 4 #include <iostream> 5 6 using std::cout; 7 using std::endl; 8 9 #include <cctype> // prototypes for islower and toupper 10 11 void convertToUppercase( char * ); 12 13 int main() 14 { 15 char phrase[] = "characters and $32.98"; 16 17 cout << "The phrase before conversion is: " << phrase; 18 convertToUppercase( phrase ); 19 cout << "\nThe phrase after conversion is: " 20 << phrase << endl; 21 22 return0; // indicates successful termination 23 24 } // end main 25 Parameter is non-constant pointer to non-constant data convertToUppercase modifies variable phrase

  17. 26 // convert string to uppercase letters 27 void convertToUppercase( char *sPtr ) 28 { 29 while ( *sPtr != '\0' ) { // current character is not '\0' 30 31 if ( islower( *sPtr ) ) // if character is lowercase, 32 *sPtr = toupper( *sPtr ); // convert to uppercase 33 34 ++sPtr; // move sPtr to next character in string 35 36 } // end while 37 38 } // end function convertToUppercase Parameter sPtr non-constant pointer to non-constant data Function islower returns true if character is lowercase Function toupper returns corresponding uppercase character if original character lowercase; otherwise toupper returns original (uppercase) character When operator ++ applied to pointer that points to array, memory address stored in pointer modified to point to next element of array. The phrase before conversion is: characters and $32.98 The phrase after conversion is: CHARACTERS AND $32.98

  18. 1 // Fig. 8.11: fig08_11.cpp 2 // Printing a string one character at a time using 3 // a non-constant pointer to constant data. 4 #include <iostream> 5 6 using std::cout; 7 using std::endl; 8 9 void printCharacters( const char * ); 10 11 int main() 12 { 13 const char phrase[] = "print characters of a string"; 14 15 cout << "The string is:\n"; 16 printCharacters( phrase ); 17 cout << endl; 18 19 return0; // indicates successful termination 20 21 } // end main 22 Parameter is non-constant pointer to constant data. Pass pointer phrase to function printCharacters.  a non-constant pointer to constant data is a pointer that can be modified to point to any data item of the appropriate type, but the data to which it points cannot be modified through that pointer

  19. 23 // sPtr cannot modify the character to which it points, 24 // i.e., sPtr is a "read-only" pointer 25 void printCharacters( const char *sPtr ) 26 { 27 for ( ; *sPtr != '\0'; sPtr++ ) // no initialization 28 cout << *sPtr; 29 30 } // end function printCharacters Increment sPtr to point to next character. sPtr is non-constant pointer to constant data; cannot modify character to which sPtr points. The string is: print characters of a string

  20. 1 // Fig. 8.12: fig08_12.cpp 2 // Attempting to modify data through a 3 // non-constant pointer to constant data. 4 5 void f( const int * ); // prototype 6 7 int main() 8 { 9 int y; 10 11 f( &y ); // f attempts illegal modification 12 13 return0; // indicates successful termination 14 15 } // end main 16 17 // xPtr cannot modify the value of the variable 18 // to which it points 19 void f( const int *xPtr ) 20 { 21 *xPtr = 100; // error: cannot modify a const object 22 23 } // end function f Parameter is non-constant pointer to constant data. Pass address of int variable y to attempt illegal modification. Attempt to modify const object pointed to by xPtr. Error produced when attempting to compile. d:\cpphtp8_examples\ch08\Fig08_12.cpp(21) : error C2166: l-value specifies const object

  21. Using constwith Pointers • constpointers • Always point to same memory location • Default for array name • Must be initialized when declared

  22. 1 // Fig. 8.13: fig08_13.cpp 2 // Attempting to modify a constant pointer to 3 // non-constant data. 4 5 int main() 6 { 7 int x, y; 8 9 // ptr is a constant pointer to an integer that can 10 // be modified through ptr, but ptr always points to the 11 // same memory location. 12 int * const ptr = &x; 13 14 *ptr = 7; // allowed: *ptr is not const 15 ptr = &y; // error: ptr is const; cannot assign new address 16 17 return0; // indicates successful termination 18 19 } // end main ptr is constant pointer to integer. Can modify x (pointed to by ptr) since x not constant. Cannot modify ptr to point to new address since ptr is constant. Line 15 generates compiler error by attempting to assign new address to constant pointer. d:\cpphtp8_examples\ch08\Fig08_13.cpp(15) : error C2166: l-value specifies const object  a constant pointer to non-constant data is a pointer that always points to the same memory location. Data at that location can be changed through the pointer. (e.g., array name)

  23. 1 // Fig. 8.14: fig08_14.cpp 2 // Attempting to modify a constant pointer to constant data. 3 #include <iostream> 4 5 using std::cout; 6 using std::endl; 7 8 int main() 9 { 10 int x = 5, y; 11 12 // ptr is a constant pointer to a constant integer. 13 // ptr always points to the same location; the integer 14 // at that location cannot be modified. 15 const int * const ptr = &x; 16 17 cout << *ptr << endl; 18 19 *ptr = 7; // error: *ptr is const; cannot assign new value 20 ptr = &y; // error: ptr is const; cannot assign new address 21 22 return0; // indicates successful termination 23 24 } // end main ptr is constant pointer to integer constant. Cannot modify ptr to point to new address since ptr is constant. Cannot modify x (pointed to by ptr) since *ptr declared constant.  a constant pointer to a constant datais a pointer that always points to the same memory location and data at that location cannot be modified.

  24. d:\cpphtp8_examples\ch08\Fig08_14.cpp(19) : error C2166: l-value specifies const object d:\cpphtp8_examples\ch08\Fig08_14.cpp(20) : error C2166: l-value specifies const object Line 19 generates compiler error by attempting to modify constant object. Line 20 generates compiler error by attempting to assign new address to constant pointer.

  25. Selection Sort Using Pass-by-Reference • Implement selectionSortusing pointers • Want function swapto access array elements • Individual array elements: scalars • Passed by value “by default” • Pass by reference using address operator& • Selection sort: • First iteration: select smallest element in array and swap it with first element of array • Second iteration: select second smallest and swap it with second element in array • Etc…..

  26. 1 // Fig. 8.15: fig08_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 selectionSort( int *, constint ); // prototype 14 void swap( int * const, int * const ); // prototype 15 16 int main() 17 { 18 const intarraySize = 10; 19 int a[ arraySize ] = { 2, 6, 4, 8, 10, 12, 89, 68, 45, 37 }; 20 21 cout << "Data items in original order\n"; 22 23 for ( int i = 0; i < arraySize; i++ ) 24 cout << setw( 4 ) << a[ i ]; 25

  27. 26 selectionSort( a, arraySize ); // sort the array 27 28 cout << "\nData items in ascending order\n"; 29 30 for ( int j = 0; j < arraySize; j++ ) 31 cout << setw( 4 ) << a[ j ]; 32 33 cout << endl; 34 35 return0; // indicates successful termination 36 37 } // end main 38 39 // sort an array of integers using selection sort algorithm 40 void selectionSort( int *array, const int size ) 41 { 42 int smallest; 43 for ( int i = 0; i < size - 1; i++ ) // loop over size-1 elements 44 { 45 smallest = i; 46 for ( int index = i + 1; index < size; index++ ) 47 if ( array[ index ] < array[ smallest ] ) 48 smallest = index; 49 // swap adjacent elements if they are out of order 50 swap( &array[ i ], &array[ smallest ] ); 51 } // end for 52 } // end function selectionSort Receives size of array as argument; declared const to ensure size not modified. Declare as int *array (rather than int array[]) to indicate function selectionSort receives single-subscripted array.

  28. 53 54 // swap values at memory locations to which 55 // element1Ptr and element2Ptr point 56 void swap( int * const element1Ptr, int * const element2Ptr ) 57 { 58 int hold = *element1Ptr; 59 *element1Ptr = *element2Ptr; 60 *element2Ptr = hold; 61 62 } // end function swap Pass arguments by reference, allowing function to swap values at memory locations. 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

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