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2003 Prentice Hall, Inc. All rights reserved.
1
Pointers and Strings
OutlineIntroductionPointer Variable Declarations and InitializationPointer OperatorsCalling Functions by ReferenceUsing const with PointersSelection Sort Using Pass-by-Reference
2003 Prentice Hall, Inc. All rights reserved.
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
2003 Prentice Hall, Inc. All rights reserved.
3
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;
count
7
countPtr
count
7
2003 Prentice Hall, Inc. All rights reserved.
4
Pointer Variable Declarations and Initialization
• Can declare pointers to any data type• Pointer initialization
– Initialized to 0, NULL, or address• 0 or NULL points to nothing (null pointer)• NULL defined in <iostream>
2003 Prentice Hall, Inc. All rights reserved.
5
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
yPtr
y5
yptr
500000 600000
y
600000 5
address of y is value of yptr
2003 Prentice Hall, Inc. All rights reserved.
6
Pointer Operators
• * (indirection or dereferencing operator)– Returns synonym for the object its pointer operand
points to.– *yPtr returns y (because yPtr points to y )
• 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
2003 Prentice Hall, Inc.All rights reserved.
Outline7
1 // Fig. 8.4: fig08_04.cpp2 // 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 integer11 int *aPtr; // aPtr is a pointer to an integer12 13 a = 7;14 aPtr = &a; // aPtr assigned address of a15 16 cout << "The address of a is " << &a17 << "\nThe value of aPtr is " << aPtr;18 19 cout << "\n\nThe value of a is " << a20 << "\nThe value of *aPtr is " << *aPtr;21 22 cout << "\n\nShowing that * and & are inverses of "23 << "each other.\n&*aPtr = " << &*aPtr24 << "\n*&aPtr = " << *&aPtr << endl;25
* and & are inverses of each other
2003 Prentice Hall, Inc.All rights reserved.
Outline8
26 return 0; // indicates successful termination27 28 } // end main
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 * and & are inverses; same result when both applied to aPtr
2003 Prentice Hall, Inc. All rights reserved.
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
• return can 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”
2003 Prentice Hall, Inc. All rights reserved.
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
2003 Prentice Hall, Inc.All rights reserved.
Outline11
1 // Fig. 8.6: fig08_06.cpp2 // Cube a variable using pass-by-value.3 #include <iostream>4 5 using std::cout;6 using std::endl;7 8 int cubeByValue( int ); // prototype9 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 cubeByValue17 number = cubeByValue( number );18 19 cout << "\nThe new value of number is " << number << endl;20 21 return 0; // indicates successful termination22 23 } // end main24
Pass number by value; result returned by cubeByValue
2003 Prentice Hall, Inc.All rights reserved.
Outline12
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 result29 30 } // end function cubeByValue
The original value of number is 5
The new value of number is 125
cubeByValue receives parameter passed-by-value
Cubes and returns local variable n
2003 Prentice Hall, Inc.All rights reserved.
Outline13
1 // Fig. 8.7: fig08_07.cpp2 // 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 * ); // prototype10 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 cubeByReference18 cubeByReference( &number );19 20 cout << "\nThe new value of number is " << number << endl;21 22 return 0; // indicates successful termination23 24 } // end main25
Apply address operator & to pass address of number to cubeByReference
cubeByReference modified variable number
Prototype indicates parameter is pointer to int
2003 Prentice Hall, Inc.All rights reserved.
Outline14
26 // calculate cube of *nPtr; modifies variable number in main27 void cubeByReference( int *nPtr ) 28 { 29 *nPtr = *nPtr * *nPtr * *nPtr; // cube *nPtr 30 31 } // end function cubeByReference
The original value of number is 5
The new value of number is 125
cubeByReference receives address of int variable,i.e., pointer to an int
Modify and access int variable using indirection operator *
2003 Prentice Hall, Inc. All rights reserved.
15
Using const with Pointers
• const qualifier– Value of variable should not be modified– const used 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
2003 Prentice Hall, Inc.All rights reserved.
Outline16
1 // Fig. 8.10: fig08_10.cpp2 // Converting lowercase letters to uppercase letters3 // 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 toupper10 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 return 0; // indicates successful termination23 24 } // end main25
Parameter is non-constant pointer to non-constant data
convertToUppercase modifies variable phrase
2003 Prentice Hall, Inc.All rights reserved.
Outline17
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
The phrase before conversion is: characters and $32.98
The phrase after conversion is: CHARACTERS AND $32.98
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.
2003 Prentice Hall, Inc.All rights reserved.
Outline18
1 // Fig. 8.11: fig08_11.cpp2 // Printing a string one character at a time using3 // 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 return 0; // indicates successful termination20 21 } // end main22
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
2003 Prentice Hall, Inc.All rights reserved.
Outline19
23 // sPtr cannot modify the character to which it points,24 // i.e., sPtr is a "read-only" pointer25 void printCharacters( const char *sPtr )26 {27 for ( ; *sPtr != '\0'; sPtr++ ) // no initialization28 cout << *sPtr; 29 30 } // end function printCharacters
The string is:
print characters of a string
sPtr is non-constant pointer to constant data; cannot modify character to which sPtr points.Increment sPtr to point to next character.
2003 Prentice Hall, Inc.All rights reserved.
Outline20
1 // Fig. 8.12: fig08_12.cpp2 // Attempting to modify data through a3 // non-constant pointer to constant data.4 5 void f( const int * ); // prototype6 7 int main()8 {9 int y;10 11 f( &y ); // f attempts illegal modification12 13 return 0; // indicates successful termination14 15 } // end main16 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 object22 23 } // end function f
d:\cpphtp8_examples\ch08\Fig08_12.cpp(21) : error C2166: l-value specifies const object
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.
2003 Prentice Hall, Inc. All rights reserved.
21
Using const with Pointers
• const pointers– Always point to same memory location– Default for array name– Must be initialized when declared
2003 Prentice Hall, Inc.All rights reserved.
Outline22
1 // Fig. 8.13: fig08_13.cpp2 // Attempting to modify a constant pointer to3 // 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 the11 // same memory location. 12 int * const ptr = &x; 13 14 *ptr = 7; // allowed: *ptr is not const15 ptr = &y; // error: ptr is const; cannot assign new address16 17 return 0; // indicates successful termination18 19 } // end main
d:\cpphtp8_examples\ch08\Fig08_13.cpp(15) : error C2166: l-value specifies const object
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.
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)
2003 Prentice Hall, Inc.All rights reserved.
Outline23
1 // Fig. 8.14: fig08_14.cpp2 // 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 integer14 // 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 address21 22 return 0; // indicates successful termination23 24 } // end main
ptr is constant pointer to integer constant.
Cannot modify x (pointed to by ptr) since *ptr declared constant.Cannot modify ptr to point to new address since ptr is constant.
a constant pointer to a constant data is a pointer that always points to the same memory location and data at that location cannot be modified.
2003 Prentice Hall, Inc.All rights reserved.
Outline24
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.
2003 Prentice Hall, Inc. All rights reserved.
25
Selection Sort Using Pass-by-Reference
• Implement selectionSort using pointers– Want function swap to 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…..
2003 Prentice Hall, Inc.All rights reserved.
Outline26
1 // Fig. 8.15: fig08_15.cpp2 // This program puts values into an array, sorts the values into3 // 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 *, const int ); // prototype14 void swap( int * const, int * const ); // prototype15 16 int main()17 {18 const int arraySize = 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
2003 Prentice Hall, Inc.All rights reserved.
Outline27
26 selectionSort( a, arraySize ); // sort the array27 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 return 0; // indicates successful termination36 37 } // end main38 39 // sort an array of integers using selection sort algorithm40 void selectionSort( int *array, const int size )41 {42 int smallest;43 for ( int i = 0; i < size - 1; i++ ) // loop over size-1 elements44 {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 order50 swap( &array[ i ], &array[ smallest ] );51 } // end for52 } // end function selectionSort
Declare as int *array (rather than int array[]) to indicate function selectionSort receives single-subscripted array.
Receives size of array as argument; declared const to ensure size not modified.
2003 Prentice Hall, Inc.All rights reserved.
Outline28
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
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
Pass arguments by reference, allowing function to swap values at memory locations.