Pointers and Strings

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


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

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


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>


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


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


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


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”


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


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


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


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


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 *


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


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


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.


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


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.


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.


21

Using const with Pointers

• const pointers– Always point to same memory location– Default for array name– Must be initialized when declared


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


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)


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.


Outline24



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 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…..


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


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.


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.

Pointers and Strings

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