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Lecture 12
Pointers and Strings
Section 5.4, 5.7-5.10, 5.12.1
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5.4Calling 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 More than one value “returned”
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5.4Calling 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 * operator used as alias/nickname for variable
inside of function
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fig05_06.cpp(1 of 2)
1 // Fig. 5.6: fig05_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 );
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Pass number by value; result returned by cubeByValue
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fig05_06.cpp(2 of 2)
fig05_06.cppoutput (1 of 1)
19 cout << "\nThe new value of number is " << number << endl;
20 21 return 0; // indicates successful termination
22 23 } // end main
24 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
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
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fig05_07.cpp(1 of 2)
1 // Fig. 5.7: fig05_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 );
Apply address operator & to pass address of number to cubeByReference
Prototype indicates parameter is pointer to int
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fig05_07.cpp(2 of 2)
fig05_07.cppoutput (1 of 1)
19 20 cout << "\nThe new value of number is " << number << endl;
21 22 return 0; // indicates successful termination
23 24 } // end main
25 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
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 *
cubeByReference modified variable number
int value = *nPtr;*nPtr = value * value * value;
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5.7Pointer Expressions and Pointer Arithmetic
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 meaningless unless performed on pointer to
array 5 element int array on a machine using 4 byte ints
vPtr points to first element v[ 0 ], which is at location 3000vPtr = 3000
vPtr += 2; sets vPtr to 3008vPtr points to v[ 2 ]
pointer variable vPtr
v[0] v[1] v[2] v[4]v[3]
3000 3004 3008 3012 3016location
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5.7Pointer Expressions and Pointer Arithmetic Subtracting pointers
Returns number of elements between two addressesvPtr2 = v[ 2 ];vPtr = v[ 0 ];vPtr2 - vPtr == 2
Pointer assignment Pointer can be assigned to another pointer if both of same type If not same type, cast operator must be used Exception: pointer to void (type void *)
Generic pointer, represents any type No casting needed to convert pointer to void pointer void pointers cannot be dereferenced
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5.7Pointer Expressions and Pointer Arithmetic Pointer comparison
Use equality and relational operators Comparisons meaningless unless pointers point
to members of same array Compare addresses stored in pointers Example: could show that one pointer points to
higher numbered element of array than other pointer
Common use to determine whether pointer is 0 (does not point to anything) (ptr != NULL)
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5.8Relationship Between Pointers and Arrays Arrays and pointers closely related
Array name like constant pointer Pointers can do array subscripting operations
Accessing array elements with pointers Element b[ n ] can be accessed by *( bPtr + n )
Called pointer/offset notation Addresses
&b[ 3 ] same as bPtr + 3 Array name can be treated as pointer
b[ 3 ] same as *( b + 3 ) Pointers can be subscripted (pointer/subscript notation)
bPtr[ 3 ] same as b[ 3 ]
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fig05_20.cpp(1 of 3)
1 // Fig. 5.20: fig05_20.cpp
2 // Using subscripting and pointer notations with arrays.
3 4 #include <iostream>
5 6 using std::cout;
7 using std::endl;
8 9 int main()
10 {11 int b[] = { 10, 20, 30, 40 };
12 int *bPtr = b; // set bPtr to point to array b
13 14 // output array b using array subscript notation
15 cout << "Array b printed with:\n"
16 << "Array subscript notation\n";
17 18 for ( int i = 0; i < 4; i++ )
19 cout << "b[" << i << "] = " << b[ i ] << '\n';
Using array subscript notation.
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fig05_20.cpp(2 of 3)
21 // output array b using the array name and
22 // pointer/offset notation
23 cout << "\nPointer/offset notation where "
24 << "the pointer is the array name\n";
25 26 for ( int offset1 = 0; offset1 < 4; offset1++ )
27 cout << "*(b + " << offset1 << ") = "
28 << *( b + offset1 ) << '\n';
29 30 // output array b using bPtr and array subscript notation
31 cout << "\nPointer subscript notation\n";
32 33 for ( int j = 0; j < 4; j++ )
34 cout << "bPtr[" << j << "] = " << bPtr[ j ] << '\n';
35 36 cout << "\nPointer/offset notation\n";
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Using array name and pointer/offset notation.
Using pointer subscript notation.
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fig05_20.cpp(3 of 3)
38 // output array b using bPtr and pointer/offset notation
39 for ( int offset2 = 0; offset2 < 4; offset2++ )
40 cout << "*(bPtr + " << offset2 << ") = "
41 << *( bPtr + offset2 ) << '\n';
42 43 return 0; // indicates successful termination
44 45 } // end main
Using bPtr and pointer/offset notation.
Array b printed with:
Array subscript notation
b[0] = 10
b[1] = 20
b[2] = 30
b[3] = 40
fig05_20.cppoutput (1 of 2)
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Pointer/offset notation where the pointer is the array name
*(b + 0) = 10
*(b + 1) = 20
*(b + 2) = 30
*(b + 3) = 40
Pointer subscript notation
bPtr[0] = 10
bPtr[1] = 20
bPtr[2] = 30
bPtr[3] = 40
Pointer/offset notation
*(bPtr + 0) = 10
*(bPtr + 1) = 20
*(bPtr + 2) = 30
*(bPtr + 3) = 40
fig05_20.cppoutput (2 of 2)
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fig05_21.cpp(1 of 3)
1 // Fig. 5.21: fig05_21.cpp
2 // Copying a string using array notation
3 // and pointer notation.
4 #include <iostream>
5 6 using std::cout;
7 using std::endl;
8 9 void copy1( char *, const char * ); // prototype
10 void copy2( char *, const char * ); // prototype
11 12 int main()
13 {14 char string1[ 10 ];
15 char *string2 = "Hello";
16 char string3[ 10 ];
17 char string4[] = "Good Bye";
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fig05_21.cpp(2 of 3)
19 copy1( string1, string2 );
20 cout << "string1 = " << string1 << endl;
21 22 copy2( string3, string4 );
23 cout << "string3 = " << string3 << endl;
24 25 return 0; // indicates successful termination
26 27 } // end main
28 29 // copy s2 to s1 using array notation
30 void copy1( char *s1, const char *s2 )
31 {32 for ( int i = 0; ( s1[ i ] = s2[ i ] ) != '\0'; i++ )
33 ; // do nothing in body
34 35 } // end function copy1
Use array subscript notation to copy string in s2 to character array s1.
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fig05_21.cpp(3 of 3)
fig05_21.cppoutput (1 of 1)
37 // copy s2 to s1 using pointer notation
38 void copy2( char *s1, const char *s2 )
39 {40 for ( ; ( *s1 = *s2 ) != '\0'; s1++, s2++ )
41 ; // do nothing in body
42 43 } // end function copy2
string1 = Hello
string3 = Good Bye
Use pointer notation to copy string in s2 to character array in s1.
Increment both pointers to point to next elements in corresponding arrays.
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5.9Arrays of Pointers
Arrays can contain pointers Commonly used to store array of strings
char *suit[ 4 ] = {"Hearts", "Diamonds", "Clubs", "Spades" };
Each element of suit points to char * (a string) Array does not store strings, only pointers to strings
suit array has fixed size, but strings can be of any size
suit[3]
suit[2]
suit[1]
suit[0] ’H’ ’e’ ’a’ ’r’ ’t’ ’s’ ’\0’
’D’ ’i’ ’a’ ’m’ ’o’ ’n’ ’d’ ’s’ ’\0’
’C’ ’l’ ’u’ ’b’ ’s’ ’\0’
’S’ ’p’ ’a’ ’d’ ’e’ ’s’ ’\0’
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5.10 Case Study: Card Shuffling and Dealing Simulation Card shuffling program
Use an array of pointers to strings, to store suit names Use a double scripted array (suit by value)
Place 1-52 into the array to specify the order in which the cards are dealt
deck[2][12] represents the King of Clubs
HeartsDiamondsClubsSpades
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1
2
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Ace Two ThreeFour Five Six SevenEightNine Ten Jack QueenKing0 1 2 3 4 5 6 7 8 9 10 11 12
Clubs King
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5.10 Case Study: Card Shuffling and Dealing Simulation Pseudocode for shuffling and dealing
simulation
For each of the 52 cards
Place card number in randomly selected unoccupied slot of deck
For each of the 52 cards
Find card number in deck array and print face and suit of card
Choose slot of deck randomly
While chosen slot of deck has been previously chosen
Choose slot of deck randomlyPlace card number in chosen slot of deck
For each slot of the deck array
If slot contains card number Print the face and suit of the card
Second refinement
Third refinement
First refinement
Initialize the suit arrayInitialize the face arrayInitialize the deck array
Shuffle the deck
Deal 52 cards
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fig05_24.cpp(1 of 4)
1 // Fig. 5.24: fig05_24.cpp
2 // Card shuffling dealing program.
3 #include <iostream>
4 5 using std::cout;
6 using std::left;
7 using std::right;
8 9 #include <iomanip>
10 11 using std::setw;
12 13 #include <cstdlib> // prototypes for rand and srand
14 #include <ctime> // prototype for time
15 16 // prototypes
17 void shuffle( int [][ 13 ] );
18 void deal( const int [][ 13 ], const char *[], const char *[] );
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fig05_24.cpp(2 of 4)
20 int main() {21 // initialize suit array
22 const char *suit[ 4 ] =
23 { "Hearts", "Diamonds", "Clubs", "Spades" };
24 25 // initialize face array
26 const char *face[ 13 ] = { "Ace", "Deuce", "Three", "Four", "Five",
27 "Six", "Seven", "Eight", "Nine", "Ten", "Jack", "Queen", "King" };
28 29 // initialize deck array
30 int deck[ 4 ][ 13 ] = { 0 };
31 32 srand( time( 0 ) ); // seed random number generator
33 34 shuffle( deck );
35 deal( deck, face, suit ); 36 return 0; // indicates successful termination
37 } // end main
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face array contains pointers to char arrays.
suit array contains pointers to char arrays.
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fig05_24.cpp(3 of 4)
39 // shuffle cards in deck
40 void shuffle( int wDeck[][ 13 ] ) {41 int row;
42 int column;
43 44 // for each of the 52 cards, choose slot of deck randomly
45 for ( int card = 1; card <= 52; card++ ) { 46 // choose new random location until unoccupied slot found
47 do {
48 row = rand() % 4;
49 column = rand() % 13;
50 } while ( wDeck[ row ][ column ] != 0 ); // end do/while
51 // place card number in chosen slot of deck
52 wDeck[ row ][ column ] = card;
53 54 } // end for
55 56 } // end function shuffle
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Current position is at randomly selected row and column.
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fig05_24.cpp(4 of 4)
58 // deal cards in deck
59 void deal( const int wDeck[][ 13 ], const char *wFace[],
60 const char *wSuit[] ) {61 // for each of the 52 cards
62 for ( int card = 1; card <= 52; card++ ) 63 // loop through rows of wDeck
64 for ( int row = 0; row <= 3; row++ )
65 66 // loop through columns of wDeck for current row
67 for ( int column = 0; column <= 12; column++ )
68 69 // if slot contains current card, display card
70 if ( wDeck[ row ][ column ] == card ) {
71 cout << setw( 5 ) << right << wFace[ column ]
72 << " of " << setw( 8 ) << left
73 << wSuit[ row ]
74 << ( card % 2 == 0 ? '\n' : '\t' );
75 } // end if
76 } // end function deal
Cause face to be output right justified in field of 5 characters.
Cause suit to be output left justified in field of 8 characters.
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Nine of Spades Seven of Clubs
Five of Spades Eight of Clubs
Queen of Diamonds Three of Hearts
Jack of Spades Five of Diamonds
Jack of Diamonds Three of Diamonds
Three of Clubs Six of Clubs
Ten of Clubs Nine of Diamonds
Ace of Hearts Queen of Hearts
Seven of Spades Deuce of Spades
Six of Hearts Deuce of Clubs
Ace of Clubs Deuce of Diamonds
Nine of Hearts Seven of Diamonds
Six of Spades Eight of Diamonds
Ten of Spades King of Hearts
Four of Clubs Ace of Spades
Ten of Hearts Four of Spades
Eight of Hearts Eight of Spades
Jack of Hearts Ten of Diamonds
Four of Diamonds King of Diamonds
Seven of Hearts King of Spades
Queen of Spades Four of Hearts
Nine of Clubs Six of Diamonds
Deuce of Hearts Jack of Clubs
King of Clubs Three of Spades
Queen of Clubs Five of Clubs
Five of Hearts Ace of Diamonds
fig05_24.cppoutput (1 of 1)
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5.12.1 Fundamentals of Characters and Strings Character constant
Integer value represented as character in single quotes 'z' is integer value of z
122 in ASCII String
Series of characters treated as single unit Can include letters, digits, special characters +, -, * ... String literal (string constants)
Enclosed in double quotes, for example:"I like C++"
Array of characters, ends with null character '\0' String is constant pointer
Pointer to string’s first character Like arrays
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5.12.1 Fundamentals of Characters and Strings
String assignment Character array
char color[] = "blue"; Creates 5 element char array color
last element is '\0' Variable of type char *
char *colorPtr = "blue"; Creates pointer colorPtr to letter b in string “blue”
“blue” somewhere in memory
Alternative for character array char color[] = { ‘b’, ‘l’, ‘u’, ‘e’, ‘\0’ };
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5.12.1 Fundamentals of Characters and Strings Reading strings
Assign input to character array word[ 20 ]cin >> word
Reads characters until whitespace or EOF String could exceed array size
cin >> setw( 20 ) >> word; Reads 19 characters (space reserved for '\0')
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5.12.1 Fundamentals of Characters and Strings cin.getline
Read line of text cin.getline( array, size, delimiter ); Copies input into specified array until either
One less than size is reached delimiter character is input
Examplechar sentence[ 80 ];cin.getline( sentence, 80, '\n' );orstring sent;getline(cin, sent, ‘\n’);