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C++. Lecture 3 Monday, 14 July 2003. Arrays, Pointers, and Strings. Use of array in C++, multi-dimensional array, array argument passing Pointers Relation between pointer and array String, string processing functions. Arrays. Declaration of an array int c[12]; or const int max = 12;

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C

C++

Lecture 3

Monday, 14 July 2003


Arrays pointers and strings

Arrays, Pointers, and Strings

  • Use of array in C++, multi-dimensional array, array argument passing

  • Pointers

  • Relation between pointer and array

  • String, string processing functions


Arrays

Arrays

  • Declaration of an array

    int c[12];

  • or

    const int max = 12;

    int c[max];

  • References to an array c[ ]

    c[0], c[1], …, c[max-1]


Array initialization

Array Initialization

int n[5] = {1, 2, 3, 4, 5};

  • or

    int n[ ] = {1, ,2, 3, 4, 5};

  • n[0], n[1], …, n[4] are valid references

C.f. Fig. 4.3


Array of characters string

Array of Characters (String)

char string1[20];

char string2[ ] = "string literal";

  • Individual array element is a character

    string2[3] = 'y';

C.f. Fig. 4.12


Passing array

Passing Array

  • Passing an array name is equivalent to passing the address of the array

    int a[5]; // declare array

    func(a); // use function

    void func(int a[ ]) // define function

    {

    }

C.f Fig. 4.14


Array name is a constant address

Array Name is a constant Address

int a[100];

a is the address of a[0],

a+k is the address of a[k].

a–1, or a+100, is also a valid address; C/C++ cannot check array index out of bound error during compile or run time.


Multiple subscripted arrays

Multiple-Subscripted Arrays

int a[2][5];

  • A 2 by 5 array.

  • Initialization

    int a[2][5] = { {0,1,2,3,4},

    {0,2,4,6,8} };

  • Row major convention in C/C++


Argument passing for multi dimensional array

Argument Passing for Multi-Dimensional Array

int a[3][5];

func(a);

  • or

    func(a+1);

    void func(int b[ ][5])

    { ...

C.f Fig. 4.22


Pointers

Pointers

  • Pointer value holds the address of a variable.

  • Pointers are also typed, i.e, pointer to int is considered different from pointer to double.

  • E.g.,

    int *p;

    double *fpt;


Declaration of pointer variables

Declaration of Pointer Variables

int *p; // a pointer to int

int c; // an int variable

p = &c; // address of c is given to p

  • The value of variable p is the address of c.

  • The value at the address stored in p is the value of c.

    *p = 2; // dereferencing


Address operator

Address Operator &

int c, *p, a[100];

p = &c; // address of c

p = a; // address of array

p = &a[0]; // same as above


Dereferencing operator

Dereferencing Operator *

int *p, c;

c = 1;

p = &c; // let p point to c

j = *p; // j gets 1,


Operator precedence and associativity

Operator Precedence and Associativity

( ) [ ] left to right highest

++, --, &, * right to left unary

* / % left to right multiply

+ , - left to right additive

<<, >> left to right insertion

<, <=, >, >= left to right relational

==, != left to right equality

, left to right comma


Left to right association

Left to Right Association

  • a + b + c means (a+b) + c

  • Precedence

  • a + b*c means a + (b*c)


Adding parentheses to show the order of evaluation

Adding Parentheses ( … ) to show the order of evaluation

  • b += x = y - k*a[ j ] && 3 | u < v--;

  • See Appendix A, page 1214 for the complete "Operator Precedence Chart".


Answer

Answer

b += (x = ((y - (k*(a[ j ]))) && (3 | (u < (v--)))));


Call by reference with pointer

Call-by-Reference with Pointer

  • Passing a pointer can modify the value in calling program.

    int c;

    cube_it(&c);

    void cube_it(int *p)

    { ...

C.f. Fig.5.7


Pointer to const data and const pointers

Pointer to const data and const Pointers

void f(const int *p);

  • The data pointed by p cannot be modified.

    int * const p = &x; // p always

    // pointing to x

    const int *const p = &x; // p always

    // points to x, x cannot change


Relationship between pointers and arrays

Relationship between Pointers and Arrays

  • An expression of

    *(a + k)

  • is considered totally equivalent to

    a[k].

  • E.g., a[0] is the same as *a.


Multi dimensional array

Multi-Dimensional Array

  • In the declaration

    int a[3][7];

    we can view a as pointer to array of a[0], a[1], a[2], which themselves are pointers to int array a[i][j].

  • **a is the same as a[0][0].


Pointer of pointers

Pointer of Pointers

a

a[2][0] a[2][1]

a[0] a[1] a[2]

a[1][0] a[1][1] ...

a[0][0] a[0][1] a[0][2] ...


Arrays of pointers

Arrays of Pointers

char *suit[4] = { "Hearts", "Diamonds", "Clubs", "Spade"};

  • suit[0][0] refers to 'H',

  • suit[3][4] refers to 'e' in "Spade",

  • suit[2][20] is an illegal access.


Function pointer

Function Pointer

void (*f)(int);

  • f is a pointer to a function of the form void g(int).

  • Function names are like address, e.g,

    f = g;

    (*f)(5); // call the function


Fundamentals of characters and strings

Fundamentals of Characters and Strings

char c, s[10]; // declare variables

c = 'a'; // can be used as small int

s = "John Q"; // don't work, why?

  • You must do

    s[0] = 'J';

    s[1] = 'o';

    s[2] = 'h'; // etc

  • Or use string copy function


String initialization

String Initialization

char color[ ] = "blue";

char *colorPtr = "blue";

char color[ ] = {'b', 'l', 'u', 'e', '\0'};

  • What is the difference between the array color and the pointer colorPtr?


Read a string

Read a String

char word[20];

cin >> word; // cause problem

cin >> setw(20) >> word; // OK

  • Read an entire line

    char sentence[80];

    cin.getline(sentence, 80);


String processing functions string h

String Processing Functions <string.h>

char *strcpy(char *t, const char *s);

char *strncpy(char *t, const char *s, size_t n);

char *strcat(char *t, const char *s);

int strcmp(const char *s, const char *t);

size_t strlen(const char *s);


String processing

String Processing

  • Examples from Fig.5.30 and 5.31.

C.f. Fig 5.30 and 5.31


Exercise 4 29

Exercise 4.29

  • (The Sieve of Eratosthenes) A prime integer is any integer that is evenly divisible only by itself and 1. Use the sieve method to find prime.

    • Initialize a[1000] all to 1 (for true).

    • Starting from 2, 3, …, k, ..., set array indices which are all multiple of k to 0,

    • print the nonzero indices.


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