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Pointers

Contents

Basics of PointerArray and pointer

using array and pointer interchangeably Passing array(pointer) to functions

Dynamic memory allocationString and pointerReference vs. pointer

Pointer

- variables storing memory address

Intimately tied to array and stringOperators

*: deference or indirection operator, e.g. *p &: address operator, e.g. &x

Pointer: Declaration and Usage

void main() {

int i = 5;

// Q: 想儲存 i的位址 ? How?

// A: 宣告一個指標 pint *p = &i ;

// Q: 如何利用 p取出 i的值 ?

// A: 使用 * operatorcout << *p ;

// Q: 我可以印出 i的位址 (p的值 )?

// A: it’s OK!, 如下 :

cout << p;

}

Pointer: Declaration and Usage

void main() {

int i=5, j=6;

int *p = &i ;

// Q: 可以利用 p 來改變 i 的值 ?

// A: yes

*p = *p + 12;

// A: 此外， p 也可以改存其它變數的位址p = &j ;

j = *p + 2; cout << j;

}

Pointer vs. Array

在 C/C++ 程式寫作上， Array和 Pointer 好像雙 生兄弟，經常交換使用。

Array Name Pointer to First Element

main() {

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

cout << &a[0];

cout << a ;

// 既然 a 記錄 a[0] 的位址，我就可以 ...

cout << *a; // 印出 a[0]

cout << *(a+1) ; // 印出 a[1]

*(a+2) = 0;

}

Array Name Constant Pointer

main() {

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

int *p;

p = a ; // p = &a[0]

// 利用 p 印出 a[] 的內容for (int i=0;i<=4; i++)

{ cout << *p; p++;}

for (int i=0;i<=4; i++)

{ cout << *a; a++;}

}

Insight Pointer Arithmetic

1

600

a[0] a[1] a[2] a[3] a[4]

Addr

2 3 4 5

604 608 612 616

int a[5] = {1,2,3,4,5};p = a ;p++ ; p+= 2;

p

char a[5] = {‘a’,’b’,’c’,’d’};p = a ; p+= 3;cout << *p;

a

600

a[0] a[1] a[2] a[3] a[4]

Addr

b c d

601 602 603 604

p

p p

Using Array and Pointer Interchangeably

main() {

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

int *p, i ;

for (p=a, i=0 ;i<size; i++)

cout << *p++;

for (p=a, i=0 ; i<size; i++)

cout << p[i];

}

Passing Array to Function

const int size = 5 ;

main() {

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

arr_mul2(a, size); // a &a[0]}

void arr_mul2(int* p, int n) {

for(int i = 0; i<n; i++)

p[i] *= 2;

}

Passing Array to Function

EX:what’s the result?

const int size = 5 ;

void arr_mul2(int *, int);

main() {

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

arr_mul2(&a[1], size-1);

arr_mul2(a+2, size-1);

}

void arr_mul2(int* p, int n) { …… }

Passing Arrays to Functions: Parameter-list Declaration

float arr_mul2(int* , int n) ;// prototypefloat arr_mul2(int *p, int n){ …… } //define

float arr_mul2(int[] , int n) ;// prototypefloat arr_mul2(int p[], int n){ …… } //define

float arr_mul2(int* , int n) ;// prototypefloat arr_mul2(int p[], int n){ …… } //define

float arr_mul2(int[] , int n) ;// prototypefloat arr_mul2(int *p, int n){ …… } //define

Dynamic Memory Allocation

#include <iostream.h>#include <stdlib.h>void main() {

int *p;p = (int *)calloc(10,sizeof(int));for (int i = 0; i<=9; i++)

p[i] = i ;}

Why dynamic memory allocation?

Dynamic Memory Allocation

Allocation:C: calloc(), malloc() C++: new

De-allocation:C: free

free(p);C++: delete

New

void main() { // C version int *p, *q, *r ; p = (int *)malloc(sizeof(int)); q = (int *)calloc(10,sizeof(int)); r = (int *)malloc(sizeof(int)); *r=10; }

void main() { // C++ version int *p, *q, *r ; p = new int ; //allocate one q = new int[10] ; //allocate array r = new int(10) ; //allocate&initialize}

Delete

void main() { int *p, *q, *r ;p = new int ;q = new int[10] ;r = new int(10) ;

…… delete p; // free one elementdelete []q; // free an arraydelete r;

}

Dynamic Memory Allocation

why? Advantages are …..

// int a[10] ;int *p ;int size ;cin >> size;p = new int[size];…….

Array of Pointers

char *sname[50]; // how about char sname[50][20];

s[0]

s[49]

char *sname[50], s[100] ;for (int i = 0 ; i<50; i++) {

cin >> s ;sname[i] = new char[strlen(s)+1];strcpy(sname,s);

}

Array of Pointer

char **sname ; // pointer to pointerint sno; char s[100];

cin >> sno;sname = new char*[sno];

for (int i = 0 ; i<sno; i++) {cin >> s ;sname[i] = new char[strlen(s)+1];strcpy(sname,s);

}

String(char *): pointer to char

void main() {char s1[10] = “hello” ;char s2[] = “hello” ;char *s3 = “hello”;char s4[] = {‘h’,’e’,’l’,’l’,’o’} ;// difference among s1,s2, s3 and s4

}

Utility Functions of String

strcmp: 比較 if (s1 == s2) {….} // ???

strcpy: 複製 s1 = s2 ; // ???

strcat: 連結 s1 = s1+s2; // ???

strlen: strlen(s1); // not including ‘\0’

[NOTE]: must include <string.h>

String

#include <string.h>

void main() {

char s1[] = "Hello";

char s2[] = "C++" ;

char s3[20];

if (strcmp(s1,s2) != 0) {

strcpy(s3,s1) ;

strcat(s3,s2);

}

cout << s3;

}

Self Test

int fun(const char* s1, const char* s2) {

int i ;for (i = 0 ; s1[i] && s2[i] && (s1[i]==s2[i]); ++i) ; // when to exit?return (s1[i]-s2[i]) ;

}

ANSI C++: string type

#include <iostream>#include <string>using namespace std;void main() {

string s1 = "Hello”, s2 = “World” ;string s3 ;s3 = s1 + " " + s2 ;cout << s3 << endl; cout << s3.length() << endl;if (s1 > s2) cout << s1 ;else cout << s2 ;

}

Reference Declaration- pointer free

void main() {int x=5 ;int *p = x ;int& xx = x ; // alias of x

xx = *p + 2; cout << x ;xx++ ;*p++ ;cout << x ;

}

(*p)++;

Reference to Array Elements

void main() {int x[5]= {1,2,3,4,5} ;int& first = x[0] ;int& last = x[4] ;

first *= 2 ;cout << x[0] << endl ;last = first + 2;cout << x[4] << endl;

}

Call by Reference

void change(int *, int&, int ) ;void main() {

int i=5, j=6, k = 7 ;cout << i << j << k << endl;change(&i, j, k);cout << i << j << k << endl;

}void change(int* p,int& q, int r) {

q = *p + r;*p = q + r;r = *p + q;

}

Reference to pointer

int x = 1, y = 2;

int *p = &x;

int*& pp = p ; // alias of p

(*p)++; cout << x << y << endl;

pp = &y ;

(*p)++ ; cout << x << y << endl;

Chapter 4 Implementing ADTs Using Base Language

- final review for C-base features- prepare marching to Class

Built-in Aggregate Data Type

Array

Enumeration

Structure

Union: self-reading

Array

Array of build-in data type int a[10], double x[100]; int n=10; char a[n]; // not ok! const int n =10; char a[n] ; // ok

Array of composite date type struct complex { int x, y; }; complex x[100] ;

Enumeration

(1) Declare a type of a subset of integer(2) Each element with a name

// type name: test// range: 0-3// name of each element: // Spring(0), Summer(1), Fall(2), Winter(3)enum test {Spring, Summer, Fall, Winter} ;main() {

test x ;if (input == ‘s’) x = Spring ; // x = 0x = 0 ; x = 50; // illegal

}

Enumeration

enum season {Spring=1, Summer, Fall, Winter};enum tbound {lb = 18, avg=25, ub=38} ; main() {

season w_type ;tbound b ;….if (b >=lb && b <=avg)

w_type = Spring ;….

}

Structure

nested structure

pointer to structure dynamic memory allocation

passing structure to function by value by reference & pointer