Lecture 25: Practical Training on Pointers. 2 Lecture Contents: t Reminder on Arrays t Reminder on Strings t Reminder on Pointers t Demo programs, Exercises

Lecture 25:Practical Training on Pointers

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Lecture Contents:

Reminder on Arrays Reminder on Strings Reminder on Pointers Demo programs, Exercises Quiz on Pointers and Strings

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Reminder on Arrays

Reminder on Arrays

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Array: a collection of data items of the same type

How to define (declare) an array:

int a[6];

float b[10][20];

char c[7][12][2];

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Arrays Language restrictions

– Subscripts are denoted as integer valued expressions within brackets: [ ]

– Base type can be • any fundamental type, or• library-defined type, or• programmer-defined type

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Arrays– The index type is always integer and the

index range must be0 ... n-1• where n is a programmer-defined constant

expression.

– Parameter passing style• Always call by reference (no indication

necessary)

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Remember Arrays are always passed by reference

– Artifact of C Can use const if array elements are not to be

modified You do not need to include the array size within

the brackets when defining an array parameter Initialize array with 0 or some other known value

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Reminder on Strings

Reminder on Strings

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Reminder on Strings

C style strings– 1D char arrays– char name[20];– Run time library functions (#include <cstring>)

C++ style strings– STL class string (#include <string>)– string name;– Operator + and Methods

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Reminder on Strings

C style strings– 1D array of characters ending with null

terminator ‘\0’ – first char in ASCII table– char name[20];– char city[7] = “Dallas”; // C-string– Char city2[] = {‘D’,’a’,’l’,’l’,’a’,’s’}; // array of

chars– Run time library functions (#include

<cstring>)

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Reminder on Strings

C style strings. Input Output of C strings– Suppose s is array for C string– cout << s; // to display on the console– Reading string from keyboard– char city[9]; cin >> city; // New York– Problem: input ends with whitespace character– Solution: use cin.getline() fun from

<iostream>– cin.getline(city, 80, ’\n’);

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Reminder on Strings

C++ style strings– STL class string (#include <string>)– string name;– string message = “Programming”;– Operator + and Methods– length() message.length()– size() mssage.size()– at(index) message.at(0)

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Reminder on Strings

C++ style strings– By default initialized as empty string or

string with noo characters– Empty string literal is “”– String index .at(i)– Subsript operator: cout<< message[0];– Comparing strings: all six rel operators

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Reminder on Strings

C++ style strings– Reading strings– String city; cin >> city; // New York– Problem: input ends with whitespace

character– Solution: use getline fun from header

<string>– getline(cin, city, ’\n’);

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Reminder on Pointers

Reminder on Pointers

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Pointer basics

Pointers are variables that contain address values.

Pointers are variables used to store address values.

The basic concept of a pointer is:

indirect access to data values

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Pointer basics

Indirect access to data values:int alpha=5, beta, *ptr;

// & - address of operatorptr = α

//indirection or dereferencing operator

beta = *ptr;

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Declaration of pointers

How to define (declare) pointers as variables?

int *p1; char *p2; float *p3;

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More onArrays, Strings,

Pointers

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More on Pointers and Arrays

loop to traverse all array elements using direct access based on array subscripting expressions

int a[10]; for (I=0;I<10;I++) { a[I]=I; cout << endl << a[I]; }

loop to traverse all array elements using indirect access based on pointers

int a[10]; int *pa; pa = &a[0]; for (I=0;I<10;I++) { *pa=I; cout << endl << *pa; pa++; }

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char amessage[] = “Now is the time!”;

char *pmessage;

pmessage = “Now is the time!”;

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char amessage[] = “Now is the time!”;

int I=0;

while(amessage[I] != ‘\0’)

{

cout << endl << amessage[I];

I++;

}

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char *pmessage = “Now is the time!”; while(*pmessage != ‘\0’) {

cout << *pmessage; pmessage++; }

=================================================== char *pmessage = “Now is the time!”; char *q; q = pmessage + strlen(pmessage); while( pmessage < q ) {

cout << *pmessage; pmessage++; }

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More on Pointers

// Array of pointers

char *pname[] = { “Illegal”, “Jan”, “Feb”,

. . . “Nov”, “Dec” };

// 2D array

char aname[][15] ={ “Illegal”, “Jan”, “Feb”,

. . . “Nov”, “Dec” };

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More on Pointers

Multidimensional arrays and pointers

int a[10][20];

int *b[10];

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Pointers and function arguments

Problem: function to swap contents of two variables: void swap(int, int);

swap(a, b);

void swap(int p1, int p2){

int temp; temp=p1; p1=p2; p2=temp;}

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Pointers and function arguments

The solution: addresses specified as actual arguments void swap(int *, int *);

swap(&a, &b);

void swap(int *p1, int *p2){

int temp; temp=*p1; *p1=*p2; *p2=temp;}

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More on Pointers

Address arithmetic:

Given array: char a[10]; int I – array subscript with range of valid values 0…9

a ≡ a + 0 ≡ &a[0]

a + I ≡ &a[I]

*(a+I) ≡ *&a[I] ≡ a[I]

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More on Pointers

Pointers to functions int fact(int n) { if(n==0) return 1; return n*fact(n-1); }

int (*pf)(int); // pointer to function that has// one int param and returns int

Direct function call Indirect function callcout << fact(5); pf = fact;

cout << (*pf)(5);

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13.9 Common Programming Errors

Use the * de-referencing operator Operator -> member *p refers to the entire node p->x refers to member x new operator to allocate storage delete de-allocates storage Watch out for run-time errors with loops Don’t try to access a node returned to heap

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Exercise 25.1-25.6

Build programs based on pointers: Exchange values of two integer variables (function swap); Display a character string symbol by symbol on separate

lines in forward and backward order; Define the length of a character string (own version of

strlen function); Catenate two character strings (own version of strcat

function); Define a function returning the name of a month as a

character string; Operate as demo programs for pointers to functions.

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Exercise 25.1-25.6

Build programs based on pointers:

Display a C-style string symbol by symbol on separate lines in forward and backward order;– Using array subscript– Using pointer and test null terminator– Using pointer and test end array address

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Exercise 23.1

char str[] = “AUBG Blagoevgrad”;

void main(){ int I=0;

cout << endl << str << endl;while (str[I] != ‘\0’){

cout << endl << str[I];I++;

}}

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Exercise 23.1


void main(){ char *p = str;

cout << endl << str << endl << p << endl;while ( *p != ‘\0’){

cout << endl << *p;p++;

}}

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Exercise 23.1


void main(){ char *p = str;

cout << endl << str << endl << p << endl;while ( p < str + strlen(str)){

cout << endl << *p;p++;

}}

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Exercise 25.1-25.6

Build programs: Display a C++-style string symbol by symbol on

separate lines in forward and backward order;– Using methods .at(i) and .length()

string b="Sofia";

cout << endl << endl;

for(i=0; i<b.length(); i++) cout << b.at(i) << " ";

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Exercise 25.1-25.6


Define the length of a character string (own version of strlen function);

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Exercise 23.1

char str[] = “AUBG Blagoevgrad”; int strlenm(char m[]);

void main(){

cout << endl << strlenm(str) << endl;}int strlenm(char m[]){

int I=0, len;while (m[I] != 0x00) I++;len = I;return len;

}

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Exercise 23.1

char str[] = “AUBG Blagoevgrad”; int strlenm(char *pm);

void main(){

char *p = str;cout << endl << strlenm(str) << “ “ << strlenm(p) << endl;

}int strlenm(char *pm){

int len = 0;while (*pm != 0x00) { Ien++; pm++; )return len;

}

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Exercise 25.1-25.6


Copy a character string (own version of strcpy function);

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Exercise 23.1


void copym(char dst[], char src[]);

void main()

{ char newstr[20];

copym(newstr, str);

cout << endl << newstr << endl;

}

void copym(char dst[], char src[])

{

int I=0;

while( ( dst[I] = src[I] ) != ‘\0’ ) I++;

}

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Exercise 23.1


void copym(char *dst, char *src);

void main()

{ char *newstr; newstr = new char[20];

copym(newstr, str);

cout << endl << newstr << endl;

}

void copym(char *dst, char *src)

{

while( ( *dst = *src ) != ‘\0’ ) { dst++; src++; }

}

43Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

13.1 Pointers and the new Operator• Pointer Variable Declarations

– pointer variable of type “pointer to float”– can store the address of a float in p

float *p;

• The new operator creates (allocates memory for) a variable of type float & puts the address of the variable in pointer p

p = new float;

• Dynamic allocation occurs during program execution


Pointers

• Actual address has no meaning for us

• Form: type *variable;

• Example: float *p;

?

P


new Operator

• Actually allocates storage

• Form: new type;

new type [n];

• Example: new float;


Accessing Data with Pointers

• indirection operator **p = 15.5;

• Stores floating value 15.5 in memory location *p - the location pointed to by p

15.5

p


Pointers to Structs

struct electric{ string current; int volts;};electric *p, *q;• p and q are pointers to a struct of type

electric


Pointers to Structs

p = new electric;

• Allocates storage for struct of type electric and places address into pointer p

current voltsp? ?


struct Member Access through a Pointer

p ->current = “AC”;p ->volts = 115;

• Could also be referenced as(*p).current = “AC”;(*p).volts = 115;

current voltspAC 115


struct Member Access through a Pointer

• Form: p ->m

• Example: p ->voltscout << p->current << p->volts << endl;

• OutputAC115


Pointers and Structs

q = new electric;

• Allocates storage for struct of type electric and places address into pointer q

• Copy contents of p struct to q struct

*q = *p;current voltsp

AC 115

current voltsqAC 115


Pointers and Structs

q ->volts = 220;

q = p;

current voltsAC 220

q

AC 115

AC 220

p q->current q->voltsp->current p->volts

q


13.2 Manipulating the Heap

• When new executes where is struct stored ?

• Heap– C++ storage pool available to new operator

• Effect of

p = new electric;


Figure 13.1 Heap before and after execution of p - new node;


Returning Cells to the Heap• Operation

delete p;

• Returns cells back to heap for re-use• When finished with a pointer, delete it• Watch

– multiple pointers pointing to same address– only pointers created with new are deleted

• Form: delete variable;• Example: delete p;


13.4 The Stack Abstract Data Type

• A stack is a data structure in which only the top element can be accessed

• LIFO (Last-In First-Out)

• Operations– push– pop


A Stack of Characters

*

C

+

2

s


The C++ stack Class

• Must include stack library#include <stack>

• Declare the stackstack <type> stack-name;

• E.g.stack <string> nameStack;stack <char> s;


Some stack Member Functions

void push(const T&)

T top( ) const

void pop( )

bool empty( ) const


Example

x = s.top( ); // stores ‘*’ into x, stack unchanged

s.pop( ); // removes top of stack

s.push(‘/’); // adds ‘/’ to top of stack

*

C

+

2

s

C

+

2

s

/

C

+

2

s


13.5 The Queue ADT

• List-like structure where items are inserted at one end and removed from the other

• First-In-First-Out (FIFO)

• E.g. a customer waiting line


Figure 13.12 Queue of customers


The C++ queue Class

• Must include queue library#include <queue>

• Declare the stackqueue <type> queue-name;

• E.g. queue <string> customers;


Member Functions of queue Class

void push(const T&)

T top( ) const

void pop( )

bool empty( ) const

int size( ) const


13.6 Binary Trees

• Like a list with additional pointer

• Nodes contain 2 pointers– right pointer– left pointer– 0 (leaf nodes), 1, or 2 successor nodes

• Binary Tree – empty– root

• left and right sub-trees


Figure 13.13 Binary trees

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For

Your Attention

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Lecture 25: Practical Training on Pointers. 2 Lecture Contents: t Reminder on Arrays t Reminder on Strings t Reminder on Pointers t Demo programs, Exercises