itei303- lecture 2 pointers in c++

8/6/2019 ITEI303- Lecture 2 Pointers in C++

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Starting Out with C++, 3rdEdition1

Review on Pointers in C++

Engr. Julius S. Cansino


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9.1 Getting the address of a Variable

The address operator (&) returns the

memory address of a variable.


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Figure

9-1

1200 1201 1203

letter number amount


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Program 9-1// This programuses the & operatorto determinea variables

// address and the sizeofoperatorto determine its size.

#include

void main(void)

{int x = 25;

cout


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3URJUDP2XWSXW

The address of x is 0x8f05The size of x is 2 bytes

The value in x is 25


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Pointer Variables

Pointer variables, which are often just called

pointers, are designed to hold memory

addresses. With pointer variables you can

indirectly manipulate data stored in other

variables.


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Pointers are useful for the following:

Working with memory locations that

regular variables dont give you access to

Working with strings and arrays

Creating new variables in memory while the

program is running

Creating arbitrarily-sized lists of values in

memory


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Starting Out with C++, 3rdEdition

8

Program 9-2

// This program stores theaddress ofa variable ina pointer.#include

void main(void)

{

int x = 25;

int *ptr;

ptr = &x; // Storetheaddress of x in ptr

cout


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The value in x is 25

The address of x is 0x7e00


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F

igure9-2

0x7e00

25

ptr

x

Address of x: 0x7e00


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Program 9-3// This program demonstrates theuseofthe indirection

// operator.

#include

void main(void)

{

int x = 25;int *ptr;

ptr = &x; // Storetheaddress of x in ptr

cout


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Here is the value in x, printed twice:25 25Once again, here is the value in x:

100 100


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Program 9-4#include

void main(void)

{

int x = 25, y = 50, z = 75;

int *ptr;

cout


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Here are the values of x, y, and z:25 50 75Once again, here are the values of x, y , and z:

50 100 150


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9.3 Relationship Between Arrays and

Pointers

array names can be used as pointers, and

vice-versa.


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Program 9-5

// This program shows anarraynamebeing dereferenced

// with the * operator.

#include

void main(void)

{

shortnumbers[] = {10, 20, 30, 40, 50};

cout


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The first element in the array is 10


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F

igure9-3

numbers

numbers[0] numbers[1] numbers[2] numbers[3] numbers[4]


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F

igure9-4

numbers

numbers[0] numbers[1] numbers[2] numbers[3] numbers[4]

(numbers+1) (numbers+2) (numbers+3) (numbers+4)


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Program 9-6// This program processes thecontents ofanarray. Pointer

// notation is used.

#include

void main(void)

{

intnumbers[5];

cout > *(numbers + count);

cout


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3URJUDP2XWSXWZLWK([DPSOH,QSXW

Enter five numbers: >(QWHU@Here are the numbers you entered:

510152025


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Program 9-7// This programuses subscriptnotationwith a pointerand

// pointernotationwith anarrayname.

#include

void main(void)

{

floatcoins[5] = {0.05, 0.1, 0.25, 0.5, 1.0};float *floatPtr; // Pointertoafloat

intcount; // array index

floatPtr = coins; // floatPtrnow points tocoins array

cout.precision(2);

cout


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Program continues

for(count = 0; count < 5; count++)

cout


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Here are the values in the coins array:0.050.10.250.51

And here they are again:

0.050.10.250.51


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Program 9-8// This programuses theaddress ofeach element in

thearray.

#include

#include

void main(void)

{

floatcoins[5] = {0.05, 0.1, 0.25, 0.5, 1.0};

float *floatPtr; // Pointertoafloat

intcount; // array index

cout.precision(2);cout


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Program continues

for(count = 0; count < 5; count++)

{

floatPtr = &coins[count];

cout


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Here are the values in the coins array:

0.050.10.250.51


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9.4 Pointer Arithmetic

Some mathematical operations may beperformed on pointers.

The ++ and operators may be used toincrement or decrement a pointer variable.

An integer may be added to or subtracted froma pointer variable. This may be performed with

the +, - +=, or-= operators. A pointer may be subtracted from another

pointer.


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Program 9-9// This programuses a pointerto displaythecontents

// ofan integerarray.

#include

void main(void)

{

int set[8] = {5, 10, 15, 20, 25, 30, 35, 40};

int *nums, index;nums = set;

cout


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Program continues

cout


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The numbers in set are:5101520253035 40The numbers in set backwards are:

403530252015105


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9.5 Initializing Pointers

Pointers may be initialized with the address

of an existing object.


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9.6 Comparing Pointers

If one address comes before another address

in memory, the first address is considered

less than the second. C++s relationaloperators maybe used to compare pointer

values.


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Figure 9-5

0x5A00

array[0] array[1] array[2] array[3] array[4]

0x5A04 0x5A08 0x5A0C 0x5A0F

(Addresses)

An array of five integers


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Program 9-10

// This programuses a pointerto displaythecontents

// ofan integerarray.

#include

void main(void)

{

int set[8] = {5, 10, 15, 20, 25, 30, 35, 40};

int *nums = set; // Makenums pointto set

cout


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Program continues

cout


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The numbers in set are:5101520253035 40The numbers in set backwards are:

403530252015105


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9.7 Pointers as Function Parameters

A pointer can be used as a function

parameter. It gives the function access to

the original argument, much like a referenceparameter does.


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Program 9-11

// This programuses twofunctions thatacceptaddresses of

// variables as arguments.#include

// Function prototypes

void getNumber(int *);

void doubleValue(int *);

void main(void){

intnumber;

getNumber(&number) // Pass address ofnumbertogetNumber

doubleValue(&number); // and doubleValue.

cout


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Program continues

// DefinitionofgetNumber. The parameter, Input, is a pointer.

// This functionasks theuserforanumber. The valueentered// is stored inthe variable pointed toby Input.

void getNumber(int *input)

{

cout > *input;

}

// Definitionof doubleValue. The parameter, val, is a pointer.

// This functionmultiplies the variable pointed toby valby

// two.

void doubleValue(int *val)

{

*val *= 2;

}


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Enter an integer number: >(QWHU@

That value doubled is 20


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Program 9-12

// This program demonstrates thata pointermaybeused as a// parametertoaccepttheaddress ofanarray. Either subscript

// or pointernotationmaybeused.

#include

#include

// Function prototypes

void getSales(float *);floattotalSales(float *);

void main(void)

{

float sales[4];

getSales(sales);cout.precision(2);


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Program continues

cout.setf(ios::fixed | ios::showpoint);

cout


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Program continues

// DefinitionoftotalSales. This functionuses a pointerto

// accepttheaddress ofanarrayoffourfloats. Thefunction// gets thetotaloftheelements inthearrayand returns that

// value.(Pointernotation is used inthis function.)

floattotalSales(float *array)

{

float sum = 0.0;

for(intcount = 0; count < 4; count++)

{

sum += *array;

array++;

}return sum;

}


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Enter the sales figure for quarter1: >(QWHU@Enter the sales figure for quarter2: >(QWHU@Enter the sales figure for quarter3: >(QWHU@

Ente

r the

sale

s figure

for quarte

r 4:>(QWHU@

The total sales for the year are $48967.86


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9.8 Focus on Software Engineering:

Dynamic Memory Allocation

Variables may be created and destroyedwhile a program is running.

A pointer than contains the address 0 iscalled a null pointer.

Use the new operator to dynamicallyallocate memory.

Use delete to dynamically deallocatememory.


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Program 9-13// This programtotals and averages the sales figures forany

// numberof days. Thefigures are stored ina dynamically// allocated array.

#include

#include

void main(void)

{float *sales, total = 0, average;

intnumDays;

cout numDays;

sales = newfloat[numDays]; // Allocatememory


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Program continues

if(sales == NULL) // Testfornull pointer

{

cout


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Program continues

// Calculatetheaverage sales per day

average = total / numDays;

// Displaytheresults

cout.precision(2);cout.setf(ios::fixed | ios::showpoint);

cout


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How many days of sales figures do you wish to process? >(QWHU@Enter the sales figures below.Day 1: >(QWHU@Day 2: >(QWHU@Day 3: >(QWHU@Day 4: >(QWHU@Day 5: >(QWHU@total sales: $4067.13

average sales: $813.43


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St ti O t ith C++ 3rd Editi

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9.9 Focus on Software Engineering:

Returning Pointers from Functions

Functions can return pointers, but you must

be sure the object the pointer references still

exists.

You should only return a pointer from a

function if it is:

A pointer to an object that was passed into thefunction as an argument.

A pointer to a dynamically allocated object.

itei303- lecture 2 pointers in c++

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