Files and Streams

Files and Streams

Chapter 9

C++ An Introduction to Computing, 3rd ed. 2

Objectives

Use OCD to solve a problem involving files

Study C++'s support for I/O with files

Examine how interactive and file streams carry out I/O

Look at string streams, how used for file I/O

Learn about files in context of database-management systems

Show how I/O operations can be used for classes

Study ho class converters can use string streams


Problem: Processing Meteorological Data

Consider large amounts of weather related data. • Pressure readings recorded every 15 min• Data stored in file named pressure.dat• Must compute minimum, maximum, average

Program is to be created to • Read the data, • Calculate the statistics• Write the results to an output file


Objects

DescriptionSoftware Objects

Type Kind Namename of input file string varying inputFileName

pressure reading double varying reading

number of readings int varying count

minimum reading double varying minimum

maximum reading double varying maximum

sum of readings double varying sum

average reading double varying average

name of output file string varying outputFileName


Operationsi. Prompt for, read string (file names) from

keyboardii. Initialize count, sum, maximum, minimumiii. Read a real value (reading) from fileiv. Increment integer (count) by 1v. Add real value (reading) to real (sum)vi. Update minimum or maximum as necessary

with readingvii. Repeat iii – vi until end of fileviii. Write real values (minimum, maximum,

sum/count) to output file


Additional Operations, Objects

Operations• Open the input, output files• Close the files as necessary

Objects

DescriptionSoftware Objects

Type Kind Nameinput stream ifstream varying inStream

output stream ofstream varying outStream


Algorithm1. Prompt for, read name of input file into inputFileName2. Open ifstream named inStream to file given by inputFileName3. Initialize count to 0, sum to 0.0, maximum to smallest possible value,

minimum to maximum possible value4. Loop

a. Read real value for reading from inStreamb. If eof mark read, exit loopc. Increment countd. Add reading to sume. If reading < minimum, set minimum to readingf. If reading > maximum, set maximum to reading

End loop5. Close inStream6. Prompt for, read name of output file into outputFileName7. Open an ofstream named outStream to file given by outputFileName8. Write count to outStream9. If count > 0, write minimum, maximum, sum/count to outStream10. Close outStream


Coding and Testing

View source code in Figure 9.1

Test runs Figure 9.2• Note that screen output is only the prompting

and the entry of the names• All input data came from the input files• All output reporting went to the output files


Declaring fstream Objects

An istream object named cin connects program and keyboard

An ostream object named cout connects the program and the screen

These streams are constructed automatically.

These streams are constructed automatically.


Declaring fstream Objects

For doing I/O from/to a file a program must explicitly open a stream• Creates a connection between a program in

memory and a text file


Basic fstream Operations

open() Establishes connection program to file

is_open() Returns true/false

>> Operator, inputs value from file

getline() Reads line of text into string object

<< Operator, outputs value to file

eof() Returns true/false, end of file

close() Terminates connection betweenprogram, file


The open() Operation

Given:ifstream inStream;inStream.open( "pressure.dat");

Parameter can also be a variable• If it is a string variable ( string fileName ) must

use fileName.data() for correct parameter type

When input file is opened, read position pointer set to beginning of sequence of characters in the file



When output file is opened, file is created on the disk, with write-position pointer pointing at the eof marker

Opening an ofstream to a file will create a new file• If file existed before, it is now (by default) destroyed• Otherwise, new file is created



Possible to open the file with a mode argument as a second parameter

Mode Description

ios::inopen for input, non destructive, read pointer at beginning

ios::trunc Open file, delete contents it contains

ios::out Open for output, use ios::trunc

ios::appOpen for output, nondestructive, write position at end of file

ios::ateOpen existing file with read (or write) position at end of file

ios::binary Open file in binary mode


Initialization at Declaration

Possible to open at declaration of variableofstream outStream ("pressure.out");ifstream inStream ("pressure.in");

ExecutingProgram


Programming Defensively

The success or failure of a call to open a file should always be tested• Use inStream.open() • Us in an assert( ) mechanism• Call before proceeding with additional

operations on the file

Consider application of some of these concepts in Figure 9.3• Note the overloading of interactiveOpen()


The Input Operator

We have used cin >> x;• Value entered via the keyboard

C++ uses the same operator to bring values into variables from a stream inStream >> reading;

The reading pointer keeps track of where in the stream the program is currently reading


The getline() Function

Requires an istream object, a string object getline (nameStream, name);

Reads entire name into variable• Reads until it hits a newline character• Newline character read, not added to variable

Note: the >> operator does not read the newline. The next >> skips it as white space. But if a getline is used next, it sees

the newline and terminates.

Think about what happens if you mix >> and getline calls.

Note: the >> operator does not read the newline. The next >> skips it as white space. But if a getline is used next, it sees

the newline and terminates.

Think about what happens if you mix >> and getline calls.


Can be used as a sentinel value to control an input loopfor ( ; ; ) { inStream >> reading; if (inStream.eof() ) break; // . . . process the input }

inStream.eof() returns true following execution of the input statement at this point

The eof() Message


The Output Operator <<

Overloaded to perform with ostream, ofstream objectsoutStream <<"\n--> There were a total of" << count << "values.";

Note that the write pointer is pushed forward, keeps pointing at the eof marker.


The close() Message

The file stream is disconnected when• The program leaves the scope of the fstream

object (implicitly)• The close() message is executed (explicitly)

It is good practice to explicitly close a file when the program is done using it• If many files are accessed, the operating

system may place a limit on how many files are open simultaneously


File Streams as Parameters

Parameters corresponding to file stream arguments must be reference parameters.

Because:• Reading from an ifstream alters the read

position in that ifstream• Writing to the ofstream alters the write

position in that ofstream


File I/O Example:Scanning for a Virus

A virus is a program that hides itself within other programs• It tries to proliferate by attaching itself to as

many other programs as possible

It can be malicious or simply annoying• Deleting files• Displaying a goofy message on the screen


Combating Viruses

Virus detection and recovery• Identify a virus in a system• Remove the virus

Virus prevention• Keep new viruses from infecting a computer• Watch for behavior characteristic of viruses

Detection – see Figure 9.4• Read lines from a file• Scan the line for specified string of text• If text found, display message


Manipulators for Random Access

Sequential access• Values in the file processed in sequence• Start at first values, read through to last

Random or direct access• Value accessed at any location in the file• Specify the location, then do the read

Two-Pass file process – Figure 9.5• Read through values, calculate the average• Then go back through file to find deviations

from the average


Status Operations

To determine the status of a stream, the libraries provide these function members:good() // returns true iff stream is ok

bad() // returns true iff stream is not ok

fail() // returns true iff last operation failed

eof() // returns true iff last file-read failed


Change-State Operations

To change the state of a stream, the libraries provide these function members:clear() // reset status to good

setstate(b) // set state bit b (one ofios_base::goodbit,ios_base::badbit,ios_base::failbit, orios_base::eofbit ).


Read-Position Operations

To manipulate the read-position within an ifstream, the libraries provide these:tellg() // returns offset of current

read-position frombeginning of file

seekg(offset, base) // move read-position

offset bytes from base (one of ios_base::beg,ios_base::cur, orios_base::end)


Write-Position Operations

To manipulate the write-position within an ofstream, the libraries provide these:

tellp() // returns offset of currentwrite-position frombeginning of file

seekp(offset, base) // move write-position offset bytes from base (one of ios_base::beg,ios_base::cur, orios_base::end )


Other Operations

To look at the next character in an ifstream without advancing the read-position (i.e., without reading it), the libraries provide:peek() // returns next char in the

stream without reading itTo “unread” the last char that was read, the

libraries provide:unget() // unread char most recently

read


Another Operation

To skip a given number of chars in the stream (or until a particular char is encountered), the libraries provide:

ignore(n, stopChar) // skip past n chars, or until stopChar is encountered


Manipulators without Arguments

Format control – use format manipulators• fixed used fixed point notation, reals• showpoint show decimal pt., trailing zeros• right right justify values, pad left• left left justify values, pad right

Example:cout << "\nTotal cost = $" << fixed << showpoint << cost << endl;

Note more extensive table of format manipulators on page 538


Manipulators with Arguments

Format manipulators with arguments• setprecision(n) specify decimal digits• setw(n) specify width of field

These require#include <iomanip>

Examplecout << "\nTotal cost = $" << fixed << showpoint << setprecision(2) << cost << endl;


String Streams

C++ provides capability to • Read input from a string object• Write output to a string object

String streams provided• istringstream Input • ostringstream Output• stringstream For both input, ouput

View sample program, Figure 9.7


Database

Businesses keep large amounts of data• For reviewing trends• For making business decisions

Data must be conveniently accessible• Managing the databases done by

Database-Management Systems


Database Facilities

High-level views of the data

Access routines

Support for large databases

Security

Data sharing


Databases

Relational Databases organized into tables• Each column is a field of the table• Each row is a record in the table

The database-system takes care of• File names• Data representation

SQL – a language that provides query capabilities of tablesFurther information• www.oracle.com• www.sybase.com


OBJECTive Thinking:Objects and Streams

Objects and File I/O• Possible to pass ifstream objects as parameters

where we have used istream objects• ifstream fin ("nameFile.txt");aName.read(fin);

The ifstream class is derived from the istream class• An ifstream object is also an istream object

Similarly with ofstream and ostream objects

Note example in Figure 9.8


Converter Methods

A converter is given an object of one time and produces an object as another type• Accessor converters• Constructor converters

Common to have converters that provide conversion to and from strings

Note example, Figure 9.9• Driver to test converter operations, Figure 9.10


Converter Methods

Sphere object converter, Figure 9.11• Driver, Figure 9.12

Convertsfrom a Sphere

to a string

Convertsfrom a Sphere

to a string

Documents

Files and Streams